Avoid caching files with unsupported syntax errors (#16425)

Author: ntBre

crates/ruff/tests/lint.rs

@@ -2701,3 +2701,45 @@ match 2:
     "
     );
 }
+
+/// Regression test for <https://github.com/astral-sh/ruff/issues/16417>
+#[test]
+fn cache_syntax_errors() -> Result<()> {
+    let tempdir = TempDir::new()?;
+    fs::write(tempdir.path().join("main.py"), "match 2:\n    case 1: ...")?;
+
+    let mut cmd = Command::new(get_cargo_bin(BIN_NAME));
+    // inline STDIN_BASE_OPTIONS to remove --no-cache
+    cmd.args(["check", "--output-format", "concise"])
+        .arg("--target-version=py39")
+        .arg("--preview")
+        .arg("--quiet") // suppress `debug build without --no-cache` warnings
+        .current_dir(&tempdir);
+
+    assert_cmd_snapshot!(
+        cmd,
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    main.py:1:1: SyntaxError: Cannot use `match` statement on Python 3.9 (syntax was added in Python 3.10)
+
+    ----- stderr -----
+    "
+    );
+
+    // this should *not* be cached, like normal parse errors
+    assert_cmd_snapshot!(
+        cmd,
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    main.py:1:1: SyntaxError: Cannot use `match` statement on Python 3.9 (syntax was added in Python 3.10)
+
+    ----- stderr -----
+    "
+    );
+
+    Ok(())
+}

crates/ruff_linter/src/linter.rs

@@ -445,7 +445,7 @@ pub fn lint_only(
             &locator,
             &directives,
         ),
-        has_syntax_error: !parsed.is_valid(),
+        has_syntax_error: !parsed.is_valid() || !parsed.unsupported_syntax_errors().is_empty(),
     }
 }
 
@@ -546,7 +546,7 @@ pub fn lint_fix<'a>(
         );
 
         if iterations == 0 {
-            is_valid_syntax = parsed.is_valid();
+            is_valid_syntax = parsed.is_valid() && parsed.unsupported_syntax_errors().is_empty();
         } else {
             // If the source code was parseable on the first pass, but is no
             // longer parseable on a subsequent pass, then we've introduced a