Allow parenthesized content exceed configured line width (#7490)

crates/ruff_formatter/src/builders.rs

@@ -2155,15 +2155,18 @@ impl<Context> std::fmt::Debug for IndentIfGroupBreaks<'_, Context> {
     }
 }
 
-/// Changes the definition of *fits* for `content`. Instead of measuring it in *flat*, measure it with
-/// all line breaks expanded and test if no line exceeds the line width. The [`FitsExpanded`] acts
-/// as a expands boundary similar to best fitting, meaning that a [`hard_line_break`] will not cause the parent group to expand.
+/// Changes the definition of *fits* for `content`. It measures the width of all lines and allows
+/// the content inside of the [`fits_expanded`] to exceed the configured line width. The content
+/// coming before and after [`fits_expanded`] must fit into the configured line width.
+///
+/// The [`fits_expanded`] acts as a expands boundary similar to best fitting,
+/// meaning that a [`hard_line_break`] will not cause the parent group to expand.
 ///
 /// Useful in conjunction with a group with a condition.
 ///
 /// ## Examples
-/// The outer group with the binary expression remains *flat* regardless of the array expression
-/// that spans multiple lines.
+/// The outer group with the binary expression remains *flat* regardless of the array expression that
+/// spans multiple lines with items exceeding the configured line width.
 ///
 /// ```
 /// # use ruff_formatter::{format, format_args, LineWidth, SimpleFormatOptions, write};
@@ -2183,7 +2186,7 @@ impl<Context> std::fmt::Debug for IndentIfGroupBreaks<'_, Context> {
 ///                 token("["),
 ///                 soft_block_indent(&format_args![
 ///                     token("a,"), space(), token("# comment"), expand_parent(), soft_line_break_or_space(),
-///                     token("b")
+///                     token("'A very long string that exceeds the configured line width of 80 characters but the enclosing binary expression still fits.'")
 ///                 ]),
 ///                 token("]")
 ///             ]))
@@ -2194,7 +2197,7 @@ impl<Context> std::fmt::Debug for IndentIfGroupBreaks<'_, Context> {
 /// let formatted = format!(SimpleFormatContext::default(), [content])?;
 ///
 /// assert_eq!(
-///     "a + [\n\ta, # comment\n\tb\n]",
+///     "a + [\n\ta, # comment\n\t'A very long string that exceeds the configured line width of 80 characters but the enclosing binary expression still fits.'\n]",
 ///     formatted.print()?.as_code()
 /// );
 /// # Ok(())

crates/ruff_formatter/src/format_element/tag.rs

@@ -84,11 +84,14 @@ pub enum Tag {
     StartFitsExpanded(FitsExpanded),
     EndFitsExpanded,
 
+    /// Marks the start and end of a best-fitting variant.
     StartBestFittingEntry,
     EndBestFittingEntry,
 
     /// Parenthesizes the content but only if adding the parentheses and indenting the content
     /// makes the content fit in the configured line width.
+    ///
+    /// See [`crate::builders::best_fit_parenthesize`] for an in-depth explanation.
     StartBestFitParenthesize {
         id: Option<GroupId>,
     },

crates/ruff_formatter/src/format_extensions.rs

@@ -1,5 +1,3 @@
-#![allow(dead_code)]
-
 use crate::prelude::*;
 use std::cell::OnceCell;
 use std::marker::PhantomData;

crates/ruff_formatter/src/printer/mod.rs

@@ -1183,7 +1183,7 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
                                 // line break should be printed as regular line break
                                 return Ok(Fits::Yes);
                             }
-                            MeasureMode::AllLines => {
+                            MeasureMode::AllLines | MeasureMode::AllLinesAllowTextOverflow => {
                                 // Continue measuring on the next line
                                 self.state.line_width = 0;
                                 self.state.pending_indent = args.indention();
@@ -1354,9 +1354,11 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
             }
 
             FormatElement::Tag(StartLineSuffix { reserved_width }) => {
-                self.state.line_width += reserved_width;
-                if self.state.line_width > self.options().line_width.into() {
-                    return Ok(Fits::No);
+                if *reserved_width > 0 {
+                    self.state.line_width += reserved_width;
+                    if self.state.line_width > self.options().line_width.into() {
+                        return Ok(Fits::No);
+                    }
                 }
                 self.queue.skip_content(TagKind::LineSuffix);
                 self.state.has_line_suffix = true;
@@ -1370,32 +1372,42 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
                 condition,
                 propagate_expand,
             })) => {
-                let condition_met = match condition {
-                    Some(condition) => {
-                        let group_mode = match condition.group_id {
-                            Some(group_id) => self.group_modes().get_print_mode(group_id)?,
-                            None => args.mode(),
+                match args.mode() {
+                    PrintMode::Expanded => {
+                        // As usual, nothing to measure
+                        self.stack.push(TagKind::FitsExpanded, args);
+                    }
+                    PrintMode::Flat => {
+                        let condition_met = match condition {
+                            Some(condition) => {
+                                let group_mode = match condition.group_id {
+                                    Some(group_id) => {
+                                        self.group_modes().get_print_mode(group_id)?
+                                    }
+                                    None => args.mode(),
+                                };
+
+                                condition.mode == group_mode
+                            }
+                            None => true,
                         };
 
-                        condition.mode == group_mode
-                    }
-                    None => true,
-                };
+                        if condition_met {
+                            // Measure in fully expanded mode and allow overflows
+                            self.stack.push(
+                                TagKind::FitsExpanded,
+                                args.with_measure_mode(MeasureMode::AllLinesAllowTextOverflow)
+                                    .with_print_mode(PrintMode::Expanded),
+                            );
+                        } else {
+                            if propagate_expand.get() {
+                                return Ok(Fits::No);
+                            }
 
-                if condition_met {
-                    // Measure in fully expanded mode.
-                    self.stack.push(
-                        TagKind::FitsExpanded,
-                        args.with_print_mode(PrintMode::Expanded)
-                            .with_measure_mode(MeasureMode::AllLines),
-                    );
-                } else {
-                    if propagate_expand.get() && args.mode().is_flat() {
-                        return Ok(Fits::No);
+                            // As usual
+                            self.stack.push(TagKind::FitsExpanded, args);
+                        }
                     }
-
-                    // As usual
-                    self.stack.push(TagKind::FitsExpanded, args);
                 }
             }
 
@@ -1482,7 +1494,8 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
                                 }
                                 match args.measure_mode() {
                                     MeasureMode::FirstLine => return Fits::Yes,
-                                    MeasureMode::AllLines => {
+                                    MeasureMode::AllLines
+                                    | MeasureMode::AllLinesAllowTextOverflow => {
                                         self.state.line_width = 0;
                                         continue;
                                     }
@@ -1498,7 +1511,9 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
             }
         }
 
-        if self.state.line_width > self.options().line_width.into() {
+        if self.state.line_width > self.options().line_width.into()
+            && !args.measure_mode().allows_text_overflow()
+        {
             return Fits::No;
         }
 
@@ -1601,6 +1616,17 @@ enum MeasureMode {
     /// The content only fits if none of the lines exceed the print width. Lines are terminated by either
     /// a hard line break or a soft line break in [`PrintMode::Expanded`].
     AllLines,
+
+    /// Measures all lines and allows lines to exceed the configured line width. Useful when it only matters
+    /// whether the content *before* and *after* fits.
+    AllLinesAllowTextOverflow,
+}
+
+impl MeasureMode {
+    /// Returns `true` if this mode allows text exceeding the configured line width.
+    const fn allows_text_overflow(self) -> bool {
+        matches!(self, MeasureMode::AllLinesAllowTextOverflow)
+    }
 }
 
 impl From<BestFittingMode> for MeasureMode {

crates/ruff_formatter/src/printer/printer_options/mod.rs

@@ -120,7 +120,6 @@ impl SourceMapGeneration {
     }
 }
 
-#[allow(dead_code)]
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Default)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum LineEnding {