Rewrite placement logic (#6040)

## Summary
This is a rewrite of the main comment placement logic. `place_comment`
now has three parts:

- place own line comments
  - between branches
  - after a branch
- place end-of-line comments
  - after colon
  - after a branch
- place comments for specific nodes (that include module level comments)

The rewrite fixed three bugs: `class A: # trailing comment` comments now
stay end-of-line, `try: # comment` remains end-of-line and deeply
indented try-else-finally comments remain with the right nested
statement.

It will be much easier to give more alternative branches nodes since
this is abstracted away by `is_node_with_body` and the first/last child
helpers. Adding new node types can now be done by adding an entry to the
`place_comment` match. The code went from 1526 lines before #6033 to
1213 lines now.

It thinks it easier to just read the new `placement.rs` rather than
reviewing the diff.

## Test Plan

The existing fixtures staying the same or improving plus new ones for
the bug fixes.

crates/ruff_python_ast/src/node.rs

@@ -4648,6 +4648,8 @@ impl AnyNodeRef<'_> {
                 | AnyNodeRef::StmtClassDef(_)
                 | AnyNodeRef::StmtTry(_)
                 | AnyNodeRef::StmtTryStar(_)
+                | AnyNodeRef::ExceptHandlerExceptHandler(_)
+                | AnyNodeRef::ElifElseClause(_)
         )
     }
 

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/class_definition.py

@@ -35,5 +35,11 @@ class Test(aaaaaaaaaaaaaaa + bbbbbbbbbbbbbbbbbbbbbb + cccccccccccccccccccccccc +
 class Test(aaaaaaaaaaaaaaa + bbbbbbbbbbbbbbbbbbbbbb * cccccccccccccccccccccccc + dddddddddddddddddddddd + eeeeeeeee, ffffffffffffffffff, gggggggggggggggggg):
     pass
 
-class Test(Aaaa): # trailing comment
+class TestTrailingComment1(Aaaa): # trailing comment
     pass
+
+
+class TestTrailingComment2: # trailing comment
+    pass
+
+

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/if.py

@@ -88,13 +88,17 @@ def f():
         # comment
 
 if True:
-    def f2():
+    def f():
         pass
         # 1
-else:
+elif True:
-    def f2():
+    def f():
         pass
         # 2
+else:
+    def f():
+        pass
+        # 3
 
 if True: print("a") # 1
 elif True: print("b") # 2

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/try.py

@@ -106,16 +106,35 @@ except RuntimeError:
     raise
 
 try:
-    def f2():
+    def f():
         pass
         # a
 except:
-    def f2():
+    def f():
         pass
         # b
+else:
+    def f():
+        pass
+        # c
+finally:
+    def f():
+        pass
+        # d
 
 try: pass # a
 except ZeroDivisionError: pass # b
-except: pass # b
+except: pass # c
 else: pass # d
-finally: pass # c
+finally: pass # e
+
+try:  # 1 preceding: any, following: first in body, enclosing: try
+    print(1)  # 2 preceding: last in body, following: fist in alt body, enclosing: try
+except ZeroDivisionError:  # 3 preceding: test, following: fist in alt body, enclosing: try
+    print(2)  # 4 preceding: last in body, following: fist in alt body, enclosing: exc
+except:  # 5 preceding: last in body, following: fist in alt body, enclosing: try
+    print(2)  # 6 preceding: last in body, following: fist in alt body, enclosing: exc
+else:  # 7 preceding: last in body, following: fist in alt body, enclosing: exc
+    print(3)  # 8 preceding: last in body, following: fist in alt body, enclosing: try
+finally:  # 9 preceding: last in body, following: fist in alt body, enclosing: try
+    print(3)  # 10 preceding: last in body, following: any, enclosing: try

crates/ruff_python_formatter/src/comments/snapshots/ruff_python_formatter__comments__tests__trailing_comment_after_single_statement_body.snap

@@ -4,18 +4,18 @@ expression: comments.debug(test_case.source_code)
 ---
 {
     Node {
-        kind: StmtExpr,
+        kind: StmtPass,
-        range: 29..42,
+        range: 12..16,
-        source: `print("test")`,
+        source: `pass`,
     }: {
-        "leading": [
+        "leading": [],
+        "dangling": [],
+        "trailing": [
             SourceComment {
                 text: "# Test",
                 position: OwnLine,
                 formatted: false,
             },
         ],
-        "dangling": [],
-        "trailing": [],
     },
 }

crates/ruff_python_formatter/src/expression/expr_slice.rs

@@ -132,15 +132,13 @@ impl FormatNodeRule<ExprSlice> for FormatExprSlice {
                 let step_leading_comments = comments.leading_comments(step.as_ref());
                 leading_comments_spacing(f, step_leading_comments)?;
                 step.format().fmt(f)?;
-            } else {
+            } else if !dangling_step_comments.is_empty() {
-                if !dangling_step_comments.is_empty() {
                 // Put the colon and comments on their own lines
                 write!(
                     f,
                     [hard_line_break(), dangling_comments(dangling_step_comments)]
                 )?;
             }
-            }
         } else {
             debug_assert!(step.is_none(), "step can't exist without a second colon");
         }

crates/ruff_python_formatter/src/other/except_handler_except_handler.rs

@@ -5,7 +5,6 @@ use crate::prelude::*;
 use crate::{FormatNodeRule, PyFormatter};
 use ruff_formatter::FormatRuleWithOptions;
 use ruff_formatter::{write, Buffer, FormatResult};
-use ruff_python_ast::node::AstNode;
 use rustpython_ast::ExceptHandlerExceptHandler;
 
 #[derive(Copy, Clone, Default)]
@@ -45,7 +44,7 @@ impl FormatNodeRule<ExceptHandlerExceptHandler> for FormatExceptHandlerExceptHan
         } = item;
 
         let comments_info = f.context().comments().clone();
-        let dangling_comments = comments_info.dangling_comments(item.as_any_node_ref());
+        let dangling_comments = comments_info.dangling_comments(item);
 
         write!(
             f,
@@ -75,7 +74,7 @@ impl FormatNodeRule<ExceptHandlerExceptHandler> for FormatExceptHandlerExceptHan
             [
                 text(":"),
                 trailing_comments(dangling_comments),
-                block_indent(&body.format())
+                block_indent(&body.format()),
             ]
         )
     }

crates/ruff_python_formatter/src/statement/stmt_try.rs

@@ -1,6 +1,6 @@
 use crate::comments;
-use crate::comments::leading_alternate_branch_comments;
 use crate::comments::SourceComment;
+use crate::comments::{leading_alternate_branch_comments, trailing_comments};
 use crate::other::except_handler_except_handler::ExceptHandlerKind;
 use crate::prelude::*;
 use crate::statement::FormatRefWithRule;
@@ -134,8 +134,7 @@ impl Format<PyFormatContext<'_>> for AnyStatementTry<'_> {
         let orelse = self.orelse();
         let finalbody = self.finalbody();
 
-        write!(f, [text("try:"), block_indent(&body.format())])?;
+        (_, dangling_comments) = format_case("try", body, None, dangling_comments, f)?;
-
         let mut previous_node = body.last();
 
         for handler in handlers {
@@ -183,15 +182,18 @@ fn format_case<'a>(
         let case_comments_start =
             dangling_comments.partition_point(|comment| comment.slice().end() <= last.end());
         let (case_comments, rest) = dangling_comments.split_at(case_comments_start);
+        let partition_point =
+            case_comments.partition_point(|comment| comment.line_position().is_own_line());
         write!(
             f,
-            [leading_alternate_branch_comments(
+            [
-                case_comments,
+                leading_alternate_branch_comments(&case_comments[..partition_point], previous_node),
-                previous_node
+                text(name),
-            )]
+                text(":"),
+                trailing_comments(&case_comments[partition_point..]),
+                block_indent(&body.format())
+            ]
         )?;
-
-        write!(f, [text(name), text(":"), block_indent(&body.format())])?;
         (None, rest)
     } else {
         (None, dangling_comments)

crates/ruff_python_formatter/tests/snapshots/black_compatibility@simple_cases__fmtskip8.py.snap

@@ -74,7 +74,7 @@ async def test_async_with():
 ```diff
 --- Black
 +++ Ruff
-@@ -1,62 +1,63 @@
+@@ -1,62 +1,62 @@
  # Make sure a leading comment is not removed.
 -def some_func(  unformatted,  args  ):  # fmt: skip
 +def some_func(unformatted, args):  # fmt: skip
@@ -134,15 +134,13 @@ async def test_async_with():
  
  
 -try  :  # fmt: skip
-+try:
++try:  # fmt: skip
-+    # fmt: skip
      some_call()
 -except  UnformattedError  as  ex:  # fmt: skip
--    handle_exception()
--finally  :  # fmt: skip
 +except UnformattedError as ex:  # fmt: skip
-+    handle_exception()  # fmt: skip
+     handle_exception()
-+finally:
+-finally  :  # fmt: skip
++finally:  # fmt: skip
      finally_call()
  
  
@@ -207,12 +205,11 @@ async def test_async_for():
         print("Do something")
 
 
-try:
+try:  # fmt: skip
-    # fmt: skip
     some_call()
 except UnformattedError as ex:  # fmt: skip
-    handle_exception()  # fmt: skip
+    handle_exception()
-finally:
+finally:  # fmt: skip
     finally_call()
 
 

crates/ruff_python_formatter/tests/snapshots/black_compatibility@simple_cases__ignore_pyi.py.snap

@@ -32,14 +32,12 @@ def h():
 ```diff
 --- Black
 +++ Ruff
-@@ -1,18 +1,26 @@
+@@ -1,18 +1,25 @@
  def f():  # type: ignore
      ...
  
--class x:  # some comment
 +
-+class x:
+ class x:  # some comment
-+    # some comment
      ...
  
 -class y: ...  # comment
@@ -71,8 +69,7 @@ def f():  # type: ignore
     ...
 
 
-class x:
+class x:  # some comment
-    # some comment
     ...
 
 

crates/ruff_python_formatter/tests/snapshots/format@statement__class_definition.py.snap

@@ -41,8 +41,14 @@ class Test(aaaaaaaaaaaaaaa + bbbbbbbbbbbbbbbbbbbbbb + cccccccccccccccccccccccc +
 class Test(aaaaaaaaaaaaaaa + bbbbbbbbbbbbbbbbbbbbbb * cccccccccccccccccccccccc + dddddddddddddddddddddd + eeeeeeeee, ffffffffffffffffff, gggggggggggggggggg):
     pass
 
-class Test(Aaaa): # trailing comment
+class TestTrailingComment1(Aaaa): # trailing comment
     pass
+
+
+class TestTrailingComment2: # trailing comment
+    pass
+
+
 ```
 
 ## Output
@@ -102,7 +108,11 @@ class Test(
     pass
 
 
-class Test(Aaaa):  # trailing comment
+class TestTrailingComment1(Aaaa):  # trailing comment
+    pass
+
+
+class TestTrailingComment2:  # trailing comment
     pass
 ```
 

crates/ruff_python_formatter/tests/snapshots/format@statement__if.py.snap

@@ -94,13 +94,17 @@ def f():
         # comment
 
 if True:
-    def f2():
+    def f():
         pass
         # 1
-else:
+elif True:
-    def f2():
+    def f():
         pass
         # 2
+else:
+    def f():
+        pass
+        # 3
 
 if True: print("a") # 1
 elif True: print("b") # 2
@@ -199,13 +203,17 @@ def f():
 
 
 if True:
-    def f2():
+    def f():
         pass
         # 1
-else:
+elif True:
-    def f2():
+    def f():
         pass
         # 2
+else:
+    def f():
+        pass
+        # 3
 
 if True:
     print("a")  # 1

crates/ruff_python_formatter/tests/snapshots/format@statement__try.py.snap

@@ -112,19 +112,38 @@ except RuntimeError:
     raise
 
 try:
-    def f2():
+    def f():
         pass
         # a
 except:
-    def f2():
+    def f():
         pass
         # b
+else:
+    def f():
+        pass
+        # c
+finally:
+    def f():
+        pass
+        # d
 
 try: pass # a
 except ZeroDivisionError: pass # b
-except: pass # b
+except: pass # c
 else: pass # d
-finally: pass # c
+finally: pass # e
+
+try:  # 1 preceding: any, following: first in body, enclosing: try
+    print(1)  # 2 preceding: last in body, following: fist in alt body, enclosing: try
+except ZeroDivisionError:  # 3 preceding: test, following: fist in alt body, enclosing: try
+    print(2)  # 4 preceding: last in body, following: fist in alt body, enclosing: exc
+except:  # 5 preceding: last in body, following: fist in alt body, enclosing: try
+    print(2)  # 6 preceding: last in body, following: fist in alt body, enclosing: exc
+else:  # 7 preceding: last in body, following: fist in alt body, enclosing: exc
+    print(3)  # 8 preceding: last in body, following: fist in alt body, enclosing: try
+finally:  # 9 preceding: last in body, following: fist in alt body, enclosing: try
+    print(3)  # 10 preceding: last in body, following: any, enclosing: try
 ```
 
 ## Output
@@ -140,8 +159,7 @@ except KeyError:  # should remove brackets and be a single line
     ...
 
 
-try:
+try:  # try
-    # try
     ...
     # end of body
 # before except
@@ -160,8 +178,7 @@ finally:
 
 
 # with line breaks
-try:
+try:  # try
-    # try
     ...
     # end of body
 
@@ -213,8 +230,7 @@ except:
 
 
 # try/except*, mostly the same as try
-try:
+try:  # try
-    # try
     ...
     # end of body
 # before except
@@ -247,24 +263,43 @@ except RuntimeError:
     raise
 
 try:
-    def f2():
+    def f():
         pass
         # a
 except:
-    def f2():
+    def f():
         pass
         # b
+else:
+    def f():
+        pass
+        # c
+finally:
+    def f():
+        pass
+        # d
 
 try:
     pass  # a
 except ZeroDivisionError:
     pass  # b
 except:
-    pass  # b
+    pass  # c
 else:
     pass  # d
 finally:
-    pass  # c
+    pass  # e
+
+try:  # 1 preceding: any, following: first in body, enclosing: try
+    print(1)  # 2 preceding: last in body, following: fist in alt body, enclosing: try
+except ZeroDivisionError:  # 3 preceding: test, following: fist in alt body, enclosing: try
+    print(2)  # 4 preceding: last in body, following: fist in alt body, enclosing: exc
+except:  # 5 preceding: last in body, following: fist in alt body, enclosing: try
+    print(2)  # 6 preceding: last in body, following: fist in alt body, enclosing: exc
+else:  # 7 preceding: last in body, following: fist in alt body, enclosing: exc
+    print(3)  # 8 preceding: last in body, following: fist in alt body, enclosing: try
+finally:  # 9 preceding: last in body, following: fist in alt body, enclosing: try
+    print(3)  # 10 preceding: last in body, following: any, enclosing: try
 ```