Avoid multiline expression if format specifier is present (#11123)

## Summary

This PR fixes the bug where the formatter would format an f-string and
could potentially change the AST.

For a triple-quoted f-string, the element can't be formatted into
multiline if it has a format specifier because otherwise the newline
would be treated as part of the format specifier.

Given the following f-string:
```python
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f} ddddddddddddddd eeeeeeee"""
```

The formatter sees that the f-string is already multiline so it assumes
that it can contain line breaks i.e., broken into multiple lines. But,
in this specific case we can't format it as:

```python
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f
} ddddddddddddddd eeeeeeee"""
```
                     
Because the format specifier string would become ".3f\n", which is not
the original string (`.3f`).

If the original source code already contained a newline, they'll be
preserved. For example:
```python
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f
} ddddddddddddddd eeeeeeee"""
```

The above will be formatted as:
```py
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {variable:.3f
} ddddddddddddddd eeeeeeee"""
```

Note that the newline after `.3f` is part of the format specifier which
needs to be preserved.
The Python version is irrelevant in this case.

fixes: #10040 

## Test Plan

Add some test cases to verify this behavior.

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

@@ -129,6 +129,10 @@
     {'x': 1, 'y': 2} }"
 x = f"{    # comment 13
     {'x': 1, 'y': 2} = }"
+# But, if there's a format specifier or a conversion flag then we don't need to add
+# any whitespace at the end
+x = f"aaaaa { {'aaaaaa', 'bbbbbbb', 'ccccccccc'}!s} bbbbbb"
+x = f"aaaaa { {'aaaaaa', 'bbbbbbb', 'ccccccccc'}:.3f} bbbbbb"
 
 # But, in this case, we would split the expression itself because it exceeds the line
 # length limit so we need not add the extra space.
@@ -207,6 +211,29 @@
 }"  # comment 19
 # comment 20
 
+# Single-quoted f-strings with a format specificer can be multiline
+f"aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
+    variable:.3f} ddddddddddddddd eeeeeeee"
+
+# But, if it's triple-quoted then we can't or the format specificer will have a
+# trailing newline
+f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
+    variable:.3f} ddddddddddddddd eeeeeeee"""
+
+# But, we can break the ones which don't have a format specifier
+f"""fooooooooooooooooooo barrrrrrrrrrrrrrrrrrr {
+        xxxxxxxxxxxxxxx:.3f} aaaaaaaaaaaaaaaaa { xxxxxxxxxxxxxxxxxxxx } bbbbbbbbbbbb"""
+
+# Throw in a random comment in it but surpise, this is not a comment but just a text
+# which is part of the format specifier
+aaaaaaaaaaa = f"""asaaaaaaaaaaaaaaaa {
+        aaaaaaaaaaaa + bbbbbbbbbbbb + ccccccccccccccc + dddddddd:.3f
+     # comment
+} cccccccccc"""
+aaaaaaaaaaa = f"""asaaaaaaaaaaaaaaaa {
+        aaaaaaaaaaaa + bbbbbbbbbbbb + ccccccccccccccc + dddddddd:.3f
+     # comment} cccccccccc"""
+
 # Conversion flags
 #
 # This is not a valid Python code because of the additional whitespace between the `!`

crates/ruff_python_formatter/src/builders.rs

@@ -211,7 +211,7 @@ impl<'fmt, 'ast, 'buf> JoinCommaSeparatedBuilder<'fmt, 'ast, 'buf> {
             if let FStringState::InsideExpressionElement(context) =
                 self.fmt.context().f_string_state()
             {
-                if context.layout().is_flat() {
+                if !context.can_contain_line_breaks() {
                     return Ok(());
                 }
             }

crates/ruff_python_formatter/src/context.rs

@@ -1,5 +1,5 @@
 use crate::comments::Comments;
-use crate::other::f_string::FStringContext;
+use crate::other::f_string_element::FStringExpressionElementContext;
 use crate::PyFormatOptions;
 use ruff_formatter::{Buffer, FormatContext, GroupId, IndentWidth, SourceCode};
 use ruff_python_ast::str::Quote;
@@ -128,13 +128,13 @@ impl Debug for PyFormatContext<'_> {
     }
 }
 
-#[derive(Copy, Clone, Debug, Default)]
+#[derive(Clone, Copy, Debug, Default)]
 pub(crate) enum FStringState {
     /// The formatter is inside an f-string expression element i.e., between the
     /// curly brace in `f"foo {x}"`.
     ///
     /// The containing `FStringContext` is the surrounding f-string context.
-    InsideExpressionElement(FStringContext),
+    InsideExpressionElement(FStringExpressionElementContext),
     /// The formatter is outside an f-string.
     #[default]
     Outside,

crates/ruff_python_formatter/src/other/f_string.rs

@@ -135,7 +135,7 @@ impl FStringLayout {
         }
     }
 
-    pub(crate) const fn is_flat(self) -> bool {
-        matches!(self, Self::Flat)
+    pub(crate) const fn is_multiline(self) -> bool {
+        matches!(self, FStringLayout::Multiline)
     }
 }

crates/ruff_python_formatter/src/other/f_string_element.rs

@@ -64,15 +64,63 @@ impl Format<PyFormatContext<'_>> for FormatFStringLiteralElement<'_> {
     }
 }
 
+/// Context representing an f-string expression element.
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct FStringExpressionElementContext {
+    /// The context of the parent f-string containing this expression element.
+    parent_context: FStringContext,
+    /// Indicates whether this expression element has format specifier or not.
+    has_format_spec: bool,
+}
+
+impl FStringExpressionElementContext {
+    /// Returns the [`FStringContext`] containing this expression element.
+    pub(crate) fn f_string(self) -> FStringContext {
+        self.parent_context
+    }
+
+    /// Returns `true` if the expression element can contain line breaks.
+    pub(crate) fn can_contain_line_breaks(self) -> bool {
+        self.parent_context.layout().is_multiline()
+            // For a triple-quoted f-string, the element can't be formatted into multiline if it
+            // has a format specifier because otherwise the newline would be treated as part of the
+            // format specifier.
+            //
+            // Given the following f-string:
+            // ```python
+            // f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {variable:.3f} ddddddddddddddd eeeeeeee"""
+            // ```
+            //
+            // We can't format it as:
+            // ```python
+            // f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
+            //     variable:.3f
+            // } ddddddddddddddd eeeeeeee"""
+            // ```
+            //
+            // Here, the format specifier string would become ".3f\n", which is not what we want.
+            // But, if the original source code already contained a newline, they'll be preserved.
+            //
+            // The Python version is irrelevant in this case.
+            && !(self.parent_context.kind().is_triple_quoted() && self.has_format_spec)
+    }
+}
+
 /// Formats an f-string expression element.
 pub(crate) struct FormatFStringExpressionElement<'a> {
     element: &'a FStringExpressionElement,
-    context: FStringContext,
+    context: FStringExpressionElementContext,
 }
 
 impl<'a> FormatFStringExpressionElement<'a> {
     pub(crate) fn new(element: &'a FStringExpressionElement, context: FStringContext) -> Self {
-        Self { element, context }
+        Self {
+            element,
+            context: FStringExpressionElementContext {
+                parent_context: context,
+                has_format_spec: element.format_spec.is_some(),
+            },
+        }
     }
 }
 
@@ -153,10 +201,10 @@ impl Format<PyFormatContext<'_>> for FormatFStringExpressionElement<'_> {
                     // added to maintain consistency.
                     Expr::Dict(_) | Expr::DictComp(_) | Expr::Set(_) | Expr::SetComp(_) => {
                         Some(format_with(|f| {
-                            if self.context.layout().is_flat() {
-                                space().fmt(f)
-                            } else {
+                            if self.context.can_contain_line_breaks() {
                                 soft_line_break_or_space().fmt(f)
+                            } else {
+                                space().fmt(f)
                             }
                         }))
                     }
@@ -183,12 +231,9 @@ impl Format<PyFormatContext<'_>> for FormatFStringExpressionElement<'_> {
                     token(":").fmt(f)?;
 
                     f.join()
-                        .entries(
-                            format_spec
-                                .elements
-                                .iter()
-                                .map(|element| FormatFStringElement::new(element, self.context)),
-                        )
+                        .entries(format_spec.elements.iter().map(|element| {
+                            FormatFStringElement::new(element, self.context.f_string())
+                        }))
                         .finish()?;
 
                     // These trailing comments can only occur if the format specifier is
@@ -205,7 +250,11 @@ impl Format<PyFormatContext<'_>> for FormatFStringExpressionElement<'_> {
                     trailing_comments(comments.trailing(self.element)).fmt(f)?;
                 }
 
-                bracket_spacing.fmt(f)
+                if conversion.is_none() && format_spec.is_none() {
+                    bracket_spacing.fmt(f)?;
+                }
+
+                Ok(())
             });
 
             let open_parenthesis_comments = if dangling_item_comments.is_empty() {
@@ -219,16 +268,16 @@ impl Format<PyFormatContext<'_>> for FormatFStringExpressionElement<'_> {
             {
                 let mut f = WithNodeLevel::new(NodeLevel::ParenthesizedExpression, f);
 
-                if self.context.layout().is_flat() {
-                    let mut buffer = RemoveSoftLinesBuffer::new(&mut *f);
-
-                    write!(buffer, [open_parenthesis_comments, item])?;
-                } else {
+                if self.context.can_contain_line_breaks() {
                     group(&format_args![
                         open_parenthesis_comments,
                         soft_block_indent(&item)
                     ])
                     .fmt(&mut f)?;
+                } else {
+                    let mut buffer = RemoveSoftLinesBuffer::new(&mut *f);
+
+                    write!(buffer, [open_parenthesis_comments, item])?;
                 }
             }
 

crates/ruff_python_formatter/src/string/normalize.rs

@@ -60,7 +60,7 @@ impl StringNormalizer {
             // The reason to preserve the quotes is based on the assumption that
             // the original f-string is valid in terms of quoting, and we don't
             // want to change that to make it invalid.
-            if (context.kind().is_triple_quoted() && !string.kind().is_triple_quoted())
+            if (context.f_string().kind().is_triple_quoted() && !string.kind().is_triple_quoted())
                 || self.target_version.supports_pep_701()
             {
                 self.quoting