Avoid syntax errors when removing f-string prefixes (#5319)

Closes https://github.com/astral-sh/ruff/issues/5281.

Closes https://github.com/astral-sh/ruff/issues/4827.

crates/ruff/resources/test/fixtures/pyflakes/F541.py

@@ -37,6 +37,9 @@ f"{{test}}"
 f'{{ 40 }}'
 f"{{a {{x}}"
 f"{{{{x}}}}"
+""f""
+''f""
+(""f""r"")
 
 # To be fixed
 # Error: f-string: single '}' is not allowed at line 41 column 8

crates/ruff/src/rules/flake8_bugbear/rules/f_string_docstring.rs

@@ -1,4 +1,4 @@
-use rustpython_parser::ast::{self, Expr, Stmt};
+use rustpython_parser::ast::{self, Stmt};
 
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
@@ -50,9 +50,9 @@ pub(crate) fn f_string_docstring(checker: &mut Checker, body: &[Stmt]) {
     let Stmt::Expr(ast::StmtExpr { value, range: _ }) = stmt else {
         return;
     };
-    let Expr::JoinedStr ( _) = value.as_ref() else {
+    if !value.is_joined_str_expr() {
         return;
-    };
+    }
     checker
         .diagnostics
         .push(Diagnostic::new(FStringDocstring, stmt.identifier()));

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

@@ -79,23 +79,6 @@ fn find_useless_f_strings<'a>(
         })
 }
 
-fn unescape_f_string(content: &str) -> String {
-    content.replace("{{", "{").replace("}}", "}")
-}
-
-fn fix_f_string_missing_placeholders(
-    prefix_range: TextRange,
-    tok_range: TextRange,
-    checker: &mut Checker,
-) -> Fix {
-    let content = &checker.locator.contents()[TextRange::new(prefix_range.end(), tok_range.end())];
-    Fix::automatic(Edit::replacement(
-        unescape_f_string(content),
-        prefix_range.start(),
-        tok_range.end(),
-    ))
-}
-
 /// F541
 pub(crate) fn f_string_missing_placeholders(expr: &Expr, values: &[Expr], checker: &mut Checker) {
     if !values
@@ -105,13 +88,51 @@ pub(crate) fn f_string_missing_placeholders(expr: &Expr, values: &[Expr], checke
         for (prefix_range, tok_range) in find_useless_f_strings(expr, checker.locator) {
             let mut diagnostic = Diagnostic::new(FStringMissingPlaceholders, tok_range);
             if checker.patch(diagnostic.kind.rule()) {
-                diagnostic.set_fix(fix_f_string_missing_placeholders(
+                diagnostic.set_fix(convert_f_string_to_regular_string(
                     prefix_range,
                     tok_range,
-                    checker,
+                    checker.locator,
                 ));
             }
             checker.diagnostics.push(diagnostic);
         }
     }
 }
+
+/// Unescape an f-string body by replacing `{{` with `{` and `}}` with `}`.
+///
+/// In Python, curly-brace literals within f-strings must be escaped by doubling the braces.
+/// When rewriting an f-string to a regular string, we need to unescape any curly-brace literals.
+///  For example, given `{{Hello, world!}}`, return `{Hello, world!}`.
+fn unescape_f_string(content: &str) -> String {
+    content.replace("{{", "{").replace("}}", "}")
+}
+
+/// Generate a [`Fix`] to rewrite an f-string as a regular string.
+fn convert_f_string_to_regular_string(
+    prefix_range: TextRange,
+    tok_range: TextRange,
+    locator: &Locator,
+) -> Fix {
+    // Extract the f-string body.
+    let mut content =
+        unescape_f_string(locator.slice(TextRange::new(prefix_range.end(), tok_range.end())));
+
+    // If the preceding character is equivalent to the quote character, insert a space to avoid a
+    // syntax error. For example, when removing the `f` prefix in `""f""`, rewrite to `"" ""`
+    // instead of `""""`.
+    if locator
+        .slice(TextRange::up_to(prefix_range.start()))
+        .chars()
+        .last()
+        .map_or(false, |char| content.starts_with(char))
+    {
+        content.insert(0, ' ');
+    }
+
+    Fix::automatic(Edit::replacement(
+        content,
+        prefix_range.start(),
+        tok_range.end(),
+    ))
+}

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__F541_F541.py.snap

@@ -269,7 +269,7 @@ F541.py:37:1: F541 [*] f-string without any placeholders
    37 |+'{ 40 }'
 38 38 | f"{{a {{x}}"
 39 39 | f"{{{{x}}}}"
-40 40 | 
+40 40 | ""f""
 
 F541.py:38:1: F541 [*] f-string without any placeholders
    |
@@ -278,6 +278,7 @@ F541.py:38:1: F541 [*] f-string without any placeholders
 38 | f"{{a {{x}}"
    | ^^^^^^^^^^^^ F541
 39 | f"{{{{x}}}}"
+40 | ""f""
    |
    = help: Remove extraneous `f` prefix
 
@@ -288,8 +289,8 @@ F541.py:38:1: F541 [*] f-string without any placeholders
 38    |-f"{{a {{x}}"
    38 |+"{a {x}"
 39 39 | f"{{{{x}}}}"
-40 40 | 
+40 40 | ""f""
-41 41 | # To be fixed
+41 41 | ''f""
 
 F541.py:39:1: F541 [*] f-string without any placeholders
    |
@@ -297,8 +298,8 @@ F541.py:39:1: F541 [*] f-string without any placeholders
 38 | f"{{a {{x}}"
 39 | f"{{{{x}}}}"
    | ^^^^^^^^^^^^ F541
-40 | 
+40 | ""f""
-41 | # To be fixed
+41 | ''f""
    |
    = help: Remove extraneous `f` prefix
 
@@ -308,8 +309,70 @@ F541.py:39:1: F541 [*] f-string without any placeholders
 38 38 | f"{{a {{x}}"
 39    |-f"{{{{x}}}}"
    39 |+"{{x}}"
-40 40 | 
+40 40 | ""f""
-41 41 | # To be fixed
+41 41 | ''f""
-42 42 | # Error: f-string: single '}' is not allowed at line 41 column 8
+42 42 | (""f""r"")
+
+F541.py:40:3: F541 [*] f-string without any placeholders
+   |
+38 | f"{{a {{x}}"
+39 | f"{{{{x}}}}"
+40 | ""f""
+   |   ^^^ F541
+41 | ''f""
+42 | (""f""r"")
+   |
+   = help: Remove extraneous `f` prefix
+
+ℹ Fix
+37 37 | f'{{ 40 }}'
+38 38 | f"{{a {{x}}"
+39 39 | f"{{{{x}}}}"
+40    |-""f""
+   40 |+"" ""
+41 41 | ''f""
+42 42 | (""f""r"")
+43 43 | 
+
+F541.py:41:3: F541 [*] f-string without any placeholders
+   |
+39 | f"{{{{x}}}}"
+40 | ""f""
+41 | ''f""
+   |   ^^^ F541
+42 | (""f""r"")
+   |
+   = help: Remove extraneous `f` prefix
+
+ℹ Fix
+38 38 | f"{{a {{x}}"
+39 39 | f"{{{{x}}}}"
+40 40 | ""f""
+41    |-''f""
+   41 |+''""
+42 42 | (""f""r"")
+43 43 | 
+44 44 | # To be fixed
+
+F541.py:42:4: F541 [*] f-string without any placeholders
+   |
+40 | ""f""
+41 | ''f""
+42 | (""f""r"")
+   |    ^^^ F541
+43 | 
+44 | # To be fixed
+   |
+   = help: Remove extraneous `f` prefix
+
+ℹ Fix
+39 39 | f"{{{{x}}}}"
+40 40 | ""f""
+41 41 | ''f""
+42    |-(""f""r"")
+   42 |+("" ""r"")
+43 43 | 
+44 44 | # To be fixed
+45 45 | # Error: f-string: single '}' is not allowed at line 41 column 8