Avoid inserting trailing commas within f-strings (#8574)

Closes https://github.com/astral-sh/ruff/issues/8556.

crates/ruff_linter/resources/test/fixtures/flake8_commas/COM81.py

@@ -42,7 +42,7 @@ foo = (
 4,
 )
 
-foo = 3, 
+foo = 3,
 
 class A(object):
  foo = 3
@@ -639,3 +639,10 @@ foo = namedtuple(
         :20
     ],
 )
+
+# Make sure we don't insert commas within f-strings.
+f"""This is a test. {
+    "Another sentence."
+    if True else
+    "Alternative route!"
+}"""

crates/ruff_linter/src/rules/flake8_commas/rules/trailing_commas.rs

@@ -223,11 +223,24 @@ pub(crate) fn trailing_commas(
     tokens: &[LexResult],
     locator: &Locator,
 ) {
+    let mut fstrings = 0u32;
     let tokens = tokens
         .iter()
         .flatten()
-        // Completely ignore comments -- they just interfere with the logic.
-        .filter(|&r| !matches!(r, (Tok::Comment(_), _)))
+        .filter(|(tok, _)| match tok {
+            // Completely ignore comments -- they just interfere with the logic.
+            Tok::Comment(_) => false,
+            // Ignore content within f-strings.
+            Tok::FStringStart => {
+                fstrings = fstrings.saturating_add(1);
+                false
+            }
+            Tok::FStringEnd => {
+                fstrings = fstrings.saturating_sub(1);
+                false
+            }
+            _ => fstrings == 0,
+        })
         .map(Token::from_spanned);
     let tokens = [Token::irrelevant(), Token::irrelevant()]
         .into_iter()

crates/ruff_linter/src/rules/flake8_commas/snapshots/ruff_linter__rules__flake8_commas__tests__COM81.py.snap

@@ -106,7 +106,7 @@ COM81.py:45:8: COM818 Trailing comma on bare tuple prohibited
    |
 43 | )
 44 | 
-45 | foo = 3, 
+45 | foo = 3,
    |        ^ COM818
 46 | 
 47 | class A(object):