Modify parsing of raise with cause when exception is absent (#15049)

When confronted with `raise from exc` the parser will now create a
`StmtRaise` that has `None` for the exception and `exc` for the cause.

Before, the parser created a `StmtRaise` with `from` for the exception,
no cause, and a spurious expression `exc` afterwards.

crates/ruff_python_parser/resources/inline/err/raise_stmt_from_without_exc.py

@@ -0,0 +1,2 @@
+raise from exc
+raise from None

crates/ruff_python_parser/src/parser/statement.rs

@@ -411,31 +411,44 @@ impl<'src> Parser<'src> {
         let start = self.node_start();
         self.bump(TokenKind::Raise);
 
-        let exc = if self.at(TokenKind::Newline) {
-            None
-        } else {
-            // test_err raise_stmt_invalid_exc
-            // raise *x
-            // raise yield x
-            // raise x := 1
-            let exc = self.parse_expression_list(ExpressionContext::default());
-
-            if let Some(ast::ExprTuple {
-                parenthesized: false,
-                ..
-            }) = exc.as_tuple_expr()
-            {
-                // test_err raise_stmt_unparenthesized_tuple_exc
-                // raise x,
-                // raise x, y
-                // raise x, y from z
-                self.add_error(ParseErrorType::UnparenthesizedTupleExpression, &exc);
+        let exc = match self.current_token_kind() {
+            TokenKind::Newline => None,
+            TokenKind::From => {
+                // test_err raise_stmt_from_without_exc
+                // raise from exc
+                // raise from None
+                self.add_error(
+                    ParseErrorType::OtherError(
+                        "Exception missing in `raise` statement with cause".to_string(),
+                    ),
+                    self.current_token_range(),
+                );
+                None
             }
+            _ => {
+                // test_err raise_stmt_invalid_exc
+                // raise *x
+                // raise yield x
+                // raise x := 1
+                let exc = self.parse_expression_list(ExpressionContext::default());
 
-            Some(Box::new(exc.expr))
+                if let Some(ast::ExprTuple {
+                    parenthesized: false,
+                    ..
+                }) = exc.as_tuple_expr()
+                {
+                    // test_err raise_stmt_unparenthesized_tuple_exc
+                    // raise x,
+                    // raise x, y
+                    // raise x, y from z
+                    self.add_error(ParseErrorType::UnparenthesizedTupleExpression, &exc);
+                }
+
+                Some(Box::new(exc.expr))
+            }
         };
 
-        let cause = (exc.is_some() && self.eat(TokenKind::From)).then(|| {
+        let cause = self.eat(TokenKind::From).then(|| {
             // test_err raise_stmt_invalid_cause
             // raise x from *y
             // raise x from yield y

crates/ruff_python_parser/tests/snapshots/invalid_syntax@raise_stmt_from_without_exc.py.snap

@@ -0,0 +1,57 @@
+---
+source: crates/ruff_python_parser/tests/fixtures.rs
+input_file: crates/ruff_python_parser/resources/inline/err/raise_stmt_from_without_exc.py
+---
+## AST
+
+```
+Module(
+    ModModule {
+        range: 0..31,
+        body: [
+            Raise(
+                StmtRaise {
+                    range: 0..14,
+                    exc: None,
+                    cause: Some(
+                        Name(
+                            ExprName {
+                                range: 11..14,
+                                id: Name("exc"),
+                                ctx: Load,
+                            },
+                        ),
+                    ),
+                },
+            ),
+            Raise(
+                StmtRaise {
+                    range: 15..30,
+                    exc: None,
+                    cause: Some(
+                        NoneLiteral(
+                            ExprNoneLiteral {
+                                range: 26..30,
+                            },
+                        ),
+                    ),
+                },
+            ),
+        ],
+    },
+)
+```
+## Errors
+
+  |
+1 | raise from exc
+  |       ^^^^ Syntax Error: Exception missing in `raise` statement with cause
+2 | raise from None
+  |
+
+
+  |
+1 | raise from exc
+2 | raise from None
+  |       ^^^^ Syntax Error: Exception missing in `raise` statement with cause
+  |