Make quoting decision per f-string instead of entire f-string expression

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/fstring.py

@@ -313,4 +313,5 @@
 _ = (
     'This string should change its quotes to double quotes'
     f'This string uses double quotes in an expression {"woah"}'
+    f'This f-string does not use any quotes.'
 )

crates/ruff_python_formatter/src/other/f_string.rs

@@ -1,10 +1,12 @@
+use crate::prelude::*;
+use crate::preview::{
+    is_f_string_formatting_enabled, is_f_string_implicit_concatenated_string_literal_quotes_enabled,
+};
+use crate::string::{Quoting, StringNormalizer, StringQuotes};
 use ruff_formatter::write;
 use ruff_python_ast::{AnyStringFlags, FString, StringFlags};
 use ruff_source_file::Locator;
-
-use crate::prelude::*;
-use crate::preview::is_f_string_formatting_enabled;
-use crate::string::{Quoting, StringNormalizer, StringQuotes};
+use ruff_text_size::Ranged;
 
 use super::f_string_element::FormatFStringElement;
 
@@ -29,8 +31,17 @@ impl Format<PyFormatContext<'_>> for FormatFString<'_> {
     fn fmt(&self, f: &mut PyFormatter) -> FormatResult<()> {
         let locator = f.context().locator();
 
+        // If the preview style is enabled, make the decision on what quotes to use locally for each
+        // f-string instead of globally for the entire f-string expression.
+        let quoting =
+            if is_f_string_implicit_concatenated_string_literal_quotes_enabled(f.context()) {
+                f_string_quoting(self.value, &locator)
+            } else {
+                self.quoting
+            };
+
         let normalizer = StringNormalizer::from_context(f.context())
-            .with_quoting(self.quoting)
+            .with_quoting(quoting)
             .with_preferred_quote_style(f.options().quote_style());
 
         // If f-string formatting is disabled (not in preview), then we will
@@ -140,3 +151,20 @@ impl FStringLayout {
         matches!(self, FStringLayout::Multiline)
     }
 }
+
+fn f_string_quoting(f_string: &FString, locator: &Locator) -> Quoting {
+    let triple_quoted = f_string.flags.is_triple_quoted();
+
+    if f_string.elements.expressions().any(|expression| {
+        let string_content = locator.slice(expression.range());
+        if triple_quoted {
+            string_content.contains(r#"""""#) || string_content.contains("'''")
+        } else {
+            string_content.contains(['"', '\''])
+        }
+    }) {
+        Quoting::Preserve
+    } else {
+        Quoting::CanChange
+    }
+}

crates/ruff_python_formatter/src/other/string_literal.rs

@@ -48,8 +48,11 @@ impl StringLiteralKind {
             StringLiteralKind::String | StringLiteralKind::Docstring => Quoting::CanChange,
             #[allow(deprecated)]
             StringLiteralKind::InImplicitlyConcatenatedFString(quoting) => {
+                // Allow string literals to pick the "optimal" quote character
+                // even if any other fstring in the implicit concatenation uses an expression
+                // containing a quote character.
                 // TODO: Remove StringLiteralKind::InImplicitlyConcatenatedFString when promoting
-                //   this style to stable
+                //   this style to stable and remove the layout from `AnyStringPart::String`.
                 if is_f_string_implicit_concatenated_string_literal_quotes_enabled(context) {
                     Quoting::CanChange
                 } else {

crates/ruff_python_formatter/tests/snapshots/format@expression__fstring.py.snap

@@ -319,6 +319,7 @@ hello {
 _ = (
     'This string should change its quotes to double quotes'
     f'This string uses double quotes in an expression {"woah"}'
+    f'This f-string does not use any quotes.'
 )
 ```
 
@@ -662,6 +663,7 @@ hello {
 _ = (
     "This string should change its quotes to double quotes"
     f'This string uses double quotes in an expression {"woah"}'
+    f"This f-string does not use any quotes."
 )
 ```
 
@@ -993,6 +995,7 @@ hello {
 _ = (
     'This string should change its quotes to double quotes'
     f'This string uses double quotes in an expression {"woah"}'
+    f'This f-string does not use any quotes.'
 )
 ```
 
@@ -1300,7 +1303,7 @@ _ = (
      # comment 27
      # comment 28
  } woah {x}"
-@@ -287,26 +299,26 @@
+@@ -287,27 +299,27 @@
          if indent2:
              foo = f"""hello world
  hello {
@@ -1342,5 +1345,7 @@ _ = (
 -    'This string should change its quotes to double quotes'
 +    "This string should change its quotes to double quotes"
      f'This string uses double quotes in an expression {"woah"}'
+-    f'This f-string does not use any quotes.'
++    f"This f-string does not use any quotes."
  )
 ```