Fix fstring formatting removing overlong implicit concatenated string in expression part (#14811)

## Summary

Fixes https://github.com/astral-sh/ruff/issues/14778


The formatter incorrectly removed the inner implicitly concatenated
string for following single-line f-string:

```py
f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"

# formatted
f"{ if True else ''}"
```

This happened because I changed the `RemoveSoftlinesBuffer` in
https://github.com/astral-sh/ruff/pull/14489 to remove any content
wrapped in `if_group_breaks`. After all, it emulates an *all flat*
layout. This works fine when `if_group_breaks` is only used to **add**
content if the gorup breaks. It doesn't work if the same content is
rendered differently depending on if the group fits using
`if_group_breaks` and `if_groups_fits` because the enclosing `group`
might still *break* if the entire content exceeds the line-length limit.

This PR fixes this by unwrapping any `if_group_fits` content by removing
the `if_group_fits` start and end tags.


## Test Plan

added test

crates/ruff_formatter/src/buffer.rs

@@ -299,7 +299,8 @@ where
 ///
 /// - Removes [`lines`](FormatElement::Line) with the mode [`Soft`](LineMode::Soft).
 /// - Replaces [`lines`](FormatElement::Line) with the mode [`Soft`](LineMode::SoftOrSpace) with a [`Space`](FormatElement::Space)
-/// - Removes [`if_group_breaks`](crate::builders::if_group_breaks) elements.
+/// - Removes [`if_group_breaks`](crate::builders::if_group_breaks) and all its content.
+/// - Unwraps the content of [`if_group_fits_on_line`](crate::builders::if_group_fits_on_line) elements (but retains it).
 ///
 /// # Examples
 ///
@@ -387,7 +388,7 @@ fn clean_interned(
             .iter()
             .enumerate()
             .find_map(|(index, element)| match element {
-                FormatElement::Line(LineMode::Soft | LineMode::SoftOrSpace) => {
+                FormatElement::Line(LineMode::SoftOrSpace) => {
                     let mut cleaned = Vec::new();
                     let (before, after) = interned.split_at(index);
                     cleaned.extend_from_slice(before);
@@ -427,7 +428,6 @@ fn clean_interned(
                     }
 
                     let element = match element {
-                        FormatElement::Line(LineMode::Soft) => continue,
                         FormatElement::Line(LineMode::SoftOrSpace) => FormatElement::Space,
                         FormatElement::Interned(interned) => {
                             FormatElement::Interned(clean_interned(interned, interned_cache))
@@ -458,7 +458,6 @@ impl<Context> Buffer for RemoveSoftLinesBuffer<'_, Context> {
         }
 
         let element = match element {
-            FormatElement::Line(LineMode::Soft) => return,
             FormatElement::Line(LineMode::SoftOrSpace) => FormatElement::Space,
             FormatElement::Interned(interned) => {
                 FormatElement::Interned(self.clean_interned(&interned))
@@ -508,28 +507,34 @@ enum RemoveSoftLineBreaksState {
 impl RemoveSoftLineBreaksState {
     fn should_drop(&mut self, element: &FormatElement) -> bool {
         match self {
-            Self::Default => {
+            Self::Default => match element {
-                // Entered the start of an `if_group_breaks`
+                FormatElement::Line(LineMode::Soft) => true,
-                if let FormatElement::Tag(Tag::StartConditionalContent(condition)) = element {
+
+                // Entered the start of an `if_group_breaks` or `if_group_fits`
+                // For `if_group_breaks`: Remove the start and end tag and all content in between.
+                // For `if_group_fits_on_line`: Unwrap the content. This is important because the enclosing group
+                // might still *expand* if the content exceeds the line width limit, in which case the
+                // `if_group_fits_on_line` content would be removed.
+                FormatElement::Tag(Tag::StartConditionalContent(condition)) => {
                     if condition.mode.is_expanded() {
                         *self = Self::InIfGroupBreaks {
                             conditional_content_level: NonZeroUsize::new(1).unwrap(),
                         };
-                        return true;
                     }
+                    true
                 }
-
+                FormatElement::Tag(Tag::EndConditionalContent) => true,
-                false
+                _ => false,
-            }
+            },
             Self::InIfGroupBreaks {
                 conditional_content_level,
             } => {
                 match element {
-                    // A nested `if_group_breaks` or `if_group_fits`
+                    // A nested `if_group_breaks` or `if_group_fits_on_line`
                     FormatElement::Tag(Tag::StartConditionalContent(_)) => {
                         *conditional_content_level = conditional_content_level.saturating_add(1);
                     }
-                    // The end of an `if_group_breaks` or `if_group_fits`.
+                    // The end of an `if_group_breaks` or `if_group_fits_on_line`.
                     FormatElement::Tag(Tag::EndConditionalContent) => {
                         if let Some(level) = NonZeroUsize::new(conditional_content_level.get() - 1)
                         {

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

@@ -665,6 +665,10 @@ _ = (
 # Regression test for https://github.com/astral-sh/ruff/issues/14487
 f"aaaaaaaaaaaaaaaaaaaaaaaaaa {10**27} bbbbbbbbbbbbbbbbbbbbbbbbbb ccccccccccccccccccccccccc"
 
+# Regression test for https://github.com/astral-sh/ruff/issues/14778
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
+
 # Quotes reuse
 f"{'a'}"
 

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

@@ -1,6 +1,7 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/fstring.py
+snapshot_kind: text
 ---
 ## Input
 ```python
@@ -671,6 +672,10 @@ _ = (
 # Regression test for https://github.com/astral-sh/ruff/issues/14487
 f"aaaaaaaaaaaaaaaaaaaaaaaaaa {10**27} bbbbbbbbbbbbbbbbbbbbbbbbbb ccccccccccccccccccccccccc"
 
+# Regression test for https://github.com/astral-sh/ruff/issues/14778
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
+
 # Quotes reuse
 f"{'a'}"
 
@@ -1441,6 +1446,10 @@ _ = (
 # Regression test for https://github.com/astral-sh/ruff/issues/14487
 f"aaaaaaaaaaaaaaaaaaaaaaaaaa {10**27} bbbbbbbbbbbbbbbbbbbbbbbbbb ccccccccccccccccccccccccc"
 
+# Regression test for https://github.com/astral-sh/ruff/issues/14778
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' if True else ''}"
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' if True else ''}"
+
 # Quotes reuse
 f"{'a'}"
 
@@ -2163,6 +2172,10 @@ _ = (
 # Regression test for https://github.com/astral-sh/ruff/issues/14487
 f"aaaaaaaaaaaaaaaaaaaaaaaaaa {10**27} bbbbbbbbbbbbbbbbbbbbbbbbbb ccccccccccccccccccccccccc"
 
+# Regression test for https://github.com/astral-sh/ruff/issues/14778
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
+f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
+
 # Quotes reuse
 f"{'a'}"
 
@@ -2993,7 +3006,7 @@ f'{1=: {'ab"cd"'}}'  # It's okay if the quotes are in an expression part.
  )
  
  # Indentation
-@@ -632,29 +680,29 @@
+@@ -632,37 +680,37 @@
          if indent2:
              foo = f"""hello world
  hello {
@@ -3041,7 +3054,17 @@ f'{1=: {'ab"cd"'}}'  # It's okay if the quotes are in an expression part.
  )
  
  # Regression test for https://github.com/astral-sh/ruff/issues/14487
-@@ -678,18 +726,18 @@
+ f"aaaaaaaaaaaaaaaaaaaaaaaaaa {10**27} bbbbbbbbbbbbbbbbbbbbbbbbbb ccccccccccccccccccccccccc"
+ 
+ # Regression test for https://github.com/astral-sh/ruff/issues/14778
+-f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
+-f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' 'a' if True else ""}"
++f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' if True else ''}"
++f"{'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' if True else ''}"
+ 
+ # Quotes reuse
+ f"{'a'}"
+@@ -682,18 +730,18 @@
  f'{1: hy "user"}'
  f'{1:hy "user"}'
  f'{1: abcd "{1}" }'