Allow compound statements of single ellipsis (#2837)

This allows `class C: ...`-style compound statements in stub files.

Closes #2835.

crates/ruff/resources/test/fixtures/pycodestyle/E70.py

@@ -46,3 +46,11 @@ class C: pass
  pass
 #:
 func = lambda x: x** 2 if cond else lambda x:x
+#:
+class C: ...
+#:
+def f(): ...
+#: E701:1:8 E702:1:13
+class C: ...; x = 1
+#: E701:1:8 E702:1:13
+class C: ...; ...

crates/ruff/src/rules/pycodestyle/rules/compound_statements.rs

@@ -53,6 +53,10 @@ pub fn compound_statements(lxr: &[LexResult]) -> Vec<Diagnostic> {
  let mut while_ = None;
  let mut with = None;
 
+ // As a special-case, track whether we're at the first token after a colon.
+ // This is used to allow `class C: ...`-style definitions in stubs.
+ let mut allow_ellipsis = false;
+
  // Track the bracket depth.
  let mut par_count = 0;
  let mut sqb_count = 0;
@@ -118,12 +122,17 @@ pub fn compound_statements(lxr: &[LexResult]) -> Vec<Diagnostic> {
  || with.is_some()
  {
  colon = Some((start, end));
+ allow_ellipsis = true;
  }
  }
  Tok::Semi => {
  semi = Some((start, end));
  }
  Tok::Comment(..) | Tok::Indent | Tok::Dedent | Tok::NonLogicalNewline => {}
+ Tok::Ellipsis if allow_ellipsis => {
+ // Allow `class C: ...`-style definitions in stubs.
+ allow_ellipsis = false;
+ }
  _ => {
  if let Some((start, end)) = semi {
  diagnostics.push(Diagnostic::new(

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__E701_E70.py.snap

@@ -112,4 +112,24 @@ expression: diagnostics
  column: 15
  fix: ~
  parent: ~
+- kind:
+ MultipleStatementsOnOneLineColon: ~
+ location:
+ row: 54
+ column: 7
+ end_location:
+ row: 54
+ column: 8
+ fix: ~
+ parent: ~
+- kind:
+ MultipleStatementsOnOneLineColon: ~
+ location:
+ row: 56
+ column: 7
+ end_location:
+ row: 56
+ column: 8
+ fix: ~
+ parent: ~
 

crates/ruff/src/rules/pycodestyle/snapshots/ruff__rules__pycodestyle__tests__E702_E70.py.snap

@@ -42,4 +42,24 @@ expression: diagnostics
  column: 11
  fix: ~
  parent: ~
+- kind:
+ MultipleStatementsOnOneLineSemicolon: ~
+ location:
+ row: 54
+ column: 12
+ end_location:
+ row: 54
+ column: 13
+ fix: ~
+ parent: ~
+- kind:
+ MultipleStatementsOnOneLineSemicolon: ~
+ location:
+ row: 56
+ column: 12
+ end_location:
+ row: 56
+ column: 13
+ fix: ~
+ parent: ~