Move f-string identification into rule module (#3838)

crates/ruff/src/rules/pyflakes/rules/f_string_missing_placeholders.rs

@@ -1,8 +1,9 @@
-use rustpython_parser::ast::{Expr, ExprKind};
+use rustpython_parser::ast::{Expr, ExprKind, Location};
+use rustpython_parser::{lexer, Mode, StringKind, Tok};
 
 use ruff_diagnostics::{AlwaysAutofixableViolation, Diagnostic, Edit};
 use ruff_macros::{derive_message_formats, violation};
-use ruff_python_ast::helpers::find_useless_f_strings;
+use ruff_python_ast::source_code::Locator;
 use ruff_python_ast::types::Range;
 
 use crate::checkers::ast::Checker;
@@ -47,6 +48,45 @@ impl AlwaysAutofixableViolation for FStringMissingPlaceholders {
     }
 }
 
+/// Find f-strings that don't contain any formatted values in a `JoinedStr`.
+fn find_useless_f_strings<'a>(
+    expr: &'a Expr,
+    locator: &'a Locator,
+) -> impl Iterator<Item = (Range, Range)> + 'a {
+    let contents = locator.slice(expr);
+    lexer::lex_located(contents, Mode::Module, expr.location)
+        .flatten()
+        .filter_map(|(location, tok, end_location)| match tok {
+            Tok::String {
+                kind: StringKind::FString | StringKind::RawFString,
+                ..
+            } => {
+                let first_char = locator.slice(Range {
+                    location,
+                    end_location: Location::new(location.row(), location.column() + 1),
+                });
+                // f"..."  => f_position = 0
+                // fr"..." => f_position = 0
+                // rf"..." => f_position = 1
+                let f_position = usize::from(!(first_char == "f" || first_char == "F"));
+                Some((
+                    Range {
+                        location: Location::new(location.row(), location.column() + f_position),
+                        end_location: Location::new(
+                            location.row(),
+                            location.column() + f_position + 1,
+                        ),
+                    },
+                    Range {
+                        location,
+                        end_location,
+                    },
+                ))
+            }
+            _ => None,
+        })
+}
+
 fn unescape_f_string(content: &str) -> String {
     content.replace("{{", "{").replace("}}", "}")
 }

crates/ruff_python_ast/src/helpers.rs

@@ -9,7 +9,7 @@ use rustpython_parser::ast::{
     Arguments, Constant, Excepthandler, ExcepthandlerKind, Expr, ExprKind, Keyword, KeywordData,
     Located, Location, MatchCase, Pattern, PatternKind, Stmt, StmtKind,
 };
-use rustpython_parser::{lexer, Mode, StringKind, Tok};
+use rustpython_parser::{lexer, Mode, Tok};
 use smallvec::{smallvec, SmallVec};
 
 use crate::context::Context;
@@ -1055,43 +1055,6 @@ pub fn except_range(handler: &Excepthandler, locator: &Locator) -> Range {
     range
 }
 
-/// Find f-strings that don't contain any formatted values in a `JoinedStr`.
-pub fn find_useless_f_strings(expr: &Expr, locator: &Locator) -> Vec<(Range, Range)> {
-    let contents = locator.slice(expr);
-    lexer::lex_located(contents, Mode::Module, expr.location)
-        .flatten()
-        .filter_map(|(location, tok, end_location)| match tok {
-            Tok::String {
-                kind: StringKind::FString | StringKind::RawFString,
-                ..
-            } => {
-                let first_char = locator.slice(Range {
-                    location,
-                    end_location: Location::new(location.row(), location.column() + 1),
-                });
-                // f"..."  => f_position = 0
-                // fr"..." => f_position = 0
-                // rf"..." => f_position = 1
-                let f_position = usize::from(!(first_char == "f" || first_char == "F"));
-                Some((
-                    Range {
-                        location: Location::new(location.row(), location.column() + f_position),
-                        end_location: Location::new(
-                            location.row(),
-                            location.column() + f_position + 1,
-                        ),
-                    },
-                    Range {
-                        location,
-                        end_location,
-                    },
-                ))
-            }
-            _ => None,
-        })
-        .collect()
-}
-
 /// Return the `Range` of `else` in `For`, `AsyncFor`, and `While` statements.
 pub fn else_range(stmt: &Stmt, locator: &Locator) -> Option<Range> {
     match &stmt.node {