[ruff] Suppress diagnostic for invalid f-strings before Python 3.12 (RUF027)

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

@@ -90,3 +90,31 @@ def fuzz_bug():
 def backslash_test():
     x = "test"
     print("Hello {'\\n'}{x}")  # Should not trigger RUF027 for Python < 3.12
+
+# Test case for comment handling in f-string interpolations
+# Should not trigger RUF027 for Python < 3.12 due to comments in interpolations
+def comment_test():
+    x = "!"
+    print("""{x  # }
+}""")
+
+# Test case for `#` inside a nested string literal in interpolation
+# `#` inside a string is NOT a comment — should trigger RUF027 even on Python < 3.12
+def hash_in_string_test():
+    x = "world"
+    print("Hello {'#'}{x}")   # RUF027: `#` is inside a string, not a comment
+    print("Hello {\"#\"}{x}") # RUF027: same, double-quoted
+
+# Test case for `#` in format spec (e.g., `{1:#x}`)
+# `#` in a format spec is NOT a comment — should trigger RUF027 even on Python < 3.12
+def hash_in_format_spec_test():
+    n = 255
+    print("Hex: {n:#x}")   # RUF027: `#` is in format spec, not a comment
+    print("Oct: {n:#o}")   # RUF027: same
+
+# Test case for `#` in nested interpolation inside format spec (e.g., `{1:{x #}}`)
+# The `#` is a comment inside a nested interpolation — should NOT trigger RUF027 on Python < 3.12
+def hash_in_nested_format_spec_test():
+    x = 5
+    print("""{1:{x #}}
+}""")  # Should not trigger RUF027 for Python < 3.12

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

@@ -4,7 +4,7 @@ use rustc_hash::FxHashSet;
 use ruff_macros::{ViolationMetadata, derive_message_formats};
 use ruff_python_ast::{self as ast, PythonVersion};
 use ruff_python_literal::format::FormatSpec;
-use ruff_python_parser::parse_expression;
+use ruff_python_parser::{UnsupportedSyntaxErrorKind, parse_expression};
 use ruff_python_semantic::analyze::logging::is_logger_candidate;
 use ruff_python_semantic::{Modules, SemanticModel, TypingOnlyBindingsStatus};
 use ruff_text_size::{Ranged, TextRange};
@@ -200,6 +200,18 @@ fn should_be_fstring(
         return false;
     };
 
+    // For Python < 3.12, reject if the parser detected any PEP 701 f-string
+    // features.
+    if target_version < PythonVersion::PY312 {
+        let has_pep701 = parsed
+            .unsupported_syntax_errors()
+            .iter()
+            .any(|e| matches!(e.kind, UnsupportedSyntaxErrorKind::Pep701FString(_)));
+        if has_pep701 {
+            return false;
+        }
+    }
+
     // Note: Range offsets for `value` are based on `fstring_expr`
     let ast::Expr::FString(ast::ExprFString { value, .. }) = parsed.expr() else {
         return false;
@@ -226,13 +238,6 @@ fn should_be_fstring(
     for f_string in value.f_strings() {
         let mut has_name = false;
         for element in f_string.elements.interpolations() {
-            // 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 target_version < PythonVersion::PY312 && interpolation_text.contains('\\') {
-                return false;
-            }
-
             if let ast::Expr::Name(ast::ExprName { id, .. }) = element.expression.as_ref() {
                 if arg_names.contains(id) {
                     return false;

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

@@ -328,11 +328,100 @@ RUF027 [*] Possible f-string without an `f` prefix
 91 |     x = "test"
 92 |     print("Hello {'\\n'}{x}")  # Should not trigger RUF027 for Python < 3.12
    |           ^^^^^^^^^^^^^^^^^^
+93 |
+94 | # Test case for comment handling in f-string interpolations
    |
 help: Add `f` prefix
 89 | # Should not trigger RUF027 for Python < 3.12 due to backslashes in interpolations
 90 | def backslash_test():
 91 |     x = "test"
    -     print("Hello {'\\n'}{x}")  # Should not trigger RUF027 for Python < 3.12
 92 +     print(f"Hello {'\\n'}{x}")  # Should not trigger RUF027 for Python < 3.12
+93 | 
+94 | # Test case for comment handling in f-string interpolations
+95 | # Should not trigger RUF027 for Python < 3.12 due to comments in interpolations
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+   --> RUF027_0.py:98:11
+    |
+ 96 |   def comment_test():
+ 97 |       x = "!"
+ 98 |       print("""{x  # }
+    |  ___________^
+ 99 | | }""")
+    | |____^
+100 |
+101 |   # Test case for `#` inside a nested string literal in interpolation
+    |
+help: Add `f` prefix
+95  | # Should not trigger RUF027 for Python < 3.12 due to comments in interpolations
+96  | def comment_test():
+97  |     x = "!"
+    -     print("""{x  # }
+98  +     print(f"""{x  # }
+99  | }""")
+100 | 
+101 | # Test case for `#` inside a nested string literal in interpolation
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+   --> RUF027_0.py:105:11
+    |
+103 | def hash_in_string_test():
+104 |     x = "world"
+105 |     print("Hello {'#'}{x}")   # RUF027: `#` is inside a string, not a comment
+    |           ^^^^^^^^^^^^^^^^
+106 |     print("Hello {\"#\"}{x}") # RUF027: same, double-quoted
+    |
+help: Add `f` prefix
+102 | # `#` inside a string is NOT a comment — should trigger RUF027 even on Python < 3.12
+103 | def hash_in_string_test():
+104 |     x = "world"
+    -     print("Hello {'#'}{x}")   # RUF027: `#` is inside a string, not a comment
+105 +     print(f"Hello {'#'}{x}")   # RUF027: `#` is inside a string, not a comment
+106 |     print("Hello {\"#\"}{x}") # RUF027: same, double-quoted
+107 | 
+108 | # Test case for `#` in format spec (e.g., `{1:#x}`)
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+   --> RUF027_0.py:112:11
+    |
+110 | def hash_in_format_spec_test():
+111 |     n = 255
+112 |     print("Hex: {n:#x}")   # RUF027: `#` is in format spec, not a comment
+    |           ^^^^^^^^^^^^^
+113 |     print("Oct: {n:#o}")   # RUF027: same
+    |
+help: Add `f` prefix
+109 | # `#` in a format spec is NOT a comment — should trigger RUF027 even on Python < 3.12
+110 | def hash_in_format_spec_test():
+111 |     n = 255
+    -     print("Hex: {n:#x}")   # RUF027: `#` is in format spec, not a comment
+112 +     print(f"Hex: {n:#x}")   # RUF027: `#` is in format spec, not a comment
+113 |     print("Oct: {n:#o}")   # RUF027: same
+114 | 
+115 | # Test case for `#` in nested interpolation inside format spec (e.g., `{1:{x #}}`)
+note: This is an unsafe fix and may change runtime behavior
+
+RUF027 [*] Possible f-string without an `f` prefix
+   --> RUF027_0.py:113:11
+    |
+111 |     n = 255
+112 |     print("Hex: {n:#x}")   # RUF027: `#` is in format spec, not a comment
+113 |     print("Oct: {n:#o}")   # RUF027: same
+    |           ^^^^^^^^^^^^^
+114 |
+115 | # Test case for `#` in nested interpolation inside format spec (e.g., `{1:{x #}}`)
+    |
+help: Add `f` prefix
+110 | def hash_in_format_spec_test():
+111 |     n = 255
+112 |     print("Hex: {n:#x}")   # RUF027: `#` is in format spec, not a comment
+    -     print("Oct: {n:#o}")   # RUF027: same
+113 +     print(f"Oct: {n:#o}")   # RUF027: same
+114 | 
+115 | # Test case for `#` in nested interpolation inside format spec (e.g., `{1:{x #}}`)
+116 | # The `#` is a comment inside a nested interpolation — should NOT trigger RUF027 on Python < 3.12
 note: This is an unsafe fix and may change runtime behavior

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

@@ -6,10 +6,33 @@ source: crates/ruff_linter/src/rules/ruff/mod.rs
 +linter.unresolved_target_version = 3.11
 
 --- Summary ---
-Removed: 2
+Removed: 4
 Added: 0
 
 --- Removed ---
+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:49:9
    |
@@ -40,11 +63,40 @@ RUF027 [*] Possible f-string without an `f` prefix
 91 |     x = "test"
 92 |     print("Hello {'\\n'}{x}")  # Should not trigger RUF027 for Python < 3.12
    |           ^^^^^^^^^^^^^^^^^^
+93 |
+94 | # Test case for comment handling in f-string interpolations
    |
 help: Add `f` prefix
 89 | # Should not trigger RUF027 for Python < 3.12 due to backslashes in interpolations
 90 | def backslash_test():
 91 |     x = "test"
    -     print("Hello {'\\n'}{x}")  # Should not trigger RUF027 for Python < 3.12
 92 +     print(f"Hello {'\\n'}{x}")  # Should not trigger RUF027 for Python < 3.12
+93 | 
+94 | # Test case for comment handling in f-string interpolations
+95 | # Should not trigger RUF027 for Python < 3.12 due to comments in interpolations
+note: This is an unsafe fix and may change runtime behavior
+
+
+RUF027 [*] Possible f-string without an `f` prefix
+   --> RUF027_0.py:98:11
+    |
+ 96 |   def comment_test():
+ 97 |       x = "!"
+ 98 |       print("""{x  # }
+    |  ___________^
+ 99 | | }""")
+    | |____^
+100 |
+101 |   # Test case for `#` inside a nested string literal in interpolation
+    |
+help: Add `f` prefix
+95  | # Should not trigger RUF027 for Python < 3.12 due to comments in interpolations
+96  | def comment_test():
+97  |     x = "!"
+    -     print("""{x  # }
+98  +     print(f"""{x  # }
+99  | }""")
+100 | 
+101 | # Test case for `#` inside a nested string literal in interpolation
 note: This is an unsafe fix and may change runtime behavior