reorganize compiler crates

ast/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "rustpython-ast"
+version = "0.1.0"
+authors = ["RustPython Team"]
+edition = "2021"
+
+[features]
+default = ["constant-optimization", "fold"]
+constant-optimization = ["fold"]
+fold = []
+unparse = ["rustpython-common"]
+
+[dependencies]
+num-bigint = "0.4.3"
+rustpython-common = { path = "../../common", optional = true }
+rustpython-bytecode = { path = "../bytecode"}

ast/Python.asdl

@@ -0,0 +1,144 @@
+-- ASDL's 4 builtin types are:
+-- identifier, int, string, constant
+
+module Python
+{
+    mod = Module(stmt* body, type_ignore* type_ignores)
+        | Interactive(stmt* body)
+        | Expression(expr body)
+        | FunctionType(expr* argtypes, expr returns)
+
+    stmt = FunctionDef(identifier name, arguments args,
+                       stmt* body, expr* decorator_list, expr? returns,
+                       string? type_comment)
+          | AsyncFunctionDef(identifier name, arguments args,
+                             stmt* body, expr* decorator_list, expr? returns,
+                             string? type_comment)
+
+          | ClassDef(identifier name,
+             expr* bases,
+             keyword* keywords,
+             stmt* body,
+             expr* decorator_list)
+          | Return(expr? value)
+
+          | Delete(expr* targets)
+          | Assign(expr* targets, expr value, string? type_comment)
+          | AugAssign(expr target, operator op, expr value)
+          -- 'simple' indicates that we annotate simple name without parens
+          | AnnAssign(expr target, expr annotation, expr? value, int simple)
+
+          -- use 'orelse' because else is a keyword in target languages
+          | For(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
+          | AsyncFor(expr target, expr iter, stmt* body, stmt* orelse, string? type_comment)
+          | While(expr test, stmt* body, stmt* orelse)
+          | If(expr test, stmt* body, stmt* orelse)
+          | With(withitem* items, stmt* body, string? type_comment)
+          | AsyncWith(withitem* items, stmt* body, string? type_comment)
+
+          | Match(expr subject, match_case* cases)
+
+          | Raise(expr? exc, expr? cause)
+          | Try(stmt* body, excepthandler* handlers, stmt* orelse, stmt* finalbody)
+          | Assert(expr test, expr? msg)
+
+          | Import(alias* names)
+          | ImportFrom(identifier? module, alias* names, int? level)
+
+          | Global(identifier* names)
+          | Nonlocal(identifier* names)
+          | Expr(expr value)
+          | Pass | Break | Continue
+
+          -- col_offset is the byte offset in the utf8 string the parser uses
+          attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
+
+          -- BoolOp() can use left & right?
+    expr = BoolOp(boolop op, expr* values)
+         | NamedExpr(expr target, expr value)
+         | BinOp(expr left, operator op, expr right)
+         | UnaryOp(unaryop op, expr operand)
+         | Lambda(arguments args, expr body)
+         | IfExp(expr test, expr body, expr orelse)
+         | Dict(expr* keys, expr* values)
+         | Set(expr* elts)
+         | ListComp(expr elt, comprehension* generators)
+         | SetComp(expr elt, comprehension* generators)
+         | DictComp(expr key, expr value, comprehension* generators)
+         | GeneratorExp(expr elt, comprehension* generators)
+         -- the grammar constrains where yield expressions can occur
+         | Await(expr value)
+         | Yield(expr? value)
+         | YieldFrom(expr value)
+         -- need sequences for compare to distinguish between
+         -- x < 4 < 3 and (x < 4) < 3
+         | Compare(expr left, cmpop* ops, expr* comparators)
+         | Call(expr func, expr* args, keyword* keywords)
+         | FormattedValue(expr value, int conversion, expr? format_spec)
+         | JoinedStr(expr* values)
+         | Constant(constant value, string? kind)
+
+         -- the following expression can appear in assignment context
+         | Attribute(expr value, identifier attr, expr_context ctx)
+         | Subscript(expr value, expr slice, expr_context ctx)
+         | Starred(expr value, expr_context ctx)
+         | Name(identifier id, expr_context ctx)
+         | List(expr* elts, expr_context ctx)
+         | Tuple(expr* elts, expr_context ctx)
+
+         -- can appear only in Subscript
+         | Slice(expr? lower, expr? upper, expr? step)
+
+          -- col_offset is the byte offset in the utf8 string the parser uses
+          attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
+
+    expr_context = Load | Store | Del
+
+    boolop = And | Or
+
+    operator = Add | Sub | Mult | MatMult | Div | Mod | Pow | LShift
+                 | RShift | BitOr | BitXor | BitAnd | FloorDiv
+
+    unaryop = Invert | Not | UAdd | USub
+
+    cmpop = Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn
+
+    comprehension = (expr target, expr iter, expr* ifs, int is_async)
+
+    excepthandler = ExceptHandler(expr? type, identifier? name, stmt* body)
+                    attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
+
+    arguments = (arg* posonlyargs, arg* args, arg? vararg, arg* kwonlyargs,
+                 expr* kw_defaults, arg? kwarg, expr* defaults)
+
+    arg = (identifier arg, expr? annotation, string? type_comment)
+           attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
+
+    -- keyword arguments supplied to call (NULL identifier for **kwargs)
+    keyword = (identifier? arg, expr value)
+               attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
+
+    -- import name with optional 'as' alias.
+    alias = (identifier name, identifier? asname)
+             attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
+
+    withitem = (expr context_expr, expr? optional_vars)
+
+    match_case = (pattern pattern, expr? guard, stmt* body)
+
+    pattern = MatchValue(expr value)
+            | MatchSingleton(constant value)
+            | MatchSequence(pattern* patterns)
+            | MatchMapping(expr* keys, pattern* patterns, identifier? rest)
+            | MatchClass(expr cls, pattern* patterns, identifier* kwd_attrs, pattern* kwd_patterns)
+
+            | MatchStar(identifier? name)
+            -- The optional "rest" MatchMapping parameter handles capturing extra mapping keys
+
+            | MatchAs(pattern? pattern, identifier? name)
+            | MatchOr(pattern* patterns)
+
+             attributes (int lineno, int col_offset, int end_lineno, int end_col_offset)
+
+    type_ignore = TypeIgnore(int lineno, string tag)
+}

ast/asdl.py

@@ -0,0 +1,385 @@
+#-------------------------------------------------------------------------------
+# Parser for ASDL [1] definition files. Reads in an ASDL description and parses
+# it into an AST that describes it.
+#
+# The EBNF we're parsing here: Figure 1 of the paper [1]. Extended to support
+# modules and attributes after a product. Words starting with Capital letters
+# are terminals. Literal tokens are in "double quotes". Others are
+# non-terminals. Id is either TokenId or ConstructorId.
+#
+# module        ::= "module" Id "{" [definitions] "}"
+# definitions   ::= { TypeId "=" type }
+# type          ::= product | sum
+# product       ::= fields ["attributes" fields]
+# fields        ::= "(" { field, "," } field ")"
+# field         ::= TypeId ["?" | "*"] [Id]
+# sum           ::= constructor { "|" constructor } ["attributes" fields]
+# constructor   ::= ConstructorId [fields]
+#
+# [1] "The Zephyr Abstract Syntax Description Language" by Wang, et. al. See
+#     http://asdl.sourceforge.net/
+#-------------------------------------------------------------------------------
+from collections import namedtuple
+import re
+
+__all__ = [
+    'builtin_types', 'parse', 'AST', 'Module', 'Type', 'Constructor',
+    'Field', 'Sum', 'Product', 'VisitorBase', 'Check', 'check']
+
+# The following classes define nodes into which the ASDL description is parsed.
+# Note: this is a "meta-AST". ASDL files (such as Python.asdl) describe the AST
+# structure used by a programming language. But ASDL files themselves need to be
+# parsed. This module parses ASDL files and uses a simple AST to represent them.
+# See the EBNF at the top of the file to understand the logical connection
+# between the various node types.
+
+builtin_types = {'identifier', 'string', 'int', 'constant'}
+
+class AST:
+    def __repr__(self):
+        raise NotImplementedError
+
+class Module(AST):
+    def __init__(self, name, dfns):
+        self.name = name
+        self.dfns = dfns
+        self.types = {type.name: type.value for type in dfns}
+
+    def __repr__(self):
+        return 'Module({0.name}, {0.dfns})'.format(self)
+
+class Type(AST):
+    def __init__(self, name, value):
+        self.name = name
+        self.value = value
+
+    def __repr__(self):
+        return 'Type({0.name}, {0.value})'.format(self)
+
+class Constructor(AST):
+    def __init__(self, name, fields=None):
+        self.name = name
+        self.fields = fields or []
+
+    def __repr__(self):
+        return 'Constructor({0.name}, {0.fields})'.format(self)
+
+class Field(AST):
+    def __init__(self, type, name=None, seq=False, opt=False):
+        self.type = type
+        self.name = name
+        self.seq = seq
+        self.opt = opt
+
+    def __str__(self):
+        if self.seq:
+            extra = "*"
+        elif self.opt:
+            extra = "?"
+        else:
+            extra = ""
+
+        return "{}{} {}".format(self.type, extra, self.name)
+
+    def __repr__(self):
+        if self.seq:
+            extra = ", seq=True"
+        elif self.opt:
+            extra = ", opt=True"
+        else:
+            extra = ""
+        if self.name is None:
+            return 'Field({0.type}{1})'.format(self, extra)
+        else:
+            return 'Field({0.type}, {0.name}{1})'.format(self, extra)
+
+class Sum(AST):
+    def __init__(self, types, attributes=None):
+        self.types = types
+        self.attributes = attributes or []
+
+    def __repr__(self):
+        if self.attributes:
+            return 'Sum({0.types}, {0.attributes})'.format(self)
+        else:
+            return 'Sum({0.types})'.format(self)
+
+class Product(AST):
+    def __init__(self, fields, attributes=None):
+        self.fields = fields
+        self.attributes = attributes or []
+
+    def __repr__(self):
+        if self.attributes:
+            return 'Product({0.fields}, {0.attributes})'.format(self)
+        else:
+            return 'Product({0.fields})'.format(self)
+
+# A generic visitor for the meta-AST that describes ASDL. This can be used by
+# emitters. Note that this visitor does not provide a generic visit method, so a
+# subclass needs to define visit methods from visitModule to as deep as the
+# interesting node.
+# We also define a Check visitor that makes sure the parsed ASDL is well-formed.
+
+class VisitorBase(object):
+    """Generic tree visitor for ASTs."""
+    def __init__(self):
+        self.cache = {}
+
+    def visit(self, obj, *args):
+        klass = obj.__class__
+        meth = self.cache.get(klass)
+        if meth is None:
+            methname = "visit" + klass.__name__
+            meth = getattr(self, methname, None)
+            self.cache[klass] = meth
+        if meth:
+            try:
+                meth(obj, *args)
+            except Exception as e:
+                print("Error visiting %r: %s" % (obj, e))
+                raise
+
+class Check(VisitorBase):
+    """A visitor that checks a parsed ASDL tree for correctness.
+
+    Errors are printed and accumulated.
+    """
+    def __init__(self):
+        super(Check, self).__init__()
+        self.cons = {}
+        self.errors = 0
+        self.types = {}
+
+    def visitModule(self, mod):
+        for dfn in mod.dfns:
+            self.visit(dfn)
+
+    def visitType(self, type):
+        self.visit(type.value, str(type.name))
+
+    def visitSum(self, sum, name):
+        for t in sum.types:
+            self.visit(t, name)
+
+    def visitConstructor(self, cons, name):
+        key = str(cons.name)
+        conflict = self.cons.get(key)
+        if conflict is None:
+            self.cons[key] = name
+        else:
+            print('Redefinition of constructor {}'.format(key))
+            print('Defined in {} and {}'.format(conflict, name))
+            self.errors += 1
+        for f in cons.fields:
+            self.visit(f, key)
+
+    def visitField(self, field, name):
+        key = str(field.type)
+        l = self.types.setdefault(key, [])
+        l.append(name)
+
+    def visitProduct(self, prod, name):
+        for f in prod.fields:
+            self.visit(f, name)
+
+def check(mod):
+    """Check the parsed ASDL tree for correctness.
+
+    Return True if success. For failure, the errors are printed out and False
+    is returned.
+    """
+    v = Check()
+    v.visit(mod)
+
+    for t in v.types:
+        if t not in mod.types and not t in builtin_types:
+            v.errors += 1
+            uses = ", ".join(v.types[t])
+            print('Undefined type {}, used in {}'.format(t, uses))
+    return not v.errors
+
+# The ASDL parser itself comes next. The only interesting external interface
+# here is the top-level parse function.
+
+def parse(filename):
+    """Parse ASDL from the given file and return a Module node describing it."""
+    with open(filename, encoding="utf-8") as f:
+        parser = ASDLParser()
+        return parser.parse(f.read())
+
+# Types for describing tokens in an ASDL specification.
+class TokenKind:
+    """TokenKind is provides a scope for enumerated token kinds."""
+    (ConstructorId, TypeId, Equals, Comma, Question, Pipe, Asterisk,
+     LParen, RParen, LBrace, RBrace) = range(11)
+
+    operator_table = {
+        '=': Equals, ',': Comma,    '?': Question, '|': Pipe,    '(': LParen,
+        ')': RParen, '*': Asterisk, '{': LBrace,   '}': RBrace}
+
+Token = namedtuple('Token', 'kind value lineno')
+
+class ASDLSyntaxError(Exception):
+    def __init__(self, msg, lineno=None):
+        self.msg = msg
+        self.lineno = lineno or '<unknown>'
+
+    def __str__(self):
+        return 'Syntax error on line {0.lineno}: {0.msg}'.format(self)
+
+def tokenize_asdl(buf):
+    """Tokenize the given buffer. Yield Token objects."""
+    for lineno, line in enumerate(buf.splitlines(), 1):
+        for m in re.finditer(r'\s*(\w+|--.*|.)', line.strip()):
+            c = m.group(1)
+            if c[0].isalpha():
+                # Some kind of identifier
+                if c[0].isupper():
+                    yield Token(TokenKind.ConstructorId, c, lineno)
+                else:
+                    yield Token(TokenKind.TypeId, c, lineno)
+            elif c[:2] == '--':
+                # Comment
+                break
+            else:
+                # Operators
+                try:
+                    op_kind = TokenKind.operator_table[c]
+                except KeyError:
+                    raise ASDLSyntaxError('Invalid operator %s' % c, lineno)
+                yield Token(op_kind, c, lineno)
+
+class ASDLParser:
+    """Parser for ASDL files.
+
+    Create, then call the parse method on a buffer containing ASDL.
+    This is a simple recursive descent parser that uses tokenize_asdl for the
+    lexing.
+    """
+    def __init__(self):
+        self._tokenizer = None
+        self.cur_token = None
+
+    def parse(self, buf):
+        """Parse the ASDL in the buffer and return an AST with a Module root.
+        """
+        self._tokenizer = tokenize_asdl(buf)
+        self._advance()
+        return self._parse_module()
+
+    def _parse_module(self):
+        if self._at_keyword('module'):
+            self._advance()
+        else:
+            raise ASDLSyntaxError(
+                'Expected "module" (found {})'.format(self.cur_token.value),
+                self.cur_token.lineno)
+        name = self._match(self._id_kinds)
+        self._match(TokenKind.LBrace)
+        defs = self._parse_definitions()
+        self._match(TokenKind.RBrace)
+        return Module(name, defs)
+
+    def _parse_definitions(self):
+        defs = []
+        while self.cur_token.kind == TokenKind.TypeId:
+            typename = self._advance()
+            self._match(TokenKind.Equals)
+            type = self._parse_type()
+            defs.append(Type(typename, type))
+        return defs
+
+    def _parse_type(self):
+        if self.cur_token.kind == TokenKind.LParen:
+            # If we see a (, it's a product
+            return self._parse_product()
+        else:
+            # Otherwise it's a sum. Look for ConstructorId
+            sumlist = [Constructor(self._match(TokenKind.ConstructorId),
+                                   self._parse_optional_fields())]
+            while self.cur_token.kind  == TokenKind.Pipe:
+                # More constructors
+                self._advance()
+                sumlist.append(Constructor(
+                                self._match(TokenKind.ConstructorId),
+                                self._parse_optional_fields()))
+            return Sum(sumlist, self._parse_optional_attributes())
+
+    def _parse_product(self):
+        return Product(self._parse_fields(), self._parse_optional_attributes())
+
+    def _parse_fields(self):
+        fields = []
+        self._match(TokenKind.LParen)
+        while self.cur_token.kind == TokenKind.TypeId:
+            typename = self._advance()
+            is_seq, is_opt = self._parse_optional_field_quantifier()
+            id = (self._advance() if self.cur_token.kind in self._id_kinds
+                                  else None)
+            fields.append(Field(typename, id, seq=is_seq, opt=is_opt))
+            if self.cur_token.kind == TokenKind.RParen:
+                break
+            elif self.cur_token.kind == TokenKind.Comma:
+                self._advance()
+        self._match(TokenKind.RParen)
+        return fields
+
+    def _parse_optional_fields(self):
+        if self.cur_token.kind == TokenKind.LParen:
+            return self._parse_fields()
+        else:
+            return None
+
+    def _parse_optional_attributes(self):
+        if self._at_keyword('attributes'):
+            self._advance()
+            return self._parse_fields()
+        else:
+            return None
+
+    def _parse_optional_field_quantifier(self):
+        is_seq, is_opt = False, False
+        if self.cur_token.kind == TokenKind.Asterisk:
+            is_seq = True
+            self._advance()
+        elif self.cur_token.kind == TokenKind.Question:
+            is_opt = True
+            self._advance()
+        return is_seq, is_opt
+
+    def _advance(self):
+        """ Return the value of the current token and read the next one into
+            self.cur_token.
+        """
+        cur_val = None if self.cur_token is None else self.cur_token.value
+        try:
+            self.cur_token = next(self._tokenizer)
+        except StopIteration:
+            self.cur_token = None
+        return cur_val
+
+    _id_kinds = (TokenKind.ConstructorId, TokenKind.TypeId)
+
+    def _match(self, kind):
+        """The 'match' primitive of RD parsers.
+
+        * Verifies that the current token is of the given kind (kind can
+          be a tuple, in which the kind must match one of its members).
+        * Returns the value of the current token
+        * Reads in the next token
+        """
+        if (isinstance(kind, tuple) and self.cur_token.kind in kind or
+            self.cur_token.kind == kind
+            ):
+            value = self.cur_token.value
+            self._advance()
+            return value
+        else:
+            raise ASDLSyntaxError(
+                'Unmatched {} (found {})'.format(kind, self.cur_token.kind),
+                self.cur_token.lineno)
+
+    def _at_keyword(self, keyword):
+        return (self.cur_token.kind == TokenKind.TypeId and
+                self.cur_token.value == keyword)

ast/asdl_rs.py

@@ -0,0 +1,726 @@
+#! /usr/bin/env python
+"""Generate Rust code from an ASDL description."""
+
+import sys
+import json
+import textwrap
+
+from argparse import ArgumentParser
+from pathlib import Path
+
+import asdl
+
+TABSIZE = 4
+AUTOGEN_MESSAGE = "// File automatically generated by {}.\n"
+
+builtin_type_mapping = {
+    "identifier": "Ident",
+    "string": "String",
+    "int": "usize",
+    "constant": "Constant",
+}
+assert builtin_type_mapping.keys() == asdl.builtin_types
+
+
+def get_rust_type(name):
+    """Return a string for the C name of the type.
+
+    This function special cases the default types provided by asdl.
+    """
+    if name in asdl.builtin_types:
+        return builtin_type_mapping[name]
+    elif name.islower():
+        return "".join(part.capitalize() for part in name.split("_"))
+    else:
+        return name
+
+
+def is_simple(sum):
+    """Return True if a sum is a simple.
+
+    A sum is simple if its types have no fields, e.g.
+    unaryop = Invert | Not | UAdd | USub
+    """
+    for t in sum.types:
+        if t.fields:
+            return False
+    return True
+
+
+def asdl_of(name, obj):
+    if isinstance(obj, asdl.Product) or isinstance(obj, asdl.Constructor):
+        fields = ", ".join(map(str, obj.fields))
+        if fields:
+            fields = "({})".format(fields)
+        return "{}{}".format(name, fields)
+    else:
+        if is_simple(obj):
+            types = " | ".join(type.name for type in obj.types)
+        else:
+            sep = "\n{}| ".format(" " * (len(name) + 1))
+            types = sep.join(asdl_of(type.name, type) for type in obj.types)
+        return "{} = {}".format(name, types)
+
+
+class EmitVisitor(asdl.VisitorBase):
+    """Visit that emits lines"""
+
+    def __init__(self, file):
+        self.file = file
+        self.identifiers = set()
+        super(EmitVisitor, self).__init__()
+
+    def emit_identifier(self, name):
+        name = str(name)
+        if name in self.identifiers:
+            return
+        self.emit("_Py_IDENTIFIER(%s);" % name, 0)
+        self.identifiers.add(name)
+
+    def emit(self, line, depth):
+        if line:
+            line = (" " * TABSIZE * depth) + line
+        self.file.write(line + "\n")
+
+
+class TypeInfo:
+    def __init__(self, name):
+        self.name = name
+        self.has_userdata = None
+        self.children = set()
+        self.boxed = False
+
+    def __repr__(self):
+        return f"<TypeInfo: {self.name}>"
+
+    def determine_userdata(self, typeinfo, stack):
+        if self.name in stack:
+            return None
+        stack.add(self.name)
+        for child, child_seq in self.children:
+            if child in asdl.builtin_types:
+                continue
+            childinfo = typeinfo[child]
+            child_has_userdata = childinfo.determine_userdata(typeinfo, stack)
+            if self.has_userdata is None and child_has_userdata is True:
+                self.has_userdata = True
+
+        stack.remove(self.name)
+        return self.has_userdata
+
+
+class FindUserdataTypesVisitor(asdl.VisitorBase):
+    def __init__(self, typeinfo):
+        self.typeinfo = typeinfo
+        super().__init__()
+
+    def visitModule(self, mod):
+        for dfn in mod.dfns:
+            self.visit(dfn)
+        stack = set()
+        for info in self.typeinfo.values():
+            info.determine_userdata(self.typeinfo, stack)
+
+    def visitType(self, type):
+        self.typeinfo[type.name] = TypeInfo(type.name)
+        self.visit(type.value, type.name)
+
+    def visitSum(self, sum, name):
+        info = self.typeinfo[name]
+        if is_simple(sum):
+            info.has_userdata = False
+        else:
+            if len(sum.types) > 1:
+                info.boxed = True
+            if sum.attributes:
+                # attributes means Located, which has the `custom: U` field
+                info.has_userdata = True
+        for variant in sum.types:
+            self.add_children(name, variant.fields)
+
+    def visitProduct(self, product, name):
+        info = self.typeinfo[name]
+        if product.attributes:
+            # attributes means Located, which has the `custom: U` field
+            info.has_userdata = True
+        if len(product.fields) > 2:
+            info.boxed = True
+        self.add_children(name, product.fields)
+
+    def add_children(self, name, fields):
+        self.typeinfo[name].children.update((field.type, field.seq) for field in fields)
+
+
+def rust_field(field_name):
+    if field_name == "type":
+        return "type_"
+    else:
+        return field_name
+
+
+class TypeInfoEmitVisitor(EmitVisitor):
+    def __init__(self, file, typeinfo):
+        self.typeinfo = typeinfo
+        super().__init__(file)
+
+    def has_userdata(self, typ):
+        return self.typeinfo[typ].has_userdata
+
+    def get_generics(self, typ, *generics):
+        if self.has_userdata(typ):
+            return [f"<{g}>" for g in generics]
+        else:
+            return ["" for g in generics]
+
+
+class StructVisitor(TypeInfoEmitVisitor):
+    """Visitor to generate typedefs for AST."""
+
+    def visitModule(self, mod):
+        for dfn in mod.dfns:
+            self.visit(dfn)
+
+    def visitType(self, type, depth=0):
+        self.visit(type.value, type.name, depth)
+
+    def visitSum(self, sum, name, depth):
+        if is_simple(sum):
+            self.simple_sum(sum, name, depth)
+        else:
+            self.sum_with_constructors(sum, name, depth)
+
+    def emit_attrs(self, depth):
+        self.emit("#[derive(Debug, PartialEq)]", depth)
+
+    def simple_sum(self, sum, name, depth):
+        rustname = get_rust_type(name)
+        self.emit_attrs(depth)
+        self.emit(f"pub enum {rustname} {{", depth)
+        for variant in sum.types:
+            self.emit(f"{variant.name},", depth + 1)
+        self.emit("}", depth)
+        self.emit("", depth)
+
+    def sum_with_constructors(self, sum, name, depth):
+        typeinfo = self.typeinfo[name]
+        generics, generics_applied = self.get_generics(name, "U = ()", "U")
+        enumname = rustname = get_rust_type(name)
+        # all the attributes right now are for location, so if it has attrs we
+        # can just wrap it in Located<>
+        if sum.attributes:
+            enumname = rustname + "Kind"
+        self.emit_attrs(depth)
+        self.emit(f"pub enum {enumname}{generics} {{", depth)
+        for t in sum.types:
+            self.visit(t, typeinfo, depth + 1)
+        self.emit("}", depth)
+        if sum.attributes:
+            self.emit(
+                f"pub type {rustname}<U = ()> = Located<{enumname}{generics_applied}, U>;",
+                depth,
+            )
+        self.emit("", depth)
+
+    def visitConstructor(self, cons, parent, depth):
+        if cons.fields:
+            self.emit(f"{cons.name} {{", depth)
+            for f in cons.fields:
+                self.visit(f, parent, "", depth + 1)
+            self.emit("},", depth)
+        else:
+            self.emit(f"{cons.name},", depth)
+
+    def visitField(self, field, parent, vis, depth):
+        typ = get_rust_type(field.type)
+        fieldtype = self.typeinfo.get(field.type)
+        if fieldtype and fieldtype.has_userdata:
+            typ = f"{typ}<U>"
+        # don't box if we're doing Vec<T>, but do box if we're doing Vec<Option<Box<T>>>
+        if fieldtype and fieldtype.boxed and (not field.seq or field.opt):
+            typ = f"Box<{typ}>"
+        if field.opt:
+            typ = f"Option<{typ}>"
+        if field.seq:
+            typ = f"Vec<{typ}>"
+        name = rust_field(field.name)
+        self.emit(f"{vis}{name}: {typ},", depth)
+
+    def visitProduct(self, product, name, depth):
+        typeinfo = self.typeinfo[name]
+        generics, generics_applied = self.get_generics(name, "U = ()", "U")
+        dataname = rustname = get_rust_type(name)
+        if product.attributes:
+            dataname = rustname + "Data"
+        self.emit_attrs(depth)
+        has_expr = any(f.type != "identifier" for f in product.fields)
+        if has_expr:
+            datadef = f"{dataname}{generics}"
+        else:
+            datadef = dataname
+        self.emit(f"pub struct {datadef} {{", depth)
+        for f in product.fields:
+            self.visit(f, typeinfo, "pub ", depth + 1)
+        self.emit("}", depth)
+        if product.attributes:
+            # attributes should just be location info
+            if not has_expr:
+                generics_applied = ""
+            self.emit(
+                f"pub type {rustname}<U = ()> = Located<{dataname}{generics_applied}, U>;",
+                depth,
+            )
+        self.emit("", depth)
+
+
+class FoldTraitDefVisitor(TypeInfoEmitVisitor):
+    def visitModule(self, mod, depth):
+        self.emit("pub trait Fold<U> {", depth)
+        self.emit("type TargetU;", depth + 1)
+        self.emit("type Error;", depth + 1)
+        self.emit(
+            "fn map_user(&mut self, user: U) -> Result<Self::TargetU, Self::Error>;",
+            depth + 2,
+        )
+        for dfn in mod.dfns:
+            self.visit(dfn, depth + 2)
+        self.emit("}", depth)
+
+    def visitType(self, type, depth):
+        name = type.name
+        apply_u, apply_target_u = self.get_generics(name, "U", "Self::TargetU")
+        enumname = get_rust_type(name)
+        self.emit(
+            f"fn fold_{name}(&mut self, node: {enumname}{apply_u}) -> Result<{enumname}{apply_target_u}, Self::Error> {{",
+            depth,
+        )
+        self.emit(f"fold_{name}(self, node)", depth + 1)
+        self.emit("}", depth)
+
+
+class FoldImplVisitor(TypeInfoEmitVisitor):
+    def visitModule(self, mod, depth):
+        self.emit(
+            "fn fold_located<U, F: Fold<U> + ?Sized, T, MT>(folder: &mut F, node: Located<T, U>, f: impl FnOnce(&mut F, T) -> Result<MT, F::Error>) -> Result<Located<MT, F::TargetU>, F::Error> {",
+            depth,
+        )
+        self.emit(
+            "Ok(Located { custom: folder.map_user(node.custom)?, location: node.location, node: f(folder, node.node)? })",
+            depth + 1,
+        )
+        self.emit("}", depth)
+        for dfn in mod.dfns:
+            self.visit(dfn, depth)
+
+    def visitType(self, type, depth=0):
+        self.visit(type.value, type.name, depth)
+
+    def visitSum(self, sum, name, depth):
+        apply_t, apply_u, apply_target_u = self.get_generics(
+            name, "T", "U", "F::TargetU"
+        )
+        enumname = get_rust_type(name)
+        is_located = bool(sum.attributes)
+
+        self.emit(f"impl<T, U> Foldable<T, U> for {enumname}{apply_t} {{", depth)
+        self.emit(f"type Mapped = {enumname}{apply_u};", depth + 1)
+        self.emit(
+            "fn fold<F: Fold<T, TargetU = U> + ?Sized>(self, folder: &mut F) -> Result<Self::Mapped, F::Error> {",
+            depth + 1,
+        )
+        self.emit(f"folder.fold_{name}(self)", depth + 2)
+        self.emit("}", depth + 1)
+        self.emit("}", depth)
+
+        self.emit(
+            f"pub fn fold_{name}<U, F: Fold<U> + ?Sized>(#[allow(unused)] folder: &mut F, node: {enumname}{apply_u}) -> Result<{enumname}{apply_target_u}, F::Error> {{",
+            depth,
+        )
+        if is_located:
+            self.emit("fold_located(folder, node, |folder, node| {", depth)
+            enumname += "Kind"
+        self.emit("match node {", depth + 1)
+        for cons in sum.types:
+            fields_pattern = self.make_pattern(cons.fields)
+            self.emit(
+                f"{enumname}::{cons.name} {{ {fields_pattern} }} => {{", depth + 2
+            )
+            self.gen_construction(f"{enumname}::{cons.name}", cons.fields, depth + 3)
+            self.emit("}", depth + 2)
+        self.emit("}", depth + 1)
+        if is_located:
+            self.emit("})", depth)
+        self.emit("}", depth)
+
+    def visitProduct(self, product, name, depth):
+        apply_t, apply_u, apply_target_u = self.get_generics(
+            name, "T", "U", "F::TargetU"
+        )
+        structname = get_rust_type(name)
+        is_located = bool(product.attributes)
+
+        self.emit(f"impl<T, U> Foldable<T, U> for {structname}{apply_t} {{", depth)
+        self.emit(f"type Mapped = {structname}{apply_u};", depth + 1)
+        self.emit(
+            "fn fold<F: Fold<T, TargetU = U> + ?Sized>(self, folder: &mut F) -> Result<Self::Mapped, F::Error> {",
+            depth + 1,
+        )
+        self.emit(f"folder.fold_{name}(self)", depth + 2)
+        self.emit("}", depth + 1)
+        self.emit("}", depth)
+
+        self.emit(
+            f"pub fn fold_{name}<U, F: Fold<U> + ?Sized>(#[allow(unused)] folder: &mut F, node: {structname}{apply_u}) -> Result<{structname}{apply_target_u}, F::Error> {{",
+            depth,
+        )
+        if is_located:
+            self.emit("fold_located(folder, node, |folder, node| {", depth)
+            structname += "Data"
+        fields_pattern = self.make_pattern(product.fields)
+        self.emit(f"let {structname} {{ {fields_pattern} }} = node;", depth + 1)
+        self.gen_construction(structname, product.fields, depth + 1)
+        if is_located:
+            self.emit("})", depth)
+        self.emit("}", depth)
+
+    def make_pattern(self, fields):
+        return ",".join(rust_field(f.name) for f in fields)
+
+    def gen_construction(self, cons_path, fields, depth):
+        self.emit(f"Ok({cons_path} {{", depth)
+        for field in fields:
+            name = rust_field(field.name)
+            self.emit(f"{name}: Foldable::fold({name}, folder)?,", depth + 1)
+        self.emit("})", depth)
+
+
+class FoldModuleVisitor(TypeInfoEmitVisitor):
+    def visitModule(self, mod):
+        depth = 0
+        self.emit('#[cfg(feature = "fold")]', depth)
+        self.emit("pub mod fold {", depth)
+        self.emit("use super::*;", depth + 1)
+        self.emit("use crate::fold_helpers::Foldable;", depth + 1)
+        FoldTraitDefVisitor(self.file, self.typeinfo).visit(mod, depth + 1)
+        FoldImplVisitor(self.file, self.typeinfo).visit(mod, depth + 1)
+        self.emit("}", depth)
+
+
+class ClassDefVisitor(EmitVisitor):
+    def visitModule(self, mod):
+        for dfn in mod.dfns:
+            self.visit(dfn)
+
+    def visitType(self, type, depth=0):
+        self.visit(type.value, type.name, depth)
+
+    def visitSum(self, sum, name, depth):
+        structname = "NodeKind" + get_rust_type(name)
+        self.emit(
+            f'#[pyclass(module = "_ast", name = {json.dumps(name)}, base = "AstNode")]',
+            depth,
+        )
+        self.emit(f"struct {structname};", depth)
+        self.emit("#[pyclass(flags(HAS_DICT, BASETYPE))]", depth)
+        self.emit(f"impl {structname} {{}}", depth)
+        for cons in sum.types:
+            self.visit(cons, sum.attributes, structname, depth)
+
+    def visitConstructor(self, cons, attrs, base, depth):
+        self.gen_classdef(cons.name, cons.fields, attrs, depth, base)
+
+    def visitProduct(self, product, name, depth):
+        self.gen_classdef(name, product.fields, product.attributes, depth)
+
+    def gen_classdef(self, name, fields, attrs, depth, base="AstNode"):
+        structname = "Node" + get_rust_type(name)
+        self.emit(
+            f'#[pyclass(module = "_ast", name = {json.dumps(name)}, base = {json.dumps(base)})]',
+            depth,
+        )
+        self.emit(f"struct {structname};", depth)
+        self.emit("#[pyclass(flags(HAS_DICT, BASETYPE))]", depth)
+        self.emit(f"impl {structname} {{", depth)
+        self.emit(f"#[extend_class]", depth + 1)
+        self.emit(
+            "fn extend_class_with_fields(ctx: &Context, class: &'static Py<PyType>) {",
+            depth + 1,
+        )
+        fields = ",".join(
+            f"ctx.new_str(ascii!({json.dumps(f.name)})).into()" for f in fields
+        )
+        self.emit(
+            f"class.set_attr(identifier!(ctx, _fields), ctx.new_list(vec![{fields}]).into());",
+            depth + 2,
+        )
+        attrs = ",".join(
+            f"ctx.new_str(ascii!({json.dumps(attr.name)})).into()" for attr in attrs
+        )
+        self.emit(
+            f"class.set_attr(identifier!(ctx, _attributes), ctx.new_list(vec![{attrs}]).into());",
+            depth + 2,
+        )
+        self.emit("}", depth + 1)
+        self.emit("}", depth)
+
+
+class ExtendModuleVisitor(EmitVisitor):
+    def visitModule(self, mod):
+        depth = 0
+        self.emit(
+            "pub fn extend_module_nodes(vm: &VirtualMachine, module: &PyObject) {",
+            depth,
+        )
+        self.emit("extend_module!(vm, module, {", depth + 1)
+        for dfn in mod.dfns:
+            self.visit(dfn, depth + 2)
+        self.emit("})", depth + 1)
+        self.emit("}", depth)
+
+    def visitType(self, type, depth):
+        self.visit(type.value, type.name, depth)
+
+    def visitSum(self, sum, name, depth):
+        rust_name = get_rust_type(name)
+        self.emit(
+            f"{json.dumps(name)} => NodeKind{rust_name}::make_class(&vm.ctx),", depth
+        )
+        for cons in sum.types:
+            self.visit(cons, depth)
+
+    def visitConstructor(self, cons, depth):
+        self.gen_extension(cons.name, depth)
+
+    def visitProduct(self, product, name, depth):
+        self.gen_extension(name, depth)
+
+    def gen_extension(self, name, depth):
+        rust_name = get_rust_type(name)
+        self.emit(f"{json.dumps(name)} => Node{rust_name}::make_class(&vm.ctx),", depth)
+
+
+class TraitImplVisitor(EmitVisitor):
+    def visitModule(self, mod):
+        for dfn in mod.dfns:
+            self.visit(dfn)
+
+    def visitType(self, type, depth=0):
+        self.visit(type.value, type.name, depth)
+
+    def visitSum(self, sum, name, depth):
+        enumname = get_rust_type(name)
+        if sum.attributes:
+            enumname += "Kind"
+
+        self.emit(f"impl NamedNode for ast::{enumname} {{", depth)
+        self.emit(f"const NAME: &'static str = {json.dumps(name)};", depth + 1)
+        self.emit("}", depth)
+        self.emit(f"impl Node for ast::{enumname} {{", depth)
+        self.emit(
+            "fn ast_to_object(self, _vm: &VirtualMachine) -> PyObjectRef {", depth + 1
+        )
+        self.emit("match self {", depth + 2)
+        for variant in sum.types:
+            self.constructor_to_object(variant, enumname, depth + 3)
+        self.emit("}", depth + 2)
+        self.emit("}", depth + 1)
+        self.emit(
+            "fn ast_from_object(_vm: &VirtualMachine, _object: PyObjectRef) -> PyResult<Self> {",
+            depth + 1,
+        )
+        self.gen_sum_fromobj(sum, name, enumname, depth + 2)
+        self.emit("}", depth + 1)
+        self.emit("}", depth)
+
+    def constructor_to_object(self, cons, enumname, depth):
+        fields_pattern = self.make_pattern(cons.fields)
+        self.emit(f"ast::{enumname}::{cons.name} {{ {fields_pattern} }} => {{", depth)
+        self.make_node(cons.name, cons.fields, depth + 1)
+        self.emit("}", depth)
+
+    def visitProduct(self, product, name, depth):
+        structname = get_rust_type(name)
+        if product.attributes:
+            structname += "Data"
+
+        self.emit(f"impl NamedNode for ast::{structname} {{", depth)
+        self.emit(f"const NAME: &'static str = {json.dumps(name)};", depth + 1)
+        self.emit("}", depth)
+        self.emit(f"impl Node for ast::{structname} {{", depth)
+        self.emit(
+            "fn ast_to_object(self, _vm: &VirtualMachine) -> PyObjectRef {", depth + 1
+        )
+        fields_pattern = self.make_pattern(product.fields)
+        self.emit(f"let ast::{structname} {{ {fields_pattern} }} = self;", depth + 2)
+        self.make_node(name, product.fields, depth + 2)
+        self.emit("}", depth + 1)
+        self.emit(
+            "fn ast_from_object(_vm: &VirtualMachine, _object: PyObjectRef) -> PyResult<Self> {",
+            depth + 1,
+        )
+        self.gen_product_fromobj(product, name, structname, depth + 2)
+        self.emit("}", depth + 1)
+        self.emit("}", depth)
+
+    def make_node(self, variant, fields, depth):
+        rust_variant = get_rust_type(variant)
+        self.emit(
+            f"let _node = AstNode.into_ref_with_type(_vm, Node{rust_variant}::static_type().to_owned()).unwrap();",
+            depth,
+        )
+        if fields:
+            self.emit("let _dict = _node.as_object().dict().unwrap();", depth)
+        for f in fields:
+            self.emit(
+                f"_dict.set_item({json.dumps(f.name)}, {rust_field(f.name)}.ast_to_object(_vm), _vm).unwrap();",
+                depth,
+            )
+        self.emit("_node.into()", depth)
+
+    def make_pattern(self, fields):
+        return ",".join(rust_field(f.name) for f in fields)
+
+    def gen_sum_fromobj(self, sum, sumname, enumname, depth):
+        if sum.attributes:
+            self.extract_location(sumname, depth)
+
+        self.emit("let _cls = _object.class();", depth)
+        self.emit("Ok(", depth)
+        for cons in sum.types:
+            self.emit(f"if _cls.is(Node{cons.name}::static_type()) {{", depth)
+            self.gen_construction(f"{enumname}::{cons.name}", cons, sumname, depth + 1)
+            self.emit("} else", depth)
+
+        self.emit("{", depth)
+        msg = f'format!("expected some sort of {sumname}, but got {{}}",_object.repr(_vm)?)'
+        self.emit(f"return Err(_vm.new_type_error({msg}));", depth + 1)
+        self.emit("})", depth)
+
+    def gen_product_fromobj(self, product, prodname, structname, depth):
+        if product.attributes:
+            self.extract_location(prodname, depth)
+
+        self.emit("Ok(", depth)
+        self.gen_construction(structname, product, prodname, depth + 1)
+        self.emit(")", depth)
+
+    def gen_construction(self, cons_path, cons, name, depth):
+        self.emit(f"ast::{cons_path} {{", depth)
+        for field in cons.fields:
+            self.emit(
+                f"{rust_field(field.name)}: {self.decode_field(field, name)},",
+                depth + 1,
+            )
+        self.emit("}", depth)
+
+    def extract_location(self, typename, depth):
+        row = self.decode_field(asdl.Field("int", "lineno"), typename)
+        column = self.decode_field(asdl.Field("int", "col_offset"), typename)
+        self.emit(f"let _location = ast::Location::new({row}, {column});", depth)
+
+    def wrap_located_node(self, depth):
+        self.emit(f"let node = ast::Located::new(_location, node);", depth)
+
+    def decode_field(self, field, typename):
+        name = json.dumps(field.name)
+        if field.opt and not field.seq:
+            return f"get_node_field_opt(_vm, &_object, {name})?.map(|obj| Node::ast_from_object(_vm, obj)).transpose()?"
+        else:
+            return f"Node::ast_from_object(_vm, get_node_field(_vm, &_object, {name}, {json.dumps(typename)})?)?"
+
+
+class ChainOfVisitors:
+    def __init__(self, *visitors):
+        self.visitors = visitors
+
+    def visit(self, object):
+        for v in self.visitors:
+            v.visit(object)
+            v.emit("", 0)
+
+
+def write_ast_def(mod, typeinfo, f):
+    f.write(
+        textwrap.dedent(
+            """
+        #![allow(clippy::derive_partial_eq_without_eq)]
+        
+        pub use crate::constant::*;
+        pub use crate::location::Location;
+
+        type Ident = String;
+        \n
+    """
+        )
+    )
+    StructVisitor(f, typeinfo).emit_attrs(0)
+    f.write(
+        textwrap.dedent(
+            """
+        pub struct Located<T, U = ()> {
+            pub location: Location,
+            pub custom: U,
+            pub node: T,
+        }
+    
+        impl<T> Located<T> {
+            pub fn new(location: Location, node: T) -> Self {
+                Self { location, custom: (), node }
+            }
+        }
+        \n
+    """.lstrip()
+        )
+    )
+
+    c = ChainOfVisitors(StructVisitor(f, typeinfo), FoldModuleVisitor(f, typeinfo))
+    c.visit(mod)
+
+
+def write_ast_mod(mod, f):
+    f.write(
+        textwrap.dedent(
+            """
+        #![allow(clippy::all)]
+
+        use super::*;
+        use crate::common::ascii;
+
+    """
+        )
+    )
+
+    c = ChainOfVisitors(ClassDefVisitor(f), TraitImplVisitor(f), ExtendModuleVisitor(f))
+    c.visit(mod)
+
+
+def main(input_filename, ast_mod_filename, ast_def_filename, dump_module=False):
+    auto_gen_msg = AUTOGEN_MESSAGE.format("/".join(Path(__file__).parts[-2:]))
+    mod = asdl.parse(input_filename)
+    if dump_module:
+        print("Parsed Module:")
+        print(mod)
+    if not asdl.check(mod):
+        sys.exit(1)
+
+    typeinfo = {}
+    FindUserdataTypesVisitor(typeinfo).visit(mod)
+
+    with ast_def_filename.open("w") as def_file, ast_mod_filename.open("w") as mod_file:
+        def_file.write(auto_gen_msg)
+        write_ast_def(mod, typeinfo, def_file)
+
+        mod_file.write(auto_gen_msg)
+        write_ast_mod(mod, mod_file)
+
+    print(f"{ast_def_filename}, {ast_mod_filename} regenerated.")
+
+
+if __name__ == "__main__":
+    parser = ArgumentParser()
+    parser.add_argument("input_file", type=Path)
+    parser.add_argument("-M", "--mod-file", type=Path, required=True)
+    parser.add_argument("-D", "--def-file", type=Path, required=True)
+    parser.add_argument("-d", "--dump-module", action="store_true")
+
+    args = parser.parse_args()
+    main(args.input_file, args.mod_file, args.def_file, args.dump_module)

ast/src/constant.rs

@@ -0,0 +1,223 @@
+use num_bigint::BigInt;
+pub use rustpython_bytecode::ConversionFlag;
+
+#[derive(Debug, PartialEq)]
+pub enum Constant {
+    None,
+    Bool(bool),
+    Str(String),
+    Bytes(Vec<u8>),
+    Int(BigInt),
+    Tuple(Vec<Constant>),
+    Float(f64),
+    Complex { real: f64, imag: f64 },
+    Ellipsis,
+}
+
+impl From<String> for Constant {
+    fn from(s: String) -> Constant {
+        Self::Str(s)
+    }
+}
+impl From<Vec<u8>> for Constant {
+    fn from(b: Vec<u8>) -> Constant {
+        Self::Bytes(b)
+    }
+}
+impl From<bool> for Constant {
+    fn from(b: bool) -> Constant {
+        Self::Bool(b)
+    }
+}
+impl From<BigInt> for Constant {
+    fn from(i: BigInt) -> Constant {
+        Self::Int(i)
+    }
+}
+
+#[cfg(feature = "rustpython-common")]
+impl std::fmt::Display for Constant {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Constant::None => f.pad("None"),
+            Constant::Bool(b) => f.pad(if *b { "True" } else { "False" }),
+            Constant::Str(s) => rustpython_common::str::repr(s).fmt(f),
+            Constant::Bytes(b) => f.pad(&rustpython_common::bytes::repr(b)),
+            Constant::Int(i) => i.fmt(f),
+            Constant::Tuple(tup) => {
+                if let [elt] = &**tup {
+                    write!(f, "({},)", elt)
+                } else {
+                    f.write_str("(")?;
+                    for (i, elt) in tup.iter().enumerate() {
+                        if i != 0 {
+                            f.write_str(", ")?;
+                        }
+                        elt.fmt(f)?;
+                    }
+                    f.write_str(")")
+                }
+            }
+            Constant::Float(fp) => f.pad(&rustpython_common::float_ops::to_string(*fp)),
+            Constant::Complex { real, imag } => {
+                if *real == 0.0 {
+                    write!(f, "{}j", imag)
+                } else {
+                    write!(f, "({}{:+}j)", real, imag)
+                }
+            }
+            Constant::Ellipsis => f.pad("..."),
+        }
+    }
+}
+
+#[cfg(feature = "constant-optimization")]
+#[non_exhaustive]
+#[derive(Default)]
+pub struct ConstantOptimizer {}
+
+#[cfg(feature = "constant-optimization")]
+impl ConstantOptimizer {
+    #[inline]
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+#[cfg(feature = "constant-optimization")]
+impl<U> crate::fold::Fold<U> for ConstantOptimizer {
+    type TargetU = U;
+    type Error = std::convert::Infallible;
+    #[inline]
+    fn map_user(&mut self, user: U) -> Result<Self::TargetU, Self::Error> {
+        Ok(user)
+    }
+    fn fold_expr(&mut self, node: crate::Expr<U>) -> Result<crate::Expr<U>, Self::Error> {
+        match node.node {
+            crate::ExprKind::Tuple { elts, ctx } => {
+                let elts = elts
+                    .into_iter()
+                    .map(|x| self.fold_expr(x))
+                    .collect::<Result<Vec<_>, _>>()?;
+                let expr = if elts
+                    .iter()
+                    .all(|e| matches!(e.node, crate::ExprKind::Constant { .. }))
+                {
+                    let tuple = elts
+                        .into_iter()
+                        .map(|e| match e.node {
+                            crate::ExprKind::Constant { value, .. } => value,
+                            _ => unreachable!(),
+                        })
+                        .collect();
+                    crate::ExprKind::Constant {
+                        value: Constant::Tuple(tuple),
+                        kind: None,
+                    }
+                } else {
+                    crate::ExprKind::Tuple { elts, ctx }
+                };
+                Ok(crate::Expr {
+                    node: expr,
+                    custom: node.custom,
+                    location: node.location,
+                })
+            }
+            _ => crate::fold::fold_expr(self, node),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[cfg(feature = "constant-optimization")]
+    #[test]
+    fn test_constant_opt() {
+        use super::*;
+        use crate::fold::Fold;
+        use crate::*;
+
+        let location = Location::new(0, 0);
+        let custom = ();
+        let ast = Located {
+            location,
+            custom,
+            node: ExprKind::Tuple {
+                ctx: ExprContext::Load,
+                elts: vec![
+                    Located {
+                        location,
+                        custom,
+                        node: ExprKind::Constant {
+                            value: BigInt::from(1).into(),
+                            kind: None,
+                        },
+                    },
+                    Located {
+                        location,
+                        custom,
+                        node: ExprKind::Constant {
+                            value: BigInt::from(2).into(),
+                            kind: None,
+                        },
+                    },
+                    Located {
+                        location,
+                        custom,
+                        node: ExprKind::Tuple {
+                            ctx: ExprContext::Load,
+                            elts: vec![
+                                Located {
+                                    location,
+                                    custom,
+                                    node: ExprKind::Constant {
+                                        value: BigInt::from(3).into(),
+                                        kind: None,
+                                    },
+                                },
+                                Located {
+                                    location,
+                                    custom,
+                                    node: ExprKind::Constant {
+                                        value: BigInt::from(4).into(),
+                                        kind: None,
+                                    },
+                                },
+                                Located {
+                                    location,
+                                    custom,
+                                    node: ExprKind::Constant {
+                                        value: BigInt::from(5).into(),
+                                        kind: None,
+                                    },
+                                },
+                            ],
+                        },
+                    },
+                ],
+            },
+        };
+        let new_ast = ConstantOptimizer::new()
+            .fold_expr(ast)
+            .unwrap_or_else(|e| match e {});
+        assert_eq!(
+            new_ast,
+            Located {
+                location,
+                custom,
+                node: ExprKind::Constant {
+                    value: Constant::Tuple(vec![
+                        BigInt::from(1).into(),
+                        BigInt::from(2).into(),
+                        Constant::Tuple(vec![
+                            BigInt::from(3).into(),
+                            BigInt::from(4).into(),
+                            BigInt::from(5).into(),
+                        ])
+                    ]),
+                    kind: None
+                },
+            }
+        );
+    }
+}

ast/src/fold_helpers.rs

@@ -0,0 +1,66 @@
+use crate::constant;
+use crate::fold::Fold;
+
+pub(crate) trait Foldable<T, U> {
+    type Mapped;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error>;
+}
+
+impl<T, U, X> Foldable<T, U> for Vec<X>
+where
+    X: Foldable<T, U>,
+{
+    type Mapped = Vec<X::Mapped>;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error> {
+        self.into_iter().map(|x| x.fold(folder)).collect()
+    }
+}
+
+impl<T, U, X> Foldable<T, U> for Option<X>
+where
+    X: Foldable<T, U>,
+{
+    type Mapped = Option<X::Mapped>;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error> {
+        self.map(|x| x.fold(folder)).transpose()
+    }
+}
+
+impl<T, U, X> Foldable<T, U> for Box<X>
+where
+    X: Foldable<T, U>,
+{
+    type Mapped = Box<X::Mapped>;
+    fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self::Mapped, F::Error> {
+        (*self).fold(folder).map(Box::new)
+    }
+}
+
+macro_rules! simple_fold {
+    ($($t:ty),+$(,)?) => {
+        $(impl<T, U> $crate::fold_helpers::Foldable<T, U> for $t {
+            type Mapped = Self;
+            #[inline]
+            fn fold<F: Fold<T, TargetU = U> + ?Sized>(
+                self,
+                _folder: &mut F,
+            ) -> Result<Self::Mapped, F::Error> {
+                Ok(self)
+            }
+        })+
+    };
+}
+
+simple_fold!(usize, String, bool, constant::Constant);

ast/src/impls.rs

@@ -0,0 +1,60 @@
+use crate::{Constant, ExprKind};
+
+impl<U> ExprKind<U> {
+    /// Returns a short name for the node suitable for use in error messages.
+    pub fn name(&self) -> &'static str {
+        match self {
+            ExprKind::BoolOp { .. } | ExprKind::BinOp { .. } | ExprKind::UnaryOp { .. } => {
+                "operator"
+            }
+            ExprKind::Subscript { .. } => "subscript",
+            ExprKind::Await { .. } => "await expression",
+            ExprKind::Yield { .. } | ExprKind::YieldFrom { .. } => "yield expression",
+            ExprKind::Compare { .. } => "comparison",
+            ExprKind::Attribute { .. } => "attribute",
+            ExprKind::Call { .. } => "function call",
+            ExprKind::Constant { value, .. } => match value {
+                Constant::Str(_)
+                | Constant::Int(_)
+                | Constant::Float(_)
+                | Constant::Complex { .. }
+                | Constant::Bytes(_) => "literal",
+                Constant::Tuple(_) => "tuple",
+                Constant::Bool(b) => {
+                    if *b {
+                        "True"
+                    } else {
+                        "False"
+                    }
+                }
+                Constant::None => "None",
+                Constant::Ellipsis => "ellipsis",
+            },
+            ExprKind::List { .. } => "list",
+            ExprKind::Tuple { .. } => "tuple",
+            ExprKind::Dict { .. } => "dict display",
+            ExprKind::Set { .. } => "set display",
+            ExprKind::ListComp { .. } => "list comprehension",
+            ExprKind::DictComp { .. } => "dict comprehension",
+            ExprKind::SetComp { .. } => "set comprehension",
+            ExprKind::GeneratorExp { .. } => "generator expression",
+            ExprKind::Starred { .. } => "starred",
+            ExprKind::Slice { .. } => "slice",
+            ExprKind::JoinedStr { values } => {
+                if values
+                    .iter()
+                    .any(|e| matches!(e.node, ExprKind::JoinedStr { .. }))
+                {
+                    "f-string expression"
+                } else {
+                    "literal"
+                }
+            }
+            ExprKind::FormattedValue { .. } => "f-string expression",
+            ExprKind::Name { .. } => "name",
+            ExprKind::Lambda { .. } => "lambda",
+            ExprKind::IfExp { .. } => "conditional expression",
+            ExprKind::NamedExpr { .. } => "named expression",
+        }
+    }
+}

ast/src/lib.rs

@@ -0,0 +1,13 @@
+mod ast_gen;
+mod constant;
+#[cfg(feature = "fold")]
+mod fold_helpers;
+mod impls;
+mod location;
+#[cfg(feature = "unparse")]
+mod unparse;
+
+pub use ast_gen::*;
+pub use location::Location;
+
+pub type Suite<U = ()> = Vec<Stmt<U>>;

ast/src/location.rs

@@ -0,0 +1,79 @@
+//! Datatypes to support source location information.
+
+use std::fmt;
+
+/// A location somewhere in the sourcecode.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub struct Location {
+    row: usize,
+    column: usize,
+}
+
+impl fmt::Display for Location {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "line {} column {}", self.row, self.column)
+    }
+}
+
+impl Location {
+    pub fn visualize<'a>(
+        &self,
+        line: &'a str,
+        desc: impl fmt::Display + 'a,
+    ) -> impl fmt::Display + 'a {
+        struct Visualize<'a, D: fmt::Display> {
+            loc: Location,
+            line: &'a str,
+            desc: D,
+        }
+        impl<D: fmt::Display> fmt::Display for Visualize<'_, D> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                write!(
+                    f,
+                    "{}\n{}{arrow:>pad$}",
+                    self.desc,
+                    self.line,
+                    pad = self.loc.column,
+                    arrow = "^",
+                )
+            }
+        }
+        Visualize {
+            loc: *self,
+            line,
+            desc,
+        }
+    }
+}
+
+impl Location {
+    pub fn new(row: usize, column: usize) -> Self {
+        Location { row, column }
+    }
+
+    pub fn row(&self) -> usize {
+        self.row
+    }
+
+    pub fn column(&self) -> usize {
+        self.column
+    }
+
+    pub fn reset(&mut self) {
+        self.row = 1;
+        self.column = 1;
+    }
+
+    pub fn go_right(&mut self) {
+        self.column += 1;
+    }
+
+    pub fn go_left(&mut self) {
+        self.column -= 1;
+    }
+
+    pub fn newline(&mut self) {
+        self.row += 1;
+        self.column = 1;
+    }
+}

ast/src/unparse.rs

@@ -0,0 +1,530 @@
+use crate::{
+    Arg, Arguments, Boolop, Cmpop, Comprehension, Constant, ConversionFlag, Expr, ExprKind,
+    Operator,
+};
+use std::fmt;
+
+mod precedence {
+    macro_rules! precedence {
+        ($($op:ident,)*) => {
+            precedence!(@0, $($op,)*);
+        };
+        (@$i:expr, $op1:ident, $($op:ident,)*) => {
+            pub const $op1: u8 = $i;
+            precedence!(@$i + 1, $($op,)*);
+        };
+        (@$i:expr,) => {};
+    }
+    precedence!(
+        TUPLE, TEST, OR, AND, NOT, CMP, // "EXPR" =
+        BOR, BXOR, BAND, SHIFT, ARITH, TERM, FACTOR, POWER, AWAIT, ATOM,
+    );
+    pub const EXPR: u8 = BOR;
+}
+
+#[repr(transparent)]
+struct Unparser<'a> {
+    f: fmt::Formatter<'a>,
+}
+impl<'a> Unparser<'a> {
+    fn new<'b>(f: &'b mut fmt::Formatter<'a>) -> &'b mut Unparser<'a> {
+        unsafe { &mut *(f as *mut fmt::Formatter<'a> as *mut Unparser<'a>) }
+    }
+
+    fn p(&mut self, s: &str) -> fmt::Result {
+        self.f.write_str(s)
+    }
+    fn p_if(&mut self, cond: bool, s: &str) -> fmt::Result {
+        if cond {
+            self.f.write_str(s)?;
+        }
+        Ok(())
+    }
+    fn p_delim(&mut self, first: &mut bool, s: &str) -> fmt::Result {
+        self.p_if(!std::mem::take(first), s)
+    }
+    fn write_fmt(&mut self, f: fmt::Arguments<'_>) -> fmt::Result {
+        self.f.write_fmt(f)
+    }
+
+    fn unparse_expr<U>(&mut self, ast: &Expr<U>, level: u8) -> fmt::Result {
+        macro_rules! opprec {
+            ($opty:ident, $x:expr, $enu:path, $($var:ident($op:literal, $prec:ident)),*$(,)?) => {
+                match $x {
+                    $(<$enu>::$var => (opprec!(@space $opty, $op), precedence::$prec),)*
+                }
+            };
+            (@space bin, $op:literal) => {
+                concat!(" ", $op, " ")
+            };
+            (@space un, $op:literal) => {
+                $op
+            };
+        }
+        macro_rules! group_if {
+            ($lvl:expr, $body:block) => {{
+                let group = level > $lvl;
+                self.p_if(group, "(")?;
+                let ret = $body;
+                self.p_if(group, ")")?;
+                ret
+            }};
+        }
+        match &ast.node {
+            ExprKind::BoolOp { op, values } => {
+                let (op, prec) = opprec!(bin, op, Boolop, And("and", AND), Or("or", OR));
+                group_if!(prec, {
+                    let mut first = true;
+                    for val in values {
+                        self.p_delim(&mut first, op)?;
+                        self.unparse_expr(val, prec + 1)?;
+                    }
+                })
+            }
+            ExprKind::NamedExpr { target, value } => {
+                group_if!(precedence::TUPLE, {
+                    self.unparse_expr(target, precedence::ATOM)?;
+                    self.p(" := ")?;
+                    self.unparse_expr(value, precedence::ATOM)?;
+                })
+            }
+            ExprKind::BinOp { left, op, right } => {
+                let rassoc = matches!(op, Operator::Pow);
+                let (op, prec) = opprec!(
+                    bin,
+                    op,
+                    Operator,
+                    Add("+", ARITH),
+                    Sub("-", ARITH),
+                    Mult("*", TERM),
+                    MatMult("@", TERM),
+                    Div("/", TERM),
+                    Mod("%", TERM),
+                    Pow("**", POWER),
+                    LShift("<<", SHIFT),
+                    RShift(">>", SHIFT),
+                    BitOr("|", BOR),
+                    BitXor("^", BXOR),
+                    BitAnd("&", BAND),
+                    FloorDiv("//", TERM),
+                );
+                group_if!(prec, {
+                    self.unparse_expr(left, prec + rassoc as u8)?;
+                    self.p(op)?;
+                    self.unparse_expr(right, prec + !rassoc as u8)?;
+                })
+            }
+            ExprKind::UnaryOp { op, operand } => {
+                let (op, prec) = opprec!(
+                    un,
+                    op,
+                    crate::Unaryop,
+                    Invert("~", FACTOR),
+                    Not("not ", NOT),
+                    UAdd("+", FACTOR),
+                    USub("-", FACTOR)
+                );
+                group_if!(prec, {
+                    self.p(op)?;
+                    self.unparse_expr(operand, prec)?;
+                })
+            }
+            ExprKind::Lambda { args, body } => {
+                group_if!(precedence::TEST, {
+                    let npos = args.args.len() + args.posonlyargs.len();
+                    self.p(if npos > 0 { "lambda " } else { "lambda" })?;
+                    self.unparse_args(args)?;
+                    write!(self, ": {}", **body)?;
+                })
+            }
+            ExprKind::IfExp { test, body, orelse } => {
+                group_if!(precedence::TEST, {
+                    self.unparse_expr(body, precedence::TEST + 1)?;
+                    self.p(" if ")?;
+                    self.unparse_expr(test, precedence::TEST + 1)?;
+                    self.p(" else ")?;
+                    self.unparse_expr(orelse, precedence::TEST)?;
+                })
+            }
+            ExprKind::Dict { keys, values } => {
+                self.p("{")?;
+                let mut first = true;
+                let (packed, unpacked) = values.split_at(keys.len());
+                for (k, v) in keys.iter().zip(packed) {
+                    self.p_delim(&mut first, ", ")?;
+                    write!(self, "{}: {}", *k, *v)?;
+                }
+                for d in unpacked {
+                    self.p_delim(&mut first, ", ")?;
+                    write!(self, "**{}", *d)?;
+                }
+                self.p("}")?;
+            }
+            ExprKind::Set { elts } => {
+                self.p("{")?;
+                let mut first = true;
+                for v in elts {
+                    self.p_delim(&mut first, ", ")?;
+                    self.unparse_expr(v, precedence::TEST)?;
+                }
+                self.p("}")?;
+            }
+            ExprKind::ListComp { elt, generators } => {
+                self.p("[")?;
+                self.unparse_expr(elt, precedence::TEST)?;
+                self.unparse_comp(generators)?;
+                self.p("]")?;
+            }
+            ExprKind::SetComp { elt, generators } => {
+                self.p("{")?;
+                self.unparse_expr(elt, precedence::TEST)?;
+                self.unparse_comp(generators)?;
+                self.p("}")?;
+            }
+            ExprKind::DictComp {
+                key,
+                value,
+                generators,
+            } => {
+                self.p("{")?;
+                self.unparse_expr(key, precedence::TEST)?;
+                self.p(": ")?;
+                self.unparse_expr(value, precedence::TEST)?;
+                self.unparse_comp(generators)?;
+                self.p("}")?;
+            }
+            ExprKind::GeneratorExp { elt, generators } => {
+                self.p("(")?;
+                self.unparse_expr(elt, precedence::TEST)?;
+                self.unparse_comp(generators)?;
+                self.p(")")?;
+            }
+            ExprKind::Await { value } => {
+                group_if!(precedence::AWAIT, {
+                    self.p("await ")?;
+                    self.unparse_expr(value, precedence::ATOM)?;
+                })
+            }
+            ExprKind::Yield { value } => {
+                if let Some(value) = value {
+                    write!(self, "(yield {})", **value)?;
+                } else {
+                    self.p("(yield)")?;
+                }
+            }
+            ExprKind::YieldFrom { value } => {
+                write!(self, "(yield from {})", **value)?;
+            }
+            ExprKind::Compare {
+                left,
+                ops,
+                comparators,
+            } => {
+                group_if!(precedence::CMP, {
+                    let new_lvl = precedence::CMP + 1;
+                    self.unparse_expr(left, new_lvl)?;
+                    for (op, cmp) in ops.iter().zip(comparators) {
+                        let op = match op {
+                            Cmpop::Eq => " == ",
+                            Cmpop::NotEq => " != ",
+                            Cmpop::Lt => " < ",
+                            Cmpop::LtE => " <= ",
+                            Cmpop::Gt => " > ",
+                            Cmpop::GtE => " >= ",
+                            Cmpop::Is => " is ",
+                            Cmpop::IsNot => " is not ",
+                            Cmpop::In => " in ",
+                            Cmpop::NotIn => " not in ",
+                        };
+                        self.p(op)?;
+                        self.unparse_expr(cmp, new_lvl)?;
+                    }
+                })
+            }
+            ExprKind::Call {
+                func,
+                args,
+                keywords,
+            } => {
+                self.unparse_expr(func, precedence::ATOM)?;
+                self.p("(")?;
+                if let (
+                    [Expr {
+                        node: ExprKind::GeneratorExp { elt, generators },
+                        ..
+                    }],
+                    [],
+                ) = (&**args, &**keywords)
+                {
+                    // make sure a single genexp doesn't get double parens
+                    self.unparse_expr(elt, precedence::TEST)?;
+                    self.unparse_comp(generators)?;
+                } else {
+                    let mut first = true;
+                    for arg in args {
+                        self.p_delim(&mut first, ", ")?;
+                        self.unparse_expr(arg, precedence::TEST)?;
+                    }
+                    for kw in keywords {
+                        self.p_delim(&mut first, ", ")?;
+                        if let Some(arg) = &kw.node.arg {
+                            self.p(arg)?;
+                            self.p("=")?;
+                        } else {
+                            self.p("**")?;
+                        }
+                        self.unparse_expr(&kw.node.value, precedence::TEST)?;
+                    }
+                }
+                self.p(")")?;
+            }
+            ExprKind::FormattedValue {
+                value,
+                conversion,
+                format_spec,
+            } => self.unparse_formatted(value, *conversion, format_spec.as_deref())?,
+            ExprKind::JoinedStr { values } => self.unparse_joinedstr(values, false)?,
+            ExprKind::Constant { value, kind } => {
+                if let Some(kind) = kind {
+                    self.p(kind)?;
+                }
+                assert_eq!(f64::MAX_10_EXP, 308);
+                let inf_str = "1e309";
+                match value {
+                    Constant::Float(f) if f.is_infinite() => self.p(inf_str)?,
+                    Constant::Complex { real, imag }
+                        if real.is_infinite() || imag.is_infinite() =>
+                    {
+                        self.p(&value.to_string().replace("inf", inf_str))?
+                    }
+                    _ => fmt::Display::fmt(value, &mut self.f)?,
+                }
+            }
+            ExprKind::Attribute { value, attr, .. } => {
+                self.unparse_expr(value, precedence::ATOM)?;
+                let period = if let ExprKind::Constant {
+                    value: Constant::Int(_),
+                    ..
+                } = &value.node
+                {
+                    " ."
+                } else {
+                    "."
+                };
+                self.p(period)?;
+                self.p(attr)?;
+            }
+            ExprKind::Subscript { value, slice, .. } => {
+                self.unparse_expr(value, precedence::ATOM)?;
+                let mut lvl = precedence::TUPLE;
+                if let ExprKind::Tuple { elts, .. } = &slice.node {
+                    if elts
+                        .iter()
+                        .any(|expr| matches!(expr.node, ExprKind::Starred { .. }))
+                    {
+                        lvl += 1
+                    }
+                }
+                self.p("[")?;
+                self.unparse_expr(slice, lvl)?;
+                self.p("]")?;
+            }
+            ExprKind::Starred { value, .. } => {
+                self.p("*")?;
+                self.unparse_expr(value, precedence::EXPR)?;
+            }
+            ExprKind::Name { id, .. } => self.p(id)?,
+            ExprKind::List { elts, .. } => {
+                self.p("[")?;
+                let mut first = true;
+                for elt in elts {
+                    self.p_delim(&mut first, ", ")?;
+                    self.unparse_expr(elt, precedence::TEST)?;
+                }
+                self.p("]")?;
+            }
+            ExprKind::Tuple { elts, .. } => {
+                if elts.is_empty() {
+                    self.p("()")?;
+                } else {
+                    group_if!(precedence::TUPLE, {
+                        let mut first = true;
+                        for elt in elts {
+                            self.p_delim(&mut first, ", ")?;
+                            self.unparse_expr(elt, precedence::TEST)?;
+                        }
+                        self.p_if(elts.len() == 1, ",")?;
+                    })
+                }
+            }
+            ExprKind::Slice { lower, upper, step } => {
+                if let Some(lower) = lower {
+                    self.unparse_expr(lower, precedence::TEST)?;
+                }
+                self.p(":")?;
+                if let Some(upper) = upper {
+                    self.unparse_expr(upper, precedence::TEST)?;
+                }
+                if let Some(step) = step {
+                    self.p(":")?;
+                    self.unparse_expr(step, precedence::TEST)?;
+                }
+            }
+        }
+        Ok(())
+    }
+
+    fn unparse_args<U>(&mut self, args: &Arguments<U>) -> fmt::Result {
+        let mut first = true;
+        let defaults_start = args.posonlyargs.len() + args.args.len() - args.defaults.len();
+        for (i, arg) in args.posonlyargs.iter().chain(&args.args).enumerate() {
+            self.p_delim(&mut first, ", ")?;
+            self.unparse_arg(arg)?;
+            if let Some(i) = i.checked_sub(defaults_start) {
+                write!(self, "={}", &args.defaults[i])?;
+            }
+            self.p_if(i + 1 == args.posonlyargs.len(), ", /")?;
+        }
+        if args.vararg.is_some() || !args.kwonlyargs.is_empty() {
+            self.p_delim(&mut first, ", ")?;
+            self.p("*")?;
+        }
+        if let Some(vararg) = &args.vararg {
+            self.unparse_arg(vararg)?;
+        }
+        let defaults_start = args.kwonlyargs.len() - args.kw_defaults.len();
+        for (i, kwarg) in args.kwonlyargs.iter().enumerate() {
+            self.p_delim(&mut first, ", ")?;
+            self.unparse_arg(kwarg)?;
+            if let Some(default) = i
+                .checked_sub(defaults_start)
+                .and_then(|i| args.kw_defaults.get(i))
+            {
+                write!(self, "={}", default)?;
+            }
+        }
+        if let Some(kwarg) = &args.kwarg {
+            self.p_delim(&mut first, ", ")?;
+            self.p("**")?;
+            self.unparse_arg(kwarg)?;
+        }
+        Ok(())
+    }
+    fn unparse_arg<U>(&mut self, arg: &Arg<U>) -> fmt::Result {
+        self.p(&arg.node.arg)?;
+        if let Some(ann) = &arg.node.annotation {
+            write!(self, ": {}", **ann)?;
+        }
+        Ok(())
+    }
+
+    fn unparse_comp<U>(&mut self, generators: &[Comprehension<U>]) -> fmt::Result {
+        for comp in generators {
+            self.p(if comp.is_async > 0 {
+                " async for "
+            } else {
+                " for "
+            })?;
+            self.unparse_expr(&comp.target, precedence::TUPLE)?;
+            self.p(" in ")?;
+            self.unparse_expr(&comp.iter, precedence::TEST + 1)?;
+            for cond in &comp.ifs {
+                self.p(" if ")?;
+                self.unparse_expr(cond, precedence::TEST + 1)?;
+            }
+        }
+        Ok(())
+    }
+
+    fn unparse_fstring_body<U>(&mut self, values: &[Expr<U>], is_spec: bool) -> fmt::Result {
+        for value in values {
+            self.unparse_fstring_elem(value, is_spec)?;
+        }
+        Ok(())
+    }
+
+    fn unparse_formatted<U>(
+        &mut self,
+        val: &Expr<U>,
+        conversion: usize,
+        spec: Option<&Expr<U>>,
+    ) -> fmt::Result {
+        let buffered = to_string_fmt(|f| Unparser::new(f).unparse_expr(val, precedence::TEST + 1));
+        let brace = if buffered.starts_with('{') {
+            // put a space to avoid escaping the bracket
+            "{ "
+        } else {
+            "{"
+        };
+        self.p(brace)?;
+        self.p(&buffered)?;
+        drop(buffered);
+
+        if conversion != ConversionFlag::None as usize {
+            self.p("!")?;
+            let buf = &[conversion as u8];
+            let c = std::str::from_utf8(buf).unwrap();
+            self.p(c)?;
+        }
+
+        if let Some(spec) = spec {
+            self.p(":")?;
+            self.unparse_fstring_elem(spec, true)?;
+        }
+
+        self.p("}")?;
+
+        Ok(())
+    }
+
+    fn unparse_fstring_elem<U>(&mut self, expr: &Expr<U>, is_spec: bool) -> fmt::Result {
+        match &expr.node {
+            ExprKind::Constant { value, .. } => {
+                if let Constant::Str(s) = value {
+                    self.unparse_fstring_str(s)
+                } else {
+                    unreachable!()
+                }
+            }
+            ExprKind::JoinedStr { values } => self.unparse_joinedstr(values, is_spec),
+            ExprKind::FormattedValue {
+                value,
+                conversion,
+                format_spec,
+            } => self.unparse_formatted(value, *conversion, format_spec.as_deref()),
+            _ => unreachable!(),
+        }
+    }
+
+    fn unparse_fstring_str(&mut self, s: &str) -> fmt::Result {
+        let s = s.replace('{', "{{").replace('}', "}}");
+        self.p(&s)
+    }
+
+    fn unparse_joinedstr<U>(&mut self, values: &[Expr<U>], is_spec: bool) -> fmt::Result {
+        if is_spec {
+            self.unparse_fstring_body(values, is_spec)
+        } else {
+            self.p("f")?;
+            let body = to_string_fmt(|f| Unparser::new(f).unparse_fstring_body(values, is_spec));
+            fmt::Display::fmt(&rustpython_common::str::repr(&body), &mut self.f)
+        }
+    }
+}
+
+impl<U> fmt::Display for Expr<U> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Unparser::new(f).unparse_expr(self, precedence::TEST)
+    }
+}
+
+fn to_string_fmt(f: impl FnOnce(&mut fmt::Formatter) -> fmt::Result) -> String {
+    use std::cell::Cell;
+    struct Fmt<F>(Cell<Option<F>>);
+    impl<F: FnOnce(&mut fmt::Formatter) -> fmt::Result> fmt::Display for Fmt<F> {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            self.0.take().unwrap()(f)
+        }
+    }
+    Fmt(Cell::new(Some(f))).to_string()
+}