Better error recovery for match patterns

crates/ruff_python_parser/resources/invalid/statements/match/invalid_class_pattern.py

@@ -0,0 +1,17 @@
+# Invalid keyword pattern in class argument
+match subject:
+    case Foo(x as y = 1):
+        pass
+    case Foo(x | y = 1):
+        pass
+    case Foo([x, y] = 1):
+        pass
+    case Foo({False: 0} = 1):
+        pass
+    case Foo(1=1):
+        pass
+    case Foo(Bar()=1):
+        pass
+    # Positional pattern cannot follow keyword pattern
+    # case Foo(x, y=1, z):
+    #     pass

crates/ruff_python_parser/resources/invalid/statements/match/invalid_lhs_or_rhs_pattern.py

@@ -0,0 +1,35 @@
+match invalid_lhs_pattern:
+    case Foo() + 1j:
+        pass
+    case x + 2j:
+        pass
+    case _ + 3j:
+        pass
+    case (1 | 2) + 4j:
+        pass
+    case [1, 2] + 5j:
+        pass
+    case {True: 1} + 6j:
+        pass
+    case 1j + 2j:
+        pass
+
+match invalid_rhs_pattern:
+    case 1 + Foo():
+        pass
+    case 2 + x:
+        pass
+    case 3 + _:
+        pass
+    case 4 + (1 | 2):
+        pass
+    case 5 + [1, 2]:
+        pass
+    case 6 + {True: 1}:
+        pass
+    case 1 + 2:
+        pass
+
+match invalid_lhs_rhs_pattern:
+    case Foo() + Bar():
+        pass

crates/ruff_python_parser/resources/invalid/statements/match/invalid_mapping_pattern.py

@@ -0,0 +1,17 @@
+# Starred expression is not allowed as a mapping pattern key
+match subject:
+    case {*key}:
+        pass
+    case {*key: 1}:
+        pass
+    case {*key 1}:
+        pass
+    case {*key, None: 1}:
+        pass
+
+# Double starred pattern cannot follow any other pattern
+match subject:
+    case {**rest, None: 1}:
+        pass
+    case {**rest1, **rest2, None: 1}:
+        pass

crates/ruff_python_parser/resources/valid/statement/match.py

@@ -164,6 +164,7 @@
         z = 0
 
 match x:
+    # F-strings aren't allowed as patterns but it's a soft syntax error in Python.
     case f"{y}":
         pass
 match {"test": 1}:
@@ -240,6 +241,8 @@
 
 # PatternMatchAs
 match x:
+    case a:
+        ...
     case a as b:
         ...
     case 1 | 2 as two:

crates/ruff_python_parser/src/parser/mod.rs

@@ -20,6 +20,7 @@ mod expression;
 mod helpers;
 mod pattern;
 mod progress;
+mod recovery;
 mod statement;
 #[cfg(test)]
 mod tests;
@@ -949,7 +950,11 @@ impl RecoveryContextKind {
             | RecoveryContextKind::TupleElements(_) => p.at_expr(),
             RecoveryContextKind::DictElements => p.at(TokenKind::DoubleStar) || p.at_expr(),
             RecoveryContextKind::SequenceMatchPattern(_) => p.at_pattern_start(),
-            RecoveryContextKind::MatchPatternMapping => p.at_mapping_pattern_start(),
+            RecoveryContextKind::MatchPatternMapping => {
+                // A star pattern is invalid as a mapping key and is here only for
+                // better error recovery.
+                p.at(TokenKind::Star) || p.at_mapping_pattern_start()
+            }
             RecoveryContextKind::MatchPatternClassArguments => p.at_pattern_start(),
             RecoveryContextKind::Arguments => p.at_expr(),
             RecoveryContextKind::DeleteTargets => p.at_expr(),