fix-21033

crates/ruff_linter/resources/test/fixtures/ruff/RUF027_0.py

@@ -84,3 +84,9 @@ def in_type_def():
 # https://github.com/astral-sh/ruff/issues/18860
 def fuzz_bug():
     c('{\t"i}')
+
+# Test case for backslash handling in f-strings
+# Should not trigger RUF027 for Python < 3.12 due to backslashes
+def backslash_test():
+    x = "test"
+    print("Hello {x}\\n")  # Should not trigger RUF027 for Python < 3.12

crates/ruff_linter/src/rules/ruff/rules/missing_fstring_syntax.rs

@@ -2,7 +2,7 @@ use memchr::memchr2_iter;
 use rustc_hash::FxHashSet;
 
 use ruff_macros::{ViolationMetadata, derive_message_formats};
-use ruff_python_ast as ast;
+use ruff_python_ast::{self as ast, PythonVersion};
 use ruff_python_literal::format::FormatSpec;
 use ruff_python_parser::parse_expression;
 use ruff_python_semantic::analyze::logging::is_logger_candidate;
@@ -116,7 +116,7 @@ pub(crate) fn missing_fstring_syntax(checker: &Checker, literal: &ast::StringLit
         return;
     }
 
-    if should_be_fstring(literal, checker.locator(), semantic) {
+    if should_be_fstring(literal, checker.locator(), semantic, checker) {
         checker
             .report_diagnostic(MissingFStringSyntax, literal.range())
             .set_fix(fix_fstring_syntax(literal.range()));
@@ -180,11 +180,18 @@ fn should_be_fstring(
     literal: &ast::StringLiteral,
     locator: &Locator,
     semantic: &SemanticModel,
+    checker: &Checker,
 ) -> bool {
     if !has_brackets(&literal.value) {
         return false;
     }
 
+    // Check if the string contains backslashes and target Python version is < 3.12
+    // F-strings with backslashes are only valid in Python 3.12+
+    if literal.value.contains('\\') && checker.target_version() < PythonVersion::PY312 {
+        return false;
+    }
+
     let fstring_expr = format!("f{}", locator.slice(literal));
     let Ok(parsed) = parse_expression(&fstring_expr) else {
         return false;

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF027_RUF027_0.py.snap

@@ -320,3 +320,19 @@ help: Add `f` prefix
 76 | # fstrings are never correct as type definitions
 77 | # so we should always skip those
 note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:92:11
+   |
+90 | def backslash_test():
+91 |     x = "test"
+92 |     print("Hello {x}\\n")  # Should not trigger RUF027 for Python < 3.12
+   |           ^^^^^^^^^^^^^^
+   |
+help: Add `f` prefix
+89 | # Should not trigger RUF027 for Python < 3.12 due to backslashes
+90 | def backslash_test():
+91 |     x = "test"
+   -     print("Hello {x}\\n")  # Should not trigger RUF027 for Python < 3.12
+92 +     print(f"Hello {x}\\n")  # Should not trigger RUF027 for Python < 3.12
+note: This is an unsafe fix and may change runtime behavior