[syntax-errors] Assignment expressions before Python 3.8 (#16383)

## Summary
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 the addition of 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

## Test Plan
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 +2628,45 @@ 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#"(x := 1)"#),
+        @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- 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#"(x := 1)"#),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    test.py:1:2: SyntaxError: Cannot use named assignment expression (`:=`) on Python 3.7 (syntax was added in Python 3.8)
+    Found 1 error.
+
+    ----- stderr -----
+    "
+    );
+}
+
 #[test]
 fn match_before_py310() {
     // ok on 3.10

crates/ruff_python_parser/resources/inline/err/walrus_py37.py

@@ -0,0 +1,2 @@
+# parse_options: { "target-version": "3.7" }
+(x := 1)

crates/ruff_python_parser/resources/inline/ok/walrus_py38.py

@@ -0,0 +1,2 @@
+# parse_options: { "target-version": "3.8" }
+(x := 1)

crates/ruff_python_parser/src/error.rs

@@ -444,31 +444,35 @@ pub struct UnsupportedSyntaxError {
     pub target_version: PythonVersion,
 }
 
-impl UnsupportedSyntaxError {
-    /// The earliest allowed version for the syntax associated with this error.
-    pub const fn minimum_version(&self) -> PythonVersion {
-        match self.kind {
-            UnsupportedSyntaxErrorKind::MatchBeforePy310 => PythonVersion::PY310,
-        }
-    }
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum UnsupportedSyntaxErrorKind {
+    Match,
+    Walrus,
 }
 
 impl Display for UnsupportedSyntaxError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self.kind {
-            UnsupportedSyntaxErrorKind::MatchBeforePy310 => write!(
-                f,
-                "Cannot use `match` statement on Python {} (syntax was added in Python {})",
-                self.target_version,
-                self.minimum_version(),
-            ),
-        }
+        let kind = match self.kind {
+            UnsupportedSyntaxErrorKind::Match => "`match` statement",
+            UnsupportedSyntaxErrorKind::Walrus => "named assignment expression (`:=`)",
+        };
+        write!(
+            f,
+            "Cannot use {kind} on Python {} (syntax was added in Python {})",
+            self.target_version,
+            self.minimum_version(),
+        )
     }
 }
 
-#[derive(Debug, PartialEq, Clone, Copy)]
-pub enum UnsupportedSyntaxErrorKind {
-    MatchBeforePy310,
+impl UnsupportedSyntaxError {
+    /// The earliest allowed version for the syntax associated with this error.
+    pub const fn minimum_version(&self) -> PythonVersion {
+        match self.kind {
+            UnsupportedSyntaxErrorKind::Match => PythonVersion::PY310,
+            UnsupportedSyntaxErrorKind::Walrus => PythonVersion::PY38,
+        }
+    }
 }
 
 #[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_py37
+        // # parse_options: { "target-version": "3.7" }
+        // (x := 1)
+
+        // test_ok walrus_py38
+        // # parse_options: { "target-version": "3.8" }
+        // (x := 1)
+
+        if self.options.target_version < PythonVersion::PY38 {
+            self.add_unsupported_syntax_error(UnsupportedSyntaxErrorKind::Walrus, range);
+        }
+
         ast::ExprNamed {
             target: Box::new(target),
             value: Box::new(value.expr),
-            range: self.node_range(start),
+            range,
         }
     }
 

crates/ruff_python_parser/src/parser/mod.rs

@@ -11,7 +11,7 @@ use crate::parser::progress::{ParserProgress, TokenId};
 use crate::token::TokenValue;
 use crate::token_set::TokenSet;
 use crate::token_source::{TokenSource, TokenSourceCheckpoint};
-use crate::{Mode, ParseError, ParseErrorType, TokenKind};
+use crate::{Mode, ParseError, ParseErrorType, TokenKind, UnsupportedSyntaxErrorKind};
 use crate::{Parsed, Tokens};
 
 pub use crate::parser::options::ParseOptions;
@@ -438,6 +438,16 @@ impl<'src> Parser<'src> {
         inner(&mut self.errors, error, ranged.range());
     }
 
+    /// Add an [`UnsupportedSyntaxError`] with the given [`UnsupportedSyntaxErrorKind`] and
+    /// [`TextRange`].
+    fn add_unsupported_syntax_error(&mut self, kind: UnsupportedSyntaxErrorKind, range: TextRange) {
+        self.unsupported_syntax_errors.push(UnsupportedSyntaxError {
+            kind,
+            range,
+            target_version: self.options.target_version,
+        });
+    }
+
     /// Returns `true` if the current token is of the given kind.
     fn at(&self, kind: TokenKind) -> bool {
         self.current_token_kind() == kind

crates/ruff_python_parser/src/parser/statement.rs

@@ -17,7 +17,7 @@ use crate::parser::{
 };
 use crate::token::{TokenKind, TokenValue};
 use crate::token_set::TokenSet;
-use crate::{Mode, ParseErrorType, UnsupportedSyntaxError, UnsupportedSyntaxErrorKind};
+use crate::{Mode, ParseErrorType, UnsupportedSyntaxErrorKind};
 
 use super::expression::ExpressionContext;
 use super::Parenthesized;
@@ -2278,11 +2278,7 @@ impl<'src> Parser<'src> {
         //         pass
 
         if self.options.target_version < PythonVersion::PY310 {
-            self.unsupported_syntax_errors.push(UnsupportedSyntaxError {
-                kind: UnsupportedSyntaxErrorKind::MatchBeforePy310,
-                range: match_range,
-                target_version: self.options.target_version,
-            });
+            self.add_unsupported_syntax_error(UnsupportedSyntaxErrorKind::Match, match_range);
         }
 
         ast::StmtMatch {

crates/ruff_python_parser/tests/snapshots/invalid_syntax@walrus_py37.py.snap

@@ -0,0 +1,47 @@
+---
+source: crates/ruff_python_parser/tests/fixtures.rs
+input_file: crates/ruff_python_parser/resources/inline/err/walrus_py37.py
+---
+## AST
+
+```
+Module(
+    ModModule {
+        range: 0..54,
+        body: [
+            Expr(
+                StmtExpr {
+                    range: 45..53,
+                    value: Named(
+                        ExprNamed {
+                            range: 46..52,
+                            target: Name(
+                                ExprName {
+                                    range: 46..47,
+                                    id: Name("x"),
+                                    ctx: Store,
+                                },
+                            ),
+                            value: NumberLiteral(
+                                ExprNumberLiteral {
+                                    range: 51..52,
+                                    value: Int(
+                                        1,
+                                    ),
+                                },
+                            ),
+                        },
+                    ),
+                },
+            ),
+        ],
+    },
+)
+```
+## Unsupported Syntax Errors
+
+  |
+1 | # parse_options: { "target-version": "3.7" }
+2 | (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_py38.py.snap

@@ -0,0 +1,40 @@
+---
+source: crates/ruff_python_parser/tests/fixtures.rs
+input_file: crates/ruff_python_parser/resources/inline/ok/walrus_py38.py
+---
+## AST
+
+```
+Module(
+    ModModule {
+        range: 0..54,
+        body: [
+            Expr(
+                StmtExpr {
+                    range: 45..53,
+                    value: Named(
+                        ExprNamed {
+                            range: 46..52,
+                            target: Name(
+                                ExprName {
+                                    range: 46..47,
+                                    id: Name("x"),
+                                    ctx: Store,
+                                },
+                            ),
+                            value: NumberLiteral(
+                                ExprNumberLiteral {
+                                    range: 51..52,
+                                    value: Int(
+                                        1,
+                                    ),
+                                },
+                            ),
+                        },
+                    ),
+                },
+            ),
+        ],
+    },
+)
+```