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 @@ if a := 1:
     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: ~