RustPython-Parser/src/symboltable.rs

/* Python code is pre-scanned for symbols in the ast.

This ensures that global and nonlocal keywords are picked up.
Then the compiler can use the symbol table to generate proper
load and store instructions for names.

Inspirational file: https://github.com/python/cpython/blob/master/Python/symtable.c
*/

use crate::error::{CompileError, CompileErrorType};
use indexmap::map::IndexMap;
use rustpython_parser::ast;
use rustpython_parser::location::Location;
use std::fmt;

pub fn make_symbol_table(program: &ast::Program) -> Result<SymbolTable, SymbolTableError> {
    let mut builder: SymbolTableBuilder = Default::default();
    builder.prepare();
    builder.scan_program(program)?;
    builder.finish()
}

pub fn statements_to_symbol_table(
    statements: &[ast::Statement],
) -> Result<SymbolTable, SymbolTableError> {
    let mut builder: SymbolTableBuilder = Default::default();
    builder.prepare();
    builder.scan_statements(statements)?;
    builder.finish()
}

/// Captures all symbols in the current scope, and has a list of subscopes in this scope.
#[derive(Clone)]
pub struct SymbolTable {
    /// The name of this symbol table. Often the name of the class or function.
    pub name: String,

    /// The type of symbol table
    pub typ: SymbolTableType,

    /// The line number in the sourcecode where this symboltable begins.
    pub line_number: usize,

    /// A set of symbols present on this scope level.
    pub symbols: IndexMap<String, Symbol>,

    /// A list of subscopes in the order as found in the
    /// AST nodes.
    pub sub_tables: Vec<SymbolTable>,
}

impl SymbolTable {
    fn new(name: String, typ: SymbolTableType, line_number: usize) -> Self {
        SymbolTable {
            name,
            typ,
            line_number,
            symbols: Default::default(),
            sub_tables: vec![],
        }
    }
}

#[derive(Clone, Copy, PartialEq)]
pub enum SymbolTableType {
    Module,
    Class,
    Function,
    Comprehension,
}

impl fmt::Display for SymbolTableType {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            SymbolTableType::Module => write!(f, "module"),
            SymbolTableType::Class => write!(f, "class"),
            SymbolTableType::Function => write!(f, "function"),
            SymbolTableType::Comprehension => write!(f, "comprehension"),
        }
    }
}

/// Indicator for a single symbol what the scope of this symbol is.
/// The scope can be unknown, which is unfortunate, but not impossible.
#[derive(Debug, Clone)]
pub enum SymbolScope {
    Global,
    Nonlocal,
    Local,
    Unknown,
}

/// A single symbol in a table. Has various properties such as the scope
/// of the symbol, and also the various uses of the symbol.
#[derive(Debug, Clone)]
pub struct Symbol {
    pub name: String,
    // pub table: SymbolTableRef,
    pub scope: SymbolScope,
    // TODO: Use bitflags replace
    pub is_referenced: bool,
    pub is_assigned: bool,
    pub is_parameter: bool,
    pub is_free: bool,
    pub is_annotated: bool,
    pub is_imported: bool,

    // indicates if the symbol gets a value assigned by a named expression in a comprehension
    // this is required to correct the scope in the analysis.
    pub is_assign_namedexpr_in_comprehension: bool,

    // inidicates that the symbol is used a bound iterator variable. We distinguish this case
    // from normal assignment to detect unallowed re-assignment to iterator variables.
    pub is_iter: bool,
}

impl Symbol {
    fn new(name: &str) -> Self {
        Symbol {
            name: name.to_owned(),
            // table,
            scope: SymbolScope::Unknown,
            is_referenced: false,
            is_assigned: false,
            is_parameter: false,
            is_free: false,
            is_annotated: false,
            is_imported: false,
            is_assign_namedexpr_in_comprehension: false,
            is_iter: false,
        }
    }

    pub fn is_global(&self) -> bool {
        if let SymbolScope::Global = self.scope {
            true
        } else {
            false
        }
    }

    pub fn is_local(&self) -> bool {
        if let SymbolScope::Local = self.scope {
            true
        } else {
            false
        }
    }
}

#[derive(Debug)]
pub struct SymbolTableError {
    error: String,
    location: Location,
}

impl CompileError {
    pub fn from_symbol_table_error(error: SymbolTableError, source_path: String) -> Self {
        CompileError {
            statement: None,
            error: CompileErrorType::SyntaxError(error.error),
            location: error.location,
            source_path,
        }
    }
}

type SymbolTableResult = Result<(), SymbolTableError>;

impl SymbolTable {
    pub fn lookup(&self, name: &str) -> Option<&Symbol> {
        self.symbols.get(name)
    }
}

impl std::fmt::Debug for SymbolTable {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(
            f,
            "SymbolTable({:?} symbols, {:?} sub scopes)",
            self.symbols.len(),
            self.sub_tables.len()
        )
    }
}

/* Perform some sort of analysis on nonlocals, globals etc..
  See also: https://github.com/python/cpython/blob/master/Python/symtable.c#L410
*/
fn analyze_symbol_table(symbol_table: &mut SymbolTable) -> SymbolTableResult {
    let mut analyzer = SymbolTableAnalyzer::default();
    analyzer.analyze_symbol_table(symbol_table)
}

/// Symbol table analysis. Can be used to analyze a fully
/// build symbol table structure. It will mark variables
/// as local variables for example.
#[derive(Default)]
struct SymbolTableAnalyzer<'a> {
    tables: Vec<(&'a mut IndexMap<String, Symbol>, SymbolTableType)>,
}

impl<'a> SymbolTableAnalyzer<'a> {
    fn analyze_symbol_table(&mut self, symbol_table: &'a mut SymbolTable) -> SymbolTableResult {
        let symbols = &mut symbol_table.symbols;
        let sub_tables = &mut symbol_table.sub_tables;

        self.tables.push((symbols, symbol_table.typ));
        // Analyze sub scopes:
        for sub_table in sub_tables {
            self.analyze_symbol_table(sub_table)?;
        }
        let (symbols, st_typ) = self.tables.pop().unwrap();

        // Analyze symbols:
        for symbol in symbols.values_mut() {
            self.analyze_symbol(symbol, st_typ)?;
        }
        Ok(())
    }

    fn analyze_symbol(
        &mut self,
        symbol: &mut Symbol,
        curr_st_typ: SymbolTableType,
    ) -> SymbolTableResult {
        if symbol.is_assign_namedexpr_in_comprehension
            && curr_st_typ == SymbolTableType::Comprehension
        {
            // propagate symbol to next higher level that can hold it,
            // i.e., function or module. Comprehension is skipped and
            // Class is not allowed and detected as error.
            //symbol.scope = SymbolScope::Nonlocal;
            self.analyze_symbol_comprehension(symbol, 0)?
        } else {
            match symbol.scope {
                SymbolScope::Nonlocal => {
                    let scope_depth = self.tables.len();
                    if scope_depth > 0 {
                        // check if the name is already defined in any outer scope
                        // therefore
                        if scope_depth < 2
                            || !self
                                .tables
                                .iter()
                                .skip(1) // omit the global scope as it is not non-local
                                .rev() // revert the order for better performance
                                .any(|t| t.0.contains_key(&symbol.name))
                        // true when any of symbol tables contains the name -> then negate
                        {
                            return Err(SymbolTableError {
                                error: format!("no binding for nonlocal '{}' found", symbol.name),
                                location: Default::default(),
                            });
                        }
                    } else {
                        return Err(SymbolTableError {
                            error: format!(
                                "nonlocal {} defined at place without an enclosing scope",
                                symbol.name
                            ),
                            location: Default::default(),
                        });
                    }
                }
                SymbolScope::Global => {
                    // TODO: add more checks for globals?
                }
                SymbolScope::Local => {
                    // all is well
                }
                SymbolScope::Unknown => {
                    // Try hard to figure out what the scope of this symbol is.
                    self.analyze_unknown_symbol(symbol);
                }
            }
        }
        Ok(())
    }

    fn analyze_unknown_symbol(&self, symbol: &mut Symbol) {
        if symbol.is_assigned || symbol.is_parameter {
            symbol.scope = SymbolScope::Local;
        } else if symbol.is_referenced {
            symbol.scope = SymbolScope::Unknown;
        } else {
            // Interesting stuff about the __class__ variable:
            // https://docs.python.org/3/reference/datamodel.html?highlight=__class__#creating-the-class-object
            let found_in_outer_scope = symbol.name == "__class__"
                || self.tables.iter().skip(1).any(|(symbols, typ)| {
                    *typ != SymbolTableType::Class && symbols.contains_key(&symbol.name)
                });

            if found_in_outer_scope {
                // Symbol is in some outer scope.
                symbol.is_free = true;
            } else if self.tables.is_empty() {
                // Don't make assumptions when we don't know.
                symbol.scope = SymbolScope::Unknown;
            } else {
                // If there are scopes above we can assume global.
                symbol.scope = SymbolScope::Global;
            }
        }
    }

    // Implements the symbol analysis and scope extension for names
    // assigned by a named expression in a comprehension. See:
    // https://github.com/python/cpython/blob/7b78e7f9fd77bb3280ee39fb74b86772a7d46a70/Python/symtable.c#L1435
    fn analyze_symbol_comprehension(
        &mut self,
        symbol: &mut Symbol,
        parent_offset: usize,
    ) -> SymbolTableResult {
        // TODO: quite C-ish way to implement the iteration
        // when this is called, we expect to be in the direct parent scope of the scope that contains 'symbol'
        let offs = self.tables.len() - 1 - parent_offset;
        let last = self.tables.get_mut(offs).unwrap();
        let symbols = &mut last.0;
        let table_type = last.1;

        // it is not allowed to use an iterator variable as assignee in a named expression
        if symbol.is_iter {
            return Err(SymbolTableError {
                error: format!(
                    "assignment expression cannot rebind comprehension iteration variable {}",
                    symbol.name
                ),
                location: Default::default(),
            });
        }

        match table_type {
            SymbolTableType::Module => {
                symbol.scope = SymbolScope::Global;
            }
            SymbolTableType::Class => {
                // named expressions are forbidden in comprehensions on class scope
                return Err(SymbolTableError {
                    error: "assignment expression within a comprehension cannot be used in a class body".to_string(),
                    location: Default::default(),
                } );
            }
            SymbolTableType::Function => {
                if let Some(parent_symbol) = symbols.get_mut(&symbol.name) {
                    if let SymbolScope::Unknown = parent_symbol.scope {
                        parent_symbol.is_assigned = true; // this information is new, as the asignment is done in inner scope
                                                          //self.analyze_unknown_symbol(symbol); // not needed, symbol is analyzed anyhow when its scope is analyzed
                    }

                    match symbol.scope {
                        SymbolScope::Global => {
                            symbol.scope = SymbolScope::Global;
                        }
                        _ => {
                            symbol.scope = SymbolScope::Nonlocal;
                        }
                    }
                } else {
                    let mut cloned_sym = symbol.clone();
                    cloned_sym.scope = SymbolScope::Local;
                    last.0.insert(cloned_sym.name.to_owned(), cloned_sym);
                }
            }
            SymbolTableType::Comprehension => {
                // TODO check for conflicts - requires more context information about variables
                match symbols.get_mut(&symbol.name) {
                    Some(parent_symbol) => {
                        // check if assignee is an iterator in top scope
                        if parent_symbol.is_iter {
                            return Err(SymbolTableError {
                                error: format!("assignment expression cannot rebind comprehension iteration variable {}", symbol.name),
                                location: Default::default(),
                            });
                        }

                        // we synthesize the assignment to the symbol from inner scope
                        parent_symbol.is_assigned = true; // more checks are required
                    }
                    None => {
                        // extend the scope of the inner symbol
                        // as we are in a nested comprehension, we expect that the symbol is needed
                        // ouside, too, and set it therefore to non-local scope. I.e., we expect to
                        // find a definition on a higher level
                        let mut cloned_sym = symbol.clone();
                        cloned_sym.scope = SymbolScope::Nonlocal;
                        last.0.insert(cloned_sym.name.to_owned(), cloned_sym);
                    }
                }

                self.analyze_symbol_comprehension(symbol, parent_offset + 1)?;
            }
        }
        Ok(())
    }
}

#[derive(Debug, Clone)]
enum SymbolUsage {
    Global,
    Nonlocal,
    Used,
    Assigned,
    Imported,
    AnnotationAssigned,
    Parameter,
    AnnotationParameter,
    AssignedNamedExprInCompr,
    Iter,
}

#[derive(Default)]
struct SymbolTableBuilder {
    // Scope stack.
    tables: Vec<SymbolTable>,
}

/// Enum to indicate in what mode an expression
/// was used.
/// In cpython this is stored in the AST, but I think this
/// is not logical, since it is not context free.
enum ExpressionContext {
    Load,
    Store,
    Delete,
    Iter,
    IterDefinitionExp,
}

impl SymbolTableBuilder {
    fn prepare(&mut self) {
        self.enter_scope("top", SymbolTableType::Module, 0)
    }

    fn finish(&mut self) -> Result<SymbolTable, SymbolTableError> {
        assert_eq!(self.tables.len(), 1);
        let mut symbol_table = self.tables.pop().unwrap();
        analyze_symbol_table(&mut symbol_table)?;
        Ok(symbol_table)
    }

    fn enter_scope(&mut self, name: &str, typ: SymbolTableType, line_number: usize) {
        let table = SymbolTable::new(name.to_owned(), typ, line_number);
        self.tables.push(table);
    }

    /// Pop symbol table and add to sub table of parent table.
    fn leave_scope(&mut self) {
        let table = self.tables.pop().unwrap();
        self.tables.last_mut().unwrap().sub_tables.push(table);
    }

    fn scan_program(&mut self, program: &ast::Program) -> SymbolTableResult {
        self.scan_statements(&program.statements)?;
        Ok(())
    }

    fn scan_statements(&mut self, statements: &[ast::Statement]) -> SymbolTableResult {
        for statement in statements {
            self.scan_statement(statement)?;
        }
        Ok(())
    }

    fn scan_parameters(&mut self, parameters: &[ast::Parameter]) -> SymbolTableResult {
        for parameter in parameters {
            self.scan_parameter(parameter)?;
        }
        Ok(())
    }

    fn scan_parameter(&mut self, parameter: &ast::Parameter) -> SymbolTableResult {
        let usage = if parameter.annotation.is_some() {
            SymbolUsage::AnnotationParameter
        } else {
            SymbolUsage::Parameter
        };
        self.register_name(&parameter.arg, usage)
    }

    fn scan_parameters_annotations(&mut self, parameters: &[ast::Parameter]) -> SymbolTableResult {
        for parameter in parameters {
            self.scan_parameter_annotation(parameter)?;
        }
        Ok(())
    }

    fn scan_parameter_annotation(&mut self, parameter: &ast::Parameter) -> SymbolTableResult {
        if let Some(annotation) = &parameter.annotation {
            self.scan_expression(&annotation, &ExpressionContext::Load)?;
        }
        Ok(())
    }

    fn scan_statement(&mut self, statement: &ast::Statement) -> SymbolTableResult {
        use ast::StatementType::*;
        match &statement.node {
            Global { names } => {
                for name in names {
                    self.register_name(name, SymbolUsage::Global)?;
                }
            }
            Nonlocal { names } => {
                for name in names {
                    self.register_name(name, SymbolUsage::Nonlocal)?;
                }
            }
            FunctionDef {
                name,
                body,
                args,
                decorator_list,
                returns,
                ..
            } => {
                self.scan_expressions(decorator_list, &ExpressionContext::Load)?;
                self.register_name(name, SymbolUsage::Assigned)?;
                if let Some(expression) = returns {
                    self.scan_expression(expression, &ExpressionContext::Load)?;
                }
                self.enter_function(name, args, statement.location.row())?;
                self.scan_statements(body)?;
                self.leave_scope();
            }
            ClassDef {
                name,
                body,
                bases,
                keywords,
                decorator_list,
            } => {
                self.enter_scope(name, SymbolTableType::Class, statement.location.row());
                self.register_name("__module__", SymbolUsage::Assigned)?;
                self.register_name("__qualname__", SymbolUsage::Assigned)?;
                self.scan_statements(body)?;
                self.leave_scope();
                self.scan_expressions(bases, &ExpressionContext::Load)?;
                for keyword in keywords {
                    self.scan_expression(&keyword.value, &ExpressionContext::Load)?;
                }
                self.scan_expressions(decorator_list, &ExpressionContext::Load)?;
                self.register_name(name, SymbolUsage::Assigned)?;
            }
            Expression { expression } => {
                self.scan_expression(expression, &ExpressionContext::Load)?
            }
            If { test, body, orelse } => {
                self.scan_expression(test, &ExpressionContext::Load)?;
                self.scan_statements(body)?;
                if let Some(code) = orelse {
                    self.scan_statements(code)?;
                }
            }
            For {
                target,
                iter,
                body,
                orelse,
                ..
            } => {
                self.scan_expression(target, &ExpressionContext::Store)?;
                self.scan_expression(iter, &ExpressionContext::Load)?;
                self.scan_statements(body)?;
                if let Some(code) = orelse {
                    self.scan_statements(code)?;
                }
            }
            While { test, body, orelse } => {
                self.scan_expression(test, &ExpressionContext::Load)?;
                self.scan_statements(body)?;
                if let Some(code) = orelse {
                    self.scan_statements(code)?;
                }
            }
            Break | Continue | Pass => {
                // No symbols here.
            }
            Import { names } | ImportFrom { names, .. } => {
                for name in names {
                    if let Some(alias) = &name.alias {
                        // `import mymodule as myalias`
                        self.register_name(alias, SymbolUsage::Imported)?;
                    } else {
                        // `import module`
                        self.register_name(
                            name.symbol.split('.').next().unwrap(),
                            SymbolUsage::Imported,
                        )?;
                    }
                }
            }
            Return { value } => {
                if let Some(expression) = value {
                    self.scan_expression(expression, &ExpressionContext::Load)?;
                }
            }
            Assert { test, msg } => {
                self.scan_expression(test, &ExpressionContext::Load)?;
                if let Some(expression) = msg {
                    self.scan_expression(expression, &ExpressionContext::Load)?;
                }
            }
            Delete { targets } => {
                self.scan_expressions(targets, &ExpressionContext::Delete)?;
            }
            Assign { targets, value } => {
                self.scan_expressions(targets, &ExpressionContext::Store)?;
                self.scan_expression(value, &ExpressionContext::Load)?;
            }
            AugAssign { target, value, .. } => {
                self.scan_expression(target, &ExpressionContext::Store)?;
                self.scan_expression(value, &ExpressionContext::Load)?;
            }
            AnnAssign {
                target,
                annotation,
                value,
            } => {
                // https://github.com/python/cpython/blob/master/Python/symtable.c#L1233
                if let ast::ExpressionType::Identifier { ref name } = target.node {
                    self.register_name(name, SymbolUsage::AnnotationAssigned)?;
                } else {
                    self.scan_expression(target, &ExpressionContext::Store)?;
                }
                self.scan_expression(annotation, &ExpressionContext::Load)?;
                if let Some(value) = value {
                    self.scan_expression(value, &ExpressionContext::Load)?;
                }
            }
            With { items, body, .. } => {
                for item in items {
                    self.scan_expression(&item.context_expr, &ExpressionContext::Load)?;
                    if let Some(expression) = &item.optional_vars {
                        self.scan_expression(expression, &ExpressionContext::Store)?;
                    }
                }
                self.scan_statements(body)?;
            }
            Try {
                body,
                handlers,
                orelse,
                finalbody,
            } => {
                self.scan_statements(body)?;
                for handler in handlers {
                    if let Some(expression) = &handler.typ {
                        self.scan_expression(expression, &ExpressionContext::Load)?;
                    }
                    if let Some(name) = &handler.name {
                        self.register_name(name, SymbolUsage::Assigned)?;
                    }
                    self.scan_statements(&handler.body)?;
                }
                if let Some(code) = orelse {
                    self.scan_statements(code)?;
                }
                if let Some(code) = finalbody {
                    self.scan_statements(code)?;
                }
            }
            Raise { exception, cause } => {
                if let Some(expression) = exception {
                    self.scan_expression(expression, &ExpressionContext::Load)?;
                }
                if let Some(expression) = cause {
                    self.scan_expression(expression, &ExpressionContext::Load)?;
                }
            }
        }
        Ok(())
    }

    fn scan_expressions(
        &mut self,
        expressions: &[ast::Expression],
        context: &ExpressionContext,
    ) -> SymbolTableResult {
        for expression in expressions {
            self.scan_expression(expression, context)?;
        }
        Ok(())
    }

    fn scan_expression(
        &mut self,
        expression: &ast::Expression,
        context: &ExpressionContext,
    ) -> SymbolTableResult {
        use ast::ExpressionType::*;
        match &expression.node {
            Binop { a, b, .. } => {
                self.scan_expression(a, context)?;
                self.scan_expression(b, context)?;
            }
            BoolOp { values, .. } => {
                self.scan_expressions(values, context)?;
            }
            Compare { vals, .. } => {
                self.scan_expressions(vals, context)?;
            }
            Subscript { a, b } => {
                self.scan_expression(a, &ExpressionContext::Load)?;
                self.scan_expression(b, context)?;
            }
            Attribute { value, .. } => {
                self.scan_expression(value, &ExpressionContext::Load)?;
            }
            Dict { elements } => {
                for (key, value) in elements {
                    if let Some(key) = key {
                        self.scan_expression(key, context)?;
                    } else {
                        // dict unpacking marker
                    }
                    self.scan_expression(value, context)?;
                }
            }
            Await { value } => {
                self.scan_expression(value, context)?;
            }
            Yield { value } => {
                if let Some(expression) = value {
                    self.scan_expression(expression, context)?;
                }
            }
            YieldFrom { value } => {
                self.scan_expression(value, context)?;
            }
            Unop { a, .. } => {
                self.scan_expression(a, context)?;
            }
            True | False | None | Ellipsis => {}
            Number { .. } => {}
            Starred { value } => {
                self.scan_expression(value, context)?;
            }
            Bytes { .. } => {}
            Tuple { elements } | Set { elements } | List { elements } | Slice { elements } => {
                self.scan_expressions(elements, context)?;
            }
            Comprehension { kind, generators } => {
                // Comprehensions are compiled as functions, so create a scope for them:
                let scope_name = match **kind {
                    ast::ComprehensionKind::GeneratorExpression { .. } => "genexpr",
                    ast::ComprehensionKind::List { .. } => "listcomp",
                    ast::ComprehensionKind::Set { .. } => "setcomp",
                    ast::ComprehensionKind::Dict { .. } => "dictcomp",
                };

                self.enter_scope(
                    scope_name,
                    SymbolTableType::Comprehension,
                    expression.location.row(),
                );

                // Register the passed argument to the generator function as the name ".0"
                self.register_name(".0", SymbolUsage::Parameter)?;

                match **kind {
                    ast::ComprehensionKind::GeneratorExpression { ref element }
                    | ast::ComprehensionKind::List { ref element }
                    | ast::ComprehensionKind::Set { ref element } => {
                        self.scan_expression(element, &ExpressionContext::Load)?;
                    }
                    ast::ComprehensionKind::Dict { ref key, ref value } => {
                        self.scan_expression(&key, &ExpressionContext::Load)?;
                        self.scan_expression(&value, &ExpressionContext::Load)?;
                    }
                }

                let mut is_first_generator = true;
                for generator in generators {
                    self.scan_expression(&generator.target, &ExpressionContext::Iter)?;
                    if is_first_generator {
                        is_first_generator = false;
                    } else {
                        self.scan_expression(
                            &generator.iter,
                            &ExpressionContext::IterDefinitionExp,
                        )?;
                    }

                    for if_expr in &generator.ifs {
                        self.scan_expression(if_expr, &ExpressionContext::Load)?;
                    }
                }

                self.leave_scope();

                // The first iterable is passed as an argument into the created function:
                assert!(!generators.is_empty());
                self.scan_expression(&generators[0].iter, &ExpressionContext::IterDefinitionExp)?;
            }
            Call {
                function,
                args,
                keywords,
            } => {
                match *context {
                    ExpressionContext::IterDefinitionExp => {
                        self.scan_expression(function, &ExpressionContext::IterDefinitionExp)?;
                    }
                    _ => {
                        self.scan_expression(function, &ExpressionContext::Load)?;
                    }
                }

                self.scan_expressions(args, &ExpressionContext::Load)?;
                for keyword in keywords {
                    self.scan_expression(&keyword.value, &ExpressionContext::Load)?;
                }
            }
            String { value } => {
                self.scan_string_group(value)?;
            }
            Identifier { name } => {
                // Determine the contextual usage of this symbol:
                match context {
                    ExpressionContext::Delete => {
                        self.register_name(name, SymbolUsage::Used)?;
                    }
                    ExpressionContext::Load | ExpressionContext::IterDefinitionExp => {
                        self.register_name(name, SymbolUsage::Used)?;
                    }
                    ExpressionContext::Store => {
                        self.register_name(name, SymbolUsage::Assigned)?;
                    }
                    ExpressionContext::Iter => {
                        self.register_name(name, SymbolUsage::Iter)?;
                    }
                }
            }
            Lambda { args, body } => {
                self.enter_function("lambda", args, expression.location.row())?;
                match *context {
                    ExpressionContext::IterDefinitionExp => {
                        self.scan_expression(body, &ExpressionContext::IterDefinitionExp)?;
                    }
                    _ => {
                        self.scan_expression(body, &ExpressionContext::Load)?;
                    }
                }
                self.leave_scope();
            }
            IfExpression { test, body, orelse } => {
                self.scan_expression(test, &ExpressionContext::Load)?;
                self.scan_expression(body, &ExpressionContext::Load)?;
                self.scan_expression(orelse, &ExpressionContext::Load)?;
            }

            NamedExpression { left, right } => {
                // named expressions are not allowed in the definiton of
                // comprehension iterator definitions
                if let ExpressionContext::IterDefinitionExp = *context {
                    return Err(SymbolTableError {
                        error: "assignment expression cannot be used in a comprehension iterable expression".to_string(),
                        location: Default::default(),
                    });
                }

                self.scan_expression(right, &ExpressionContext::Load)?;

                // special handling for assigned identifier in named expressions
                // that are used in comprehensions. This required to correctly
                // propagate the scope of the named assigned named and not to
                // propagate inner names.
                if let Identifier { name } = &left.node {
                    let table = self.tables.last().unwrap();
                    if table.typ == SymbolTableType::Comprehension {
                        self.register_name(name, SymbolUsage::AssignedNamedExprInCompr)?;
                    } else {
                        // omit one recursion. When the handling of an store changes for
                        // Identifiers this needs adapted - more forward safe would be
                        // calling scan_expression directly.
                        self.register_name(name, SymbolUsage::Assigned)?;
                    }
                } else {
                    self.scan_expression(left, &ExpressionContext::Store)?;
                }
            }
        }
        Ok(())
    }

    fn enter_function(
        &mut self,
        name: &str,
        args: &ast::Parameters,
        line_number: usize,
    ) -> SymbolTableResult {
        // Evaluate eventual default parameters:
        self.scan_expressions(&args.defaults, &ExpressionContext::Load)?;
        for kw_default in &args.kw_defaults {
            if let Some(expression) = kw_default {
                self.scan_expression(&expression, &ExpressionContext::Load)?;
            }
        }

        // Annotations are scanned in outer scope:
        self.scan_parameters_annotations(&args.args)?;
        self.scan_parameters_annotations(&args.kwonlyargs)?;
        if let ast::Varargs::Named(name) = &args.vararg {
            self.scan_parameter_annotation(name)?;
        }
        if let ast::Varargs::Named(name) = &args.kwarg {
            self.scan_parameter_annotation(name)?;
        }

        self.enter_scope(name, SymbolTableType::Function, line_number);

        // Fill scope with parameter names:
        self.scan_parameters(&args.args)?;
        self.scan_parameters(&args.kwonlyargs)?;
        if let ast::Varargs::Named(name) = &args.vararg {
            self.scan_parameter(name)?;
        }
        if let ast::Varargs::Named(name) = &args.kwarg {
            self.scan_parameter(name)?;
        }
        Ok(())
    }

    fn scan_string_group(&mut self, group: &ast::StringGroup) -> SymbolTableResult {
        match group {
            ast::StringGroup::Constant { .. } => {}
            ast::StringGroup::FormattedValue { value, spec, .. } => {
                self.scan_expression(value, &ExpressionContext::Load)?;
                if let Some(spec) = spec {
                    self.scan_string_group(spec)?;
                }
            }
            ast::StringGroup::Joined { values } => {
                for subgroup in values {
                    self.scan_string_group(subgroup)?;
                }
            }
        }
        Ok(())
    }

    fn register_name(&mut self, name: &str, role: SymbolUsage) -> SymbolTableResult {
        let scope_depth = self.tables.len();
        let table = self.tables.last_mut().unwrap();
        let location = Default::default();

        // Some checks:
        let containing = table.symbols.contains_key(name);
        if containing {
            // Role already set..
            match role {
                SymbolUsage::Global => {
                    let symbol = table.symbols.get(name).unwrap();
                    if let SymbolScope::Global = symbol.scope {
                        // Ok
                    } else {
                        return Err(SymbolTableError {
                            error: format!("name '{}' is used prior to global declaration", name),
                            location,
                        });
                    }
                }
                SymbolUsage::Nonlocal => {
                    return Err(SymbolTableError {
                        error: format!("name '{}' is used prior to nonlocal declaration", name),
                        location,
                    })
                }
                _ => {
                    // Ok?
                }
            }
        }

        // Some more checks:
        match role {
            SymbolUsage::Nonlocal if scope_depth < 2 => {
                return Err(SymbolTableError {
                    error: format!("cannot define nonlocal '{}' at top level.", name),
                    location,
                })
            }
            _ => {
                // Ok!
            }
        }

        // Insert symbol when required:
        if !containing {
            let symbol = Symbol::new(name);
            table.symbols.insert(name.to_owned(), symbol);
        }

        // Set proper flags on symbol:
        let symbol = table.symbols.get_mut(name).unwrap();
        match role {
            SymbolUsage::Nonlocal => {
                if let SymbolScope::Unknown = symbol.scope {
                    symbol.scope = SymbolScope::Nonlocal;
                } else {
                    return Err(SymbolTableError {
                        error: format!("Symbol {} scope cannot be set to nonlocal, since its scope was already determined otherwise.", name),
                        location,
                    });
                }
            }
            SymbolUsage::Imported => {
                symbol.is_assigned = true;
                symbol.is_imported = true;
            }
            SymbolUsage::Parameter => {
                symbol.is_parameter = true;
            }
            SymbolUsage::AnnotationParameter => {
                symbol.is_parameter = true;
                symbol.is_annotated = true;
            }
            SymbolUsage::AnnotationAssigned => {
                symbol.is_assigned = true;
                symbol.is_annotated = true;
            }
            SymbolUsage::Assigned => {
                symbol.is_assigned = true;
            }
            SymbolUsage::AssignedNamedExprInCompr => {
                symbol.is_assigned = true;
                symbol.is_assign_namedexpr_in_comprehension = true;
            }
            SymbolUsage::Global => {
                if let SymbolScope::Unknown = symbol.scope {
                    symbol.scope = SymbolScope::Global;
                } else if let SymbolScope::Global = symbol.scope {
                    // Global scope can be set to global
                } else {
                    return Err(SymbolTableError {
                        error: format!("Symbol {} scope cannot be set to global, since its scope was already determined otherwise.", name),
                        location,
                    });
                }
            }
            SymbolUsage::Used => {
                symbol.is_referenced = true;
            }
            SymbolUsage::Iter => {
                symbol.is_iter = true;
                symbol.scope = SymbolScope::Local;
            }
        }

        // and even more checking
        // it is not allowed to assign to iterator variables (by named expressions)
        if symbol.is_iter && symbol.is_assigned
        /*&& symbol.is_assign_namedexpr_in_comprehension*/
        {
            return Err(SymbolTableError {
                error:
                    "assignment expression cannot be used in a comprehension iterable expression"
                        .to_string(),
                location,
            });
        }
        Ok(())
    }
}