[`flynt`] Fix f-string quoting for mixed quote joiners (`FLY002`) (#20662)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Fixes #19837

Track quote usage across the joiner and parts to choose a safe f-string
quote or skip the fix when both appear.

## Test Plan

<!-- How was it tested? -->

Add regression coverage to FLY002.py

crates/ruff_linter/resources/test/fixtures/flynt/FLY002.py

@@ -23,3 +23,9 @@ nok9 = '\n'.join([r"raw string", '<""">', "<'''>"])  # Not OK (both triple-quote
 # Regression test for: https://github.com/astral-sh/ruff/issues/7197
 def create_file_public_url(url, filename):
     return''.join([url, filename])
+
+# Regression test for: https://github.com/astral-sh/ruff/issues/19837
+nok10 = "".join((foo, '"'))
+nok11 = ''.join((foo, "'"))
+nok12 = ''.join([foo, "'", '"'])
+nok13 = "".join([foo, "'", '"'])

crates/ruff_linter/src/rules/flynt/rules/static_join_to_fstring.rs

@@ -2,7 +2,7 @@ use ast::FStringFlags;
 use itertools::Itertools;
 
 use ruff_macros::{ViolationMetadata, derive_message_formats};
-use ruff_python_ast::{self as ast, Arguments, Expr, StringFlags};
+use ruff_python_ast::{self as ast, Arguments, Expr, StringFlags, str::Quote};
 use ruff_text_size::{Ranged, TextRange};
 
 use crate::checkers::ast::Checker;
@@ -115,6 +115,8 @@ fn build_fstring(joiner: &str, joinees: &[Expr], flags: FStringFlags) -> Option<
     }
 
     let mut f_string_elements = Vec::with_capacity(joinees.len() * 2);
+    let mut has_single_quote = joiner.contains('\'');
+    let mut has_double_quote = joiner.contains('"');
     let mut first = true;
 
     for expr in joinees {
@@ -126,14 +128,31 @@ fn build_fstring(joiner: &str, joinees: &[Expr], flags: FStringFlags) -> Option<
         if !std::mem::take(&mut first) {
             f_string_elements.push(helpers::to_interpolated_string_literal_element(joiner));
         }
-        f_string_elements.push(helpers::to_interpolated_string_element(expr)?);
+        let element = helpers::to_interpolated_string_element(expr)?;
+        if let ast::InterpolatedStringElement::Literal(ast::InterpolatedStringLiteralElement {
+            value,
+            ..
+        }) = &element
+        {
+            has_single_quote |= value.contains('\'');
+            has_double_quote |= value.contains('"');
         }
+        f_string_elements.push(element);
+    }
+
+    let quote = flags.quote_style();
+    let adjusted_quote = match quote {
+        Quote::Single if has_single_quote && !has_double_quote => quote.opposite(),
+        Quote::Double if has_double_quote && !has_single_quote => quote.opposite(),
+        _ if has_double_quote && has_single_quote => return None,
+        _ => quote,
+    };
 
     let node = ast::FString {
         elements: f_string_elements.into(),
         range: TextRange::default(),
         node_index: ruff_python_ast::AtomicNodeIndex::NONE,
-        flags,
+        flags: flags.with_quote_style(adjusted_quote),
     };
     Some(node.into())
 }

crates/ruff_linter/src/rules/flynt/snapshots/ruff_linter__rules__flynt__tests__FLY002_FLY002.py.snap

@@ -153,6 +153,8 @@ FLY002 [*] Consider `f"{url}{filename}"` instead of string join
 24 | def create_file_public_url(url, filename):
 25 |     return''.join([url, filename])
    |           ^^^^^^^^^^^^^^^^^^^^^^^^
+26 |
+27 | # Regression test for: https://github.com/astral-sh/ruff/issues/19837
    |
 help: Replace with `f"{url}{filename}"`
 22 | 
@@ -160,4 +162,47 @@ help: Replace with `f"{url}{filename}"`
 24 | def create_file_public_url(url, filename):
    -     return''.join([url, filename])
 25 +     return f"{url}{filename}"
+26 | 
+27 | # Regression test for: https://github.com/astral-sh/ruff/issues/19837
+28 | nok10 = "".join((foo, '"'))
+note: This is an unsafe fix and may change runtime behavior
+
+FLY002 [*] Consider `f'{foo}"'` instead of string join
+  --> FLY002.py:28:9
+   |
+27 | # Regression test for: https://github.com/astral-sh/ruff/issues/19837
+28 | nok10 = "".join((foo, '"'))
+   |         ^^^^^^^^^^^^^^^^^^^
+29 | nok11 = ''.join((foo, "'"))
+30 | nok12 = ''.join([foo, "'", '"'])
+   |
+help: Replace with `f'{foo}"'`
+25 |     return''.join([url, filename])
+26 | 
+27 | # Regression test for: https://github.com/astral-sh/ruff/issues/19837
+   - nok10 = "".join((foo, '"'))
+28 + nok10 = f'{foo}"'
+29 | nok11 = ''.join((foo, "'"))
+30 | nok12 = ''.join([foo, "'", '"'])
+31 | nok13 = "".join([foo, "'", '"'])
+note: This is an unsafe fix and may change runtime behavior
+
+FLY002 [*] Consider `f"{foo}'"` instead of string join
+  --> FLY002.py:29:9
+   |
+27 | # Regression test for: https://github.com/astral-sh/ruff/issues/19837
+28 | nok10 = "".join((foo, '"'))
+29 | nok11 = ''.join((foo, "'"))
+   |         ^^^^^^^^^^^^^^^^^^^
+30 | nok12 = ''.join([foo, "'", '"'])
+31 | nok13 = "".join([foo, "'", '"'])
+   |
+help: Replace with `f"{foo}'"`
+26 | 
+27 | # Regression test for: https://github.com/astral-sh/ruff/issues/19837
+28 | nok10 = "".join((foo, '"'))
+   - nok11 = ''.join((foo, "'"))
+29 + nok11 = f"{foo}'"
+30 | nok12 = ''.join([foo, "'", '"'])
+31 | nok13 = "".join([foo, "'", '"'])
 note: This is an unsafe fix and may change runtime behavior