Fix RUF027 handling for f-string backslashes in Python 3.11

Updates the MissingFStringSyntax rule to correctly skip diagnostics for f-string interpolations containing backslashes when targeting Python 3.11, matching Python's behavior. Adds a dedicated test and snapshot to verify the change.

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

@@ -126,6 +126,21 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn missing_fstring_syntax_backslash_py311() -> Result<()> {
+        let diagnostics = test_path(
+            Path::new("ruff/RUF027_0.py"),
+            &LinterSettings {
+                unresolved_target_version: PythonVersion::PY311.into(),
+                ..LinterSettings::for_rule(Rule::MissingFStringSyntax)
+            },
+        )?;
+        // With Python 3.11, backslashes in interpolations should NOT trigger RUF027
+        // (only the backslash_test function should be skipped)
+        assert_diagnostics!(diagnostics);
+        Ok(())
+    }
+
     #[test]
     fn prefer_parentheses_getitem_tuple() -> Result<()> {
         let diagnostics = test_path(

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

@@ -116,7 +116,12 @@ pub(crate) fn missing_fstring_syntax(checker: &Checker, literal: &ast::StringLit
         return;
     }
 
-    if should_be_fstring(literal, checker.locator(), semantic, checker) {
+    if should_be_fstring(
+        literal,
+        checker.locator(),
+        semantic,
+        checker.target_version(),
+    ) {
         checker
             .report_diagnostic(MissingFStringSyntax, literal.range())
             .set_fix(fix_fstring_syntax(literal.range()));
@@ -180,7 +185,7 @@ fn should_be_fstring(
     literal: &ast::StringLiteral,
     locator: &Locator,
     semantic: &SemanticModel,
-    checker: &Checker,
+    target_version: PythonVersion,
 ) -> bool {
     if !has_brackets(&literal.value) {
         return false;
@@ -220,8 +225,7 @@ fn should_be_fstring(
             // Check if the interpolation expression contains backslashes
             // F-strings with backslashes in interpolations are only valid in Python 3.12+
             let interpolation_text = &fstring_expr[element.range()];
-            if interpolation_text.contains('\\') && checker.target_version() < PythonVersion::PY312
+            if interpolation_text.contains('\\') && target_version < PythonVersion::PY312 {
-            {
                 return false;
             }
 

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__missing_fstring_syntax_backslash_py311.snap

@@ -0,0 +1,300 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+RUF027 [*] Possible f-string without an `f` prefix
+ --> RUF027_0.py:5:7
+  |
+3 | "always ignore this: {val}"
+4 |
+5 | print("but don't ignore this: {val}")  # RUF027
+  |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+help: Add `f` prefix
+2 | 
+3 | "always ignore this: {val}"
+4 | 
+  - print("but don't ignore this: {val}")  # RUF027
+5 + print(f"but don't ignore this: {val}")  # RUF027
+6 | 
+7 | 
+8 | def simple_cases():
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:10:9
+   |
+ 8 | def simple_cases():
+ 9 |     a = 4
+10 |     b = "{a}"  # RUF027
+   |         ^^^^^
+11 |     c = "{a} {b} f'{val}' "  # RUF027
+   |
+help: Add `f` prefix
+7  | 
+8  | def simple_cases():
+9  |     a = 4
+   -     b = "{a}"  # RUF027
+10 +     b = f"{a}"  # RUF027
+11 |     c = "{a} {b} f'{val}' "  # RUF027
+12 | 
+13 | 
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:11:9
+   |
+ 9 |     a = 4
+10 |     b = "{a}"  # RUF027
+11 |     c = "{a} {b} f'{val}' "  # RUF027
+   |         ^^^^^^^^^^^^^^^^^^^
+   |
+help: Add `f` prefix
+8  | def simple_cases():
+9  |     a = 4
+10 |     b = "{a}"  # RUF027
+   -     c = "{a} {b} f'{val}' "  # RUF027
+11 +     c = f"{a} {b} f'{val}' "  # RUF027
+12 | 
+13 | 
+14 | def escaped_string():
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:21:9
+   |
+19 | def raw_string():
+20 |     a = 4
+21 |     b = r"raw string with formatting: {a}"  # RUF027
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+22 |     c = r"raw string with \backslashes\ and \"escaped quotes\": {a}"  # RUF027
+   |
+help: Add `f` prefix
+18 | 
+19 | def raw_string():
+20 |     a = 4
+   -     b = r"raw string with formatting: {a}"  # RUF027
+21 +     b = fr"raw string with formatting: {a}"  # RUF027
+22 |     c = r"raw string with \backslashes\ and \"escaped quotes\": {a}"  # RUF027
+23 | 
+24 | 
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:22:9
+   |
+20 |     a = 4
+21 |     b = r"raw string with formatting: {a}"  # RUF027
+22 |     c = r"raw string with \backslashes\ and \"escaped quotes\": {a}"  # RUF027
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: Add `f` prefix
+19 | def raw_string():
+20 |     a = 4
+21 |     b = r"raw string with formatting: {a}"  # RUF027
+   -     c = r"raw string with \backslashes\ and \"escaped quotes\": {a}"  # RUF027
+22 +     c = fr"raw string with \backslashes\ and \"escaped quotes\": {a}"  # RUF027
+23 | 
+24 | 
+25 | def print_name(name: str):
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:27:11
+   |
+25 | def print_name(name: str):
+26 |     a = 4
+27 |     print("Hello, {name}!")  # RUF027
+   |           ^^^^^^^^^^^^^^^^
+28 |     print("The test value we're using today is {a}")  # RUF027
+   |
+help: Add `f` prefix
+24 | 
+25 | def print_name(name: str):
+26 |     a = 4
+   -     print("Hello, {name}!")  # RUF027
+27 +     print(f"Hello, {name}!")  # RUF027
+28 |     print("The test value we're using today is {a}")  # RUF027
+29 | 
+30 | 
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:28:11
+   |
+26 |     a = 4
+27 |     print("Hello, {name}!")  # RUF027
+28 |     print("The test value we're using today is {a}")  # RUF027
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: Add `f` prefix
+25 | def print_name(name: str):
+26 |     a = 4
+27 |     print("Hello, {name}!")  # RUF027
+   -     print("The test value we're using today is {a}")  # RUF027
+28 +     print(f"The test value we're using today is {a}")  # RUF027
+29 | 
+30 | 
+31 | def nested_funcs():
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:33:33
+   |
+31 | def nested_funcs():
+32 |     a = 4
+33 |     print(do_nothing(do_nothing("{a}")))  # RUF027
+   |                                 ^^^^^
+   |
+help: Add `f` prefix
+30 | 
+31 | def nested_funcs():
+32 |     a = 4
+   -     print(do_nothing(do_nothing("{a}")))  # RUF027
+33 +     print(do_nothing(do_nothing(f"{a}")))  # RUF027
+34 | 
+35 | 
+36 | def tripled_quoted():
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:39:19
+   |
+37 |     a = 4
+38 |     c = a
+39 |     single_line = """ {a} """  # RUF027
+   |                   ^^^^^^^^^^^
+40 |     # RUF027
+41 |     multi_line = a = """b { # comment
+   |
+help: Add `f` prefix
+36 | def tripled_quoted():
+37 |     a = 4
+38 |     c = a
+   -     single_line = """ {a} """  # RUF027
+39 +     single_line = f""" {a} """  # RUF027
+40 |     # RUF027
+41 |     multi_line = a = """b { # comment
+42 |     c}  d
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:41:22
+   |
+39 |       single_line = """ {a} """  # RUF027
+40 |       # RUF027
+41 |       multi_line = a = """b { # comment
+   |  ______________________^
+42 | |     c}  d
+43 | |     """
+   | |_______^
+   |
+help: Add `f` prefix
+38 |     c = a
+39 |     single_line = """ {a} """  # RUF027
+40 |     # RUF027
+   -     multi_line = a = """b { # comment
+41 +     multi_line = a = f"""b { # comment
+42 |     c}  d
+43 |     """
+44 | 
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:56:9
+   |
+54 | def implicit_concat():
+55 |     a = 4
+56 |     b = "{a}" "+" "{b}" r" \\ "  # RUF027 for the first part only
+   |         ^^^^^
+57 |     print(f"{a}" "{a}" f"{b}")  # RUF027
+   |
+help: Add `f` prefix
+53 | 
+54 | def implicit_concat():
+55 |     a = 4
+   -     b = "{a}" "+" "{b}" r" \\ "  # RUF027 for the first part only
+56 +     b = f"{a}" "+" "{b}" r" \\ "  # RUF027 for the first part only
+57 |     print(f"{a}" "{a}" f"{b}")  # RUF027
+58 | 
+59 | 
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:57:18
+   |
+55 |     a = 4
+56 |     b = "{a}" "+" "{b}" r" \\ "  # RUF027 for the first part only
+57 |     print(f"{a}" "{a}" f"{b}")  # RUF027
+   |                  ^^^^^
+   |
+help: Add `f` prefix
+54 | def implicit_concat():
+55 |     a = 4
+56 |     b = "{a}" "+" "{b}" r" \\ "  # RUF027 for the first part only
+   -     print(f"{a}" "{a}" f"{b}")  # RUF027
+57 +     print(f"{a}" f"{a}" f"{b}")  # RUF027
+58 | 
+59 | 
+60 | def escaped_chars():
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:62:9
+   |
+60 | def escaped_chars():
+61 |     a = 4
+62 |     b = "\"not escaped:\" '{a}' \"escaped:\": '{{c}}'"  # RUF027
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: Add `f` prefix
+59 | 
+60 | def escaped_chars():
+61 |     a = 4
+   -     b = "\"not escaped:\" '{a}' \"escaped:\": '{{c}}'"  # RUF027
+62 +     b = f"\"not escaped:\" '{a}' \"escaped:\": '{{c}}'"  # RUF027
+63 | 
+64 | 
+65 | def method_calls():
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:70:18
+   |
+68 |     first = "Wendy"
+69 |     last = "Appleseed"
+70 |     value.method("{first} {last}")  # RUF027
+   |                  ^^^^^^^^^^^^^^^^
+71 |
+72 | def format_specifiers():
+   |
+help: Add `f` prefix
+67 |     value.method = print_name
+68 |     first = "Wendy"
+69 |     last = "Appleseed"
+   -     value.method("{first} {last}")  # RUF027
+70 +     value.method(f"{first} {last}")  # RUF027
+71 | 
+72 | def format_specifiers():
+73 |     a = 4
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+  --> RUF027_0.py:74:9
+   |
+72 | def format_specifiers():
+73 |     a = 4
+74 |     b = "{a:b} {a:^5}"
+   |         ^^^^^^^^^^^^^^
+75 |
+76 | # fstrings are never correct as type definitions
+   |
+help: Add `f` prefix
+71 | 
+72 | def format_specifiers():
+73 |     a = 4
+   -     b = "{a:b} {a:^5}"
+74 +     b = f"{a:b} {a:^5}"
+75 | 
+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