perf: Reduce best fitting allocations (#7411)

crates/ruff_formatter/src/builders.rs

@@ -9,9 +9,7 @@ use Tag::*;
 use crate::format_element::tag::{Condition, Tag};
 use crate::prelude::tag::{DedentMode, GroupMode, LabelId};
 use crate::prelude::*;
-use crate::{
+use crate::{write, Argument, Arguments, FormatContext, FormatOptions, GroupId, TextSize};
-    format_element, write, Argument, Arguments, FormatContext, FormatOptions, GroupId, TextSize,
-};
 use crate::{Buffer, VecBuffer};
 
 /// A line break that only gets printed if the enclosing `Group` doesn't fit on a single line.
@@ -2543,15 +2541,12 @@ impl<Context> Format<Context> for BestFitting<'_, Context> {
     fn fmt(&self, f: &mut Formatter<Context>) -> FormatResult<()> {
         let variants = self.variants.items();
 
-        let mut formatted_variants = Vec::with_capacity(variants.len());
+        let mut buffer = VecBuffer::with_capacity(variants.len() * 8, f.state_mut());
 
         for variant in variants {
-            let mut buffer = VecBuffer::with_capacity(8, f.state_mut());
+            buffer.write_element(FormatElement::Tag(StartBestFittingEntry));
-            buffer.write_element(FormatElement::Tag(StartEntry));
             buffer.write_fmt(Arguments::from(variant))?;
-            buffer.write_element(FormatElement::Tag(EndEntry));
+            buffer.write_element(FormatElement::Tag(EndBestFittingEntry));
-
-            formatted_variants.push(buffer.into_vec().into_boxed_slice());
         }
 
         // SAFETY: The constructor guarantees that there are always at least two variants. It's, therefore,
@@ -2559,9 +2554,7 @@ impl<Context> Format<Context> for BestFitting<'_, Context> {
         #[allow(unsafe_code)]
         let element = unsafe {
             FormatElement::BestFitting {
-                variants: format_element::BestFittingVariants::from_vec_unchecked(
+                variants: BestFittingVariants::from_vec_unchecked(buffer.into_vec()),
-                    formatted_variants,
-                ),
                 mode: self.mode,
             }
         };

crates/ruff_formatter/src/format_element.rs

@@ -3,6 +3,7 @@ pub mod tag;
 
 use std::borrow::Cow;
 use std::hash::{Hash, Hasher};
+use std::iter::FusedIterator;
 use std::num::NonZeroU32;
 use std::ops::Deref;
 use std::rc::Rc;
@@ -67,6 +68,16 @@ pub enum FormatElement {
     Tag(Tag),
 }
 
+impl FormatElement {
+    pub fn tag_kind(&self) -> Option<TagKind> {
+        if let FormatElement::Tag(tag) = self {
+            Some(tag.kind())
+        } else {
+            None
+        }
+    }
+}
+
 impl std::fmt::Debug for FormatElement {
     fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
@@ -318,7 +329,7 @@ pub enum BestFittingMode {
 /// The first element is the one that takes up the most space horizontally (the most flat),
 /// The last element takes up the least space horizontally (but most horizontal space).
 #[derive(Clone, Eq, PartialEq, Debug)]
-pub struct BestFittingVariants(Box<[Box<[FormatElement]>]>);
+pub struct BestFittingVariants(Box<[FormatElement]>);
 
 impl BestFittingVariants {
     /// Creates a new best fitting IR with the given variants. The method itself isn't unsafe
@@ -331,9 +342,13 @@ impl BestFittingVariants {
     /// The slice must contain at least two variants.
     #[doc(hidden)]
     #[allow(unsafe_code)]
-    pub unsafe fn from_vec_unchecked(variants: Vec<Box<[FormatElement]>>) -> Self {
+    pub unsafe fn from_vec_unchecked(variants: Vec<FormatElement>) -> Self {
         debug_assert!(
-            variants.len() >= 2,
+            variants
+                .iter()
+                .filter(|element| matches!(element, FormatElement::Tag(Tag::StartBestFittingEntry)))
+                .count()
+                >= 2,
             "Requires at least the least expanded and most expanded variants"
         );
 
@@ -342,40 +357,85 @@ impl BestFittingVariants {
 
     /// Returns the most expanded variant
     pub fn most_expanded(&self) -> &[FormatElement] {
-        self.0.last().expect(
+        self.into_iter().last().unwrap()
-            "Most contain at least two elements, as guaranteed by the best fitting builder.",
-        )
     }
 
-    pub fn as_slice(&self) -> &[Box<[FormatElement]>] {
+    pub fn as_slice(&self) -> &[FormatElement] {
         &self.0
     }
 
     /// Returns the least expanded variant
     pub fn most_flat(&self) -> &[FormatElement] {
-        self.0.first().expect(
+        self.into_iter().next().unwrap()
-            "Most contain at least two elements, as guaranteed by the best fitting builder.",
-        )
     }
 }
 
 impl Deref for BestFittingVariants {
-    type Target = [Box<[FormatElement]>];
+    type Target = [FormatElement];
 
     fn deref(&self) -> &Self::Target {
         self.as_slice()
     }
 }
 
+pub struct BestFittingVariantsIter<'a> {
+    elements: &'a [FormatElement],
+}
+
 impl<'a> IntoIterator for &'a BestFittingVariants {
-    type Item = &'a Box<[FormatElement]>;
+    type Item = &'a [FormatElement];
-    type IntoIter = std::slice::Iter<'a, Box<[FormatElement]>>;
+    type IntoIter = BestFittingVariantsIter<'a>;
 
     fn into_iter(self) -> Self::IntoIter {
-        self.as_slice().iter()
+        BestFittingVariantsIter { elements: &self.0 }
     }
 }
 
+impl<'a> Iterator for BestFittingVariantsIter<'a> {
+    type Item = &'a [FormatElement];
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.elements.first()? {
+            FormatElement::Tag(Tag::StartBestFittingEntry) => {
+                let end = self
+                    .elements
+                    .iter()
+                    .position(|element| {
+                        matches!(element, FormatElement::Tag(Tag::EndBestFittingEntry))
+                    })
+                    .map_or(self.elements.len(), |position| position + 1);
+
+                let (variant, rest) = self.elements.split_at(end);
+                self.elements = rest;
+
+                Some(variant)
+            }
+            _ => None,
+        }
+    }
+
+    fn last(mut self) -> Option<Self::Item>
+    where
+        Self: Sized,
+    {
+        self.next_back()
+    }
+}
+
+impl<'a> DoubleEndedIterator for BestFittingVariantsIter<'a> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        let start_position = self.elements.iter().rposition(|element| {
+            matches!(element, FormatElement::Tag(Tag::StartBestFittingEntry))
+        })?;
+
+        let (rest, variant) = self.elements.split_at(start_position);
+        self.elements = rest;
+        Some(variant)
+    }
+}
+
+impl FusedIterator for BestFittingVariantsIter<'_> {}
+
 pub trait FormatElements {
     /// Returns true if this [`FormatElement`] is guaranteed to break across multiple lines by the printer.
     /// This is the case if this format element recursively contains a:

crates/ruff_formatter/src/format_element/document.rs

@@ -35,7 +35,10 @@ impl Document {
         enum Enclosing<'a> {
             Group(&'a tag::Group),
             ConditionalGroup(&'a tag::ConditionalGroup),
-            FitsExpanded(&'a tag::FitsExpanded),
+            FitsExpanded {
+                tag: &'a tag::FitsExpanded,
+                expands_before: bool,
+            },
             BestFitting,
         }
 
@@ -43,7 +46,7 @@ impl Document {
             match enclosing.last() {
                 Some(Enclosing::Group(group)) => group.propagate_expand(),
                 Some(Enclosing::ConditionalGroup(group)) => group.propagate_expand(),
-                Some(Enclosing::FitsExpanded(fits_expanded)) => fits_expanded.propagate_expand(),
+                Some(Enclosing::FitsExpanded { tag, .. }) => tag.propagate_expand(),
                 _ => {}
             }
         }
@@ -85,23 +88,24 @@ impl Document {
                     FormatElement::BestFitting { variants, mode: _ } => {
                         enclosing.push(Enclosing::BestFitting);
 
-                        for variant in variants {
+                        propagate_expands(variants, enclosing, checked_interned);
-                            propagate_expands(variant, enclosing, checked_interned);
-                        }
-
-                        // Best fitting acts as a boundary
-                        expands = false;
                         enclosing.pop();
                         continue;
                     }
                     FormatElement::Tag(Tag::StartFitsExpanded(fits_expanded)) => {
-                        enclosing.push(Enclosing::FitsExpanded(fits_expanded));
+                        enclosing.push(Enclosing::FitsExpanded {
+                            tag: fits_expanded,
+                            expands_before: expands,
+                        });
                         false
                     }
                     FormatElement::Tag(Tag::EndFitsExpanded) => {
-                        enclosing.pop();
+                        if let Some(Enclosing::FitsExpanded { expands_before, .. }) =
-                        // Fits expanded acts as a boundary
+                            enclosing.pop()
-                        expands = false;
+                        {
+                            expands = expands_before;
+                        }
+
                         continue;
                     }
                     FormatElement::Text {
@@ -338,14 +342,20 @@ impl Format<IrFormatContext<'_>> for &[FormatElement] {
                 }
 
                 FormatElement::BestFitting { variants, mode } => {
-                    write!(f, [token("best_fitting([")])?;
+                    write!(f, [token("best_fitting(")])?;
+
+                    if *mode != BestFittingMode::FirstLine {
+                        write!(f, [text(&std::format!("mode: {mode:?}, "), None)])?;
+                    }
+
+                    write!(f, [token("[")])?;
                     f.write_elements([
                         FormatElement::Tag(StartIndent),
                         FormatElement::Line(LineMode::Hard),
                     ]);
 
                     for variant in variants {
-                        write!(f, [&**variant, hard_line_break()])?;
+                        write!(f, [variant, hard_line_break()])?;
                     }
 
                     f.write_elements([
@@ -353,13 +363,7 @@ impl Format<IrFormatContext<'_>> for &[FormatElement] {
                         FormatElement::Line(LineMode::Hard),
                     ]);
 
-                    write!(f, [token("]")])?;
+                    write!(f, [token("])")])?;
-
-                    if *mode != BestFittingMode::FirstLine {
-                        write!(f, [text(&std::format!(", mode: {mode:?}"), None),])?;
-                    }
-
-                    write!(f, [token(")")])?;
                 }
 
                 FormatElement::Interned(interned) => {
@@ -594,10 +598,10 @@ impl Format<IrFormatContext<'_>> for &[FormatElement] {
                             }
                         }
 
-                        StartEntry => {
+                        StartEntry | StartBestFittingEntry { .. } => {
                             // handled after the match for all start tags
                         }
-                        EndEntry => write!(f, [ContentArrayEnd])?,
+                        EndEntry | EndBestFittingEntry => write!(f, [ContentArrayEnd])?,
 
                         EndFill
                         | EndLabelled

crates/ruff_formatter/src/format_element/tag.rs

@@ -83,6 +83,9 @@ pub enum Tag {
 
     StartFitsExpanded(FitsExpanded),
     EndFitsExpanded,
+
+    StartBestFittingEntry,
+    EndBestFittingEntry,
 }
 
 impl Tag {
@@ -103,6 +106,7 @@ impl Tag {
                 | Tag::StartVerbatim(_)
                 | Tag::StartLabelled(_)
                 | Tag::StartFitsExpanded(_)
+                | Tag::StartBestFittingEntry,
         )
     }
 
@@ -129,6 +133,7 @@ impl Tag {
             StartVerbatim(_) | EndVerbatim => TagKind::Verbatim,
             StartLabelled(_) | EndLabelled => TagKind::Labelled,
             StartFitsExpanded { .. } | EndFitsExpanded => TagKind::FitsExpanded,
+            StartBestFittingEntry { .. } | EndBestFittingEntry => TagKind::BestFittingEntry,
         }
     }
 }
@@ -152,6 +157,7 @@ pub enum TagKind {
     Verbatim,
     Labelled,
     FitsExpanded,
+    BestFittingEntry,
 }
 
 #[derive(Debug, Copy, Default, Clone, Eq, PartialEq)]

crates/ruff_formatter/src/printer/mod.rs

@@ -288,7 +288,9 @@ impl<'a> Printer<'a> {
                 stack.push(TagKind::FitsExpanded, args);
             }
 
-            FormatElement::Tag(tag @ (StartLabelled(_) | StartEntry)) => {
+            FormatElement::Tag(
+                tag @ (StartLabelled(_) | StartEntry | StartBestFittingEntry { .. }),
+            ) => {
                 stack.push(tag.kind(), args);
             }
 
@@ -305,6 +307,7 @@ impl<'a> Printer<'a> {
                 | EndFitsExpanded
                 | EndVerbatim
                 | EndLineSuffix
+                | EndBestFittingEntry
                 | EndFill),
             ) => {
                 stack.pop(tag.kind())?;
@@ -495,47 +498,64 @@ impl<'a> Printer<'a> {
 
         if args.mode().is_flat() && self.state.measured_group_fits {
             queue.extend_back(variants.most_flat());
-            self.print_entry(queue, stack, args)
+            self.print_entry(queue, stack, args, TagKind::BestFittingEntry)
         } else {
             self.state.measured_group_fits = true;
-            let normal_variants = &variants[..variants.len() - 1];
+            let mut variants_iter = variants.into_iter();
+            let mut current = variants_iter.next().unwrap();
 
-            for variant in normal_variants {
+            for next in variants_iter {
                 // Test if this variant fits and if so, use it. Otherwise try the next
                 // variant.
 
                 // Try to fit only the first variant on a single line
-                if !matches!(variant.first(), Some(&FormatElement::Tag(Tag::StartEntry))) {
+                if !matches!(
-                    return invalid_start_tag(TagKind::Entry, variant.first());
+                    current.first(),
+                    Some(&FormatElement::Tag(Tag::StartBestFittingEntry))
+                ) {
+                    return invalid_start_tag(TagKind::BestFittingEntry, current.first());
                 }
 
                 // Skip the first element because we want to override the args for the entry and the
                 // args must be popped from the stack as soon as it sees the matching end entry.
-                let content = &variant[1..];
+                let content = &current[1..];
 
                 let entry_args = args
                     .with_print_mode(PrintMode::Flat)
                     .with_measure_mode(MeasureMode::from(mode));
 
                 queue.extend_back(content);
-                stack.push(TagKind::Entry, entry_args);
+                stack.push(TagKind::BestFittingEntry, entry_args);
                 let variant_fits = self.fits(queue, stack)?;
-                stack.pop(TagKind::Entry)?;
+                stack.pop(TagKind::BestFittingEntry)?;
 
                 // Remove the content slice because printing needs the variant WITH the start entry
                 let popped_slice = queue.pop_slice();
                 debug_assert_eq!(popped_slice, Some(content));
 
                 if variant_fits {
-                    queue.extend_back(variant);
+                    queue.extend_back(current);
-                    return self.print_entry(queue, stack, args.with_print_mode(PrintMode::Flat));
+                    return self.print_entry(
-                }
+                        queue,
+                        stack,
+                        args.with_print_mode(PrintMode::Flat),
+                        TagKind::BestFittingEntry,
+                    );
                 }
 
+                current = next;
+            }
+
+            // At this stage current is the most expanded.
+
             // No variant fits, take the last (most expanded) as fallback
-            let most_expanded = variants.most_expanded();
+            queue.extend_back(current);
-            queue.extend_back(most_expanded);
+            self.print_entry(
-            self.print_entry(queue, stack, args.with_print_mode(PrintMode::Expanded))
+                queue,
+                stack,
+                args.with_print_mode(PrintMode::Expanded),
+                TagKind::BestFittingEntry,
+            )
         }
     }
 
@@ -686,7 +706,7 @@ impl<'a> Printer<'a> {
         stack: &mut PrintCallStack,
         args: PrintElementArgs,
     ) -> PrintResult<()> {
-        self.print_entry(queue, stack, args)
+        self.print_entry(queue, stack, args, TagKind::Entry)
     }
 
     /// Semantic alias for [`Self::print_entry`] for fill separators.
@@ -696,7 +716,7 @@ impl<'a> Printer<'a> {
         stack: &mut PrintCallStack,
         args: PrintElementArgs,
     ) -> PrintResult<()> {
-        self.print_entry(queue, stack, args)
+        self.print_entry(queue, stack, args, TagKind::Entry)
     }
 
     /// Fully print an element (print the element itself and all its descendants)
@@ -708,32 +728,31 @@ impl<'a> Printer<'a> {
         queue: &mut PrintQueue<'a>,
         stack: &mut PrintCallStack,
         args: PrintElementArgs,
+        kind: TagKind,
     ) -> PrintResult<()> {
         let start_entry = queue.top();
 
-        if !matches!(start_entry, Some(&FormatElement::Tag(Tag::StartEntry))) {
+        if queue
-            return invalid_start_tag(TagKind::Entry, start_entry);
+            .pop()
+            .is_some_and(|start| start.tag_kind() == Some(kind))
+        {
+            stack.push(kind, args);
+        } else {
+            return invalid_start_tag(kind, start_entry);
         }
 
-        let mut depth = 0;
+        let mut depth = 1u32;
 
         while let Some(element) = queue.pop() {
             match element {
-                FormatElement::Tag(Tag::StartEntry) => {
+                FormatElement::Tag(Tag::StartEntry | Tag::StartBestFittingEntry) => {
-                    // Handle the start of the first element by pushing the args on the stack.
-                    if depth == 0 {
-                        depth = 1;
-                        stack.push(TagKind::Entry, args);
-                        continue;
-                    }
-
                     depth += 1;
                 }
-                FormatElement::Tag(Tag::EndEntry) => {
+                FormatElement::Tag(end_tag @ (Tag::EndEntry | Tag::EndBestFittingEntry)) => {
                     depth -= 1;
                     // Reached the end entry, pop the entry from the stack and return.
                     if depth == 0 {
-                        stack.pop(TagKind::Entry)?;
+                        stack.pop(end_tag.kind())?;
                         return Ok(());
                     }
                 }
@@ -745,7 +764,7 @@ impl<'a> Printer<'a> {
             self.print_element(stack, queue, element)?;
         }
 
-        invalid_end_tag(TagKind::Entry, stack.top_kind())
+        invalid_end_tag(kind, stack.top_kind())
     }
 
     fn print_char(&mut self, char: char) {
@@ -1148,11 +1167,14 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
                     PrintMode::Expanded => (variants.most_expanded(), args),
                 };
 
-                if !matches!(slice.first(), Some(FormatElement::Tag(Tag::StartEntry))) {
+                if !matches!(
-                    return invalid_start_tag(TagKind::Entry, slice.first());
+                    slice.first(),
+                    Some(FormatElement::Tag(Tag::StartBestFittingEntry))
+                ) {
+                    return invalid_start_tag(TagKind::BestFittingEntry, slice.first());
                 }
 
-                self.stack.push(TagKind::Entry, args);
+                self.stack.push(TagKind::BestFittingEntry, args);
                 self.queue.extend_back(&slice[1..]);
             }
 
@@ -1277,7 +1299,11 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
             }
 
             FormatElement::Tag(
-                tag @ (StartFill | StartVerbatim(_) | StartLabelled(_) | StartEntry),
+                tag @ (StartFill
+                | StartVerbatim(_)
+                | StartLabelled(_)
+                | StartEntry
+                | StartBestFittingEntry { .. }),
             ) => {
                 self.stack.push(tag.kind(), args);
             }
@@ -1294,6 +1320,7 @@ impl<'a, 'print> FitsMeasurer<'a, 'print> {
                 | EndAlign
                 | EndDedent
                 | EndIndent
+                | EndBestFittingEntry
                 | EndFitsExpanded),
             ) => {
                 self.stack.pop(tag.kind())?;