Detect assignment expressions before Python 3.8

This PR is the first in a series derived from #16308, each of which add support
for detecting one version-related syntax error from #6591. This one should be
the largest because it also includes a couple of additional changes:
1. the `syntax_errors!` macro, which makes adding more variants a bit easier
2. the `Parser::add_unsupported_syntax_error` method

Otherwise I think the general structure will be the same for each syntax error:
* Detecting the error in the parser
* Inline parser tests for the new error
* New ruff CLI tests for the new error

Because of the second point here, this PR is currently stacked on #16357.

As noted above, there are new inline parser tests, as well as new ruff CLI
tests. Once #16379 is resolved, there should also be new mdtests for red-knot,
but this PR does not currently include those.

crates/ruff/tests/lint.rs

@@ -2628,6 +2628,47 @@ class A(Generic[T]):
     );
 }
 
+#[test]
+fn walrus_before_py38() {
+    // ok
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .args(STDIN_BASE_OPTIONS)
+        .args(["--stdin-filename", "test.py"])
+        .arg("--target-version=py38")
+        .arg("-")
+        .pass_stdin(r#"if x := 1: pass"#),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    test.py:1:10: E701 Multiple statements on one line (colon)
+    Found 1 error.
+
+    ----- stderr -----
+    "
+    );
+
+    // not ok on 3.7 with preview
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .args(STDIN_BASE_OPTIONS)
+        .args(["--stdin-filename", "test.py"])
+        .arg("--target-version=py37")
+        .arg("--preview")
+        .arg("-")
+        .pass_stdin(r#"if x := 1: pass"#),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    test.py:1:4: SyntaxError: Cannot use named assignment expression (`:=`) on Python 3.7 (syntax was added in Python 3.8)
+    test.py:1:10: E701 Multiple statements on one line (colon)
+    Found 2 errors.
+
+    ----- stderr -----
+    "
+    );
+}
+
 #[test]
 fn match_before_py310() {
     // ok on 3.10

crates/ruff_python_parser/resources/inline/err/walrus_before_py38.py

@@ -0,0 +1,2 @@
+# parse_options: { "target_version": "3.7" }
+if x := 1: ...

crates/ruff_python_parser/resources/inline/ok/walrus_after_py38.py

@@ -0,0 +1,2 @@
+# parse_options: { "target_version": "3.8" }
+if x := 1: ...

crates/ruff_python_parser/src/error.rs

@@ -479,6 +479,7 @@ macro_rules! syntax_errors {
 
 syntax_errors! {
     (MatchBeforePy310, PY310, "`match` statement"),
+    (WalrusBeforePy38, PY38, "named assignment expression (`:=`)"),
 }
 
 #[cfg(target_pointer_width = "64")]

crates/ruff_python_parser/src/parser/expression.rs

@@ -7,7 +7,7 @@ use rustc_hash::{FxBuildHasher, FxHashSet};
 use ruff_python_ast::name::Name;
 use ruff_python_ast::{
     self as ast, BoolOp, CmpOp, ConversionFlag, Expr, ExprContext, FStringElement, FStringElements,
-    IpyEscapeKind, Number, Operator, StringFlags, UnaryOp,
+    IpyEscapeKind, Number, Operator, PythonVersion, StringFlags, UnaryOp,
 };
 use ruff_text_size::{Ranged, TextLen, TextRange, TextSize};
 
@@ -16,7 +16,7 @@ use crate::parser::{helpers, FunctionKind, Parser};
 use crate::string::{parse_fstring_literal_element, parse_string_literal, StringType};
 use crate::token::{TokenKind, TokenValue};
 use crate::token_set::TokenSet;
-use crate::{FStringErrorType, Mode, ParseErrorType};
+use crate::{FStringErrorType, Mode, ParseErrorType, UnsupportedSyntaxErrorKind};
 
 use super::{FStringElementsKind, Parenthesized, RecoveryContextKind};
 
@@ -2161,10 +2161,24 @@ impl<'src> Parser<'src> {
 
         let value = self.parse_conditional_expression_or_higher();
 
+        let range = self.node_range(start);
+
+        // test_err walrus_before_py38
+        // # parse_options: { "target_version": "3.7" }
+        // if x := 1: ...
+
+        // test_ok walrus_after_py38
+        // # parse_options: { "target_version": "3.8" }
+        // if x := 1: ...
+
+        if self.options.target_version < PythonVersion::PY38 {
+            self.add_unsupported_syntax_error(UnsupportedSyntaxErrorKind::WalrusBeforePy38, range);
+        }
+
         ast::ExprNamed {
             target: Box::new(target),
             value: Box::new(value.expr),
-            range: self.node_range(start),
+            range,
         }
     }
 

crates/ruff_python_parser/tests/snapshots/invalid_syntax@walrus_before_py38.py.snap

@@ -0,0 +1,60 @@
+---
+source: crates/ruff_python_parser/tests/fixtures.rs
+input_file: crates/ruff_python_parser/resources/inline/err/walrus_before_py38.py
+---
+## AST
+
+```
+Module(
+    ModModule {
+        range: 0..60,
+        body: [
+            If(
+                StmtIf {
+                    range: 45..59,
+                    test: Named(
+                        ExprNamed {
+                            range: 48..54,
+                            target: Name(
+                                ExprName {
+                                    range: 48..49,
+                                    id: Name("x"),
+                                    ctx: Store,
+                                },
+                            ),
+                            value: NumberLiteral(
+                                ExprNumberLiteral {
+                                    range: 53..54,
+                                    value: Int(
+                                        1,
+                                    ),
+                                },
+                            ),
+                        },
+                    ),
+                    body: [
+                        Expr(
+                            StmtExpr {
+                                range: 56..59,
+                                value: EllipsisLiteral(
+                                    ExprEllipsisLiteral {
+                                        range: 56..59,
+                                    },
+                                ),
+                            },
+                        ),
+                    ],
+                    elif_else_clauses: [],
+                },
+            ),
+        ],
+    },
+)
+```
+## Unsupported Syntax Errors
+
+  |
+1 | # parse_options: { "target_version": "3.7" }
+2 | if x := 1: ...
+  |    ^^^^^^ Syntax Error: Cannot use named assignment expression (`:=`) on Python 3.7 (syntax was added in Python 3.8)
+  |

crates/ruff_python_parser/tests/snapshots/valid_syntax@walrus_after_py38.py.snap

@@ -0,0 +1,53 @@
+---
+source: crates/ruff_python_parser/tests/fixtures.rs
+input_file: crates/ruff_python_parser/resources/inline/ok/walrus_after_py38.py
+---
+## AST
+
+```
+Module(
+    ModModule {
+        range: 0..60,
+        body: [
+            If(
+                StmtIf {
+                    range: 45..59,
+                    test: Named(
+                        ExprNamed {
+                            range: 48..54,
+                            target: Name(
+                                ExprName {
+                                    range: 48..49,
+                                    id: Name("x"),
+                                    ctx: Store,
+                                },
+                            ),
+                            value: NumberLiteral(
+                                ExprNumberLiteral {
+                                    range: 53..54,
+                                    value: Int(
+                                        1,
+                                    ),
+                                },
+                            ),
+                        },
+                    ),
+                    body: [
+                        Expr(
+                            StmtExpr {
+                                range: 56..59,
+                                value: EllipsisLiteral(
+                                    ExprEllipsisLiteral {
+                                        range: 56..59,
+                                    },
+                                ),
+                            },
+                        ),
+                    ],
+                    elif_else_clauses: [],
+                },
+            ),
+        ],
+    },
+)
+```