perf(diagnostics): reduce graphical rendering allocations (#25711)

Keep common graphical-rendering buffers inline and add a focused
benchmark for regressions.

AI assistance: Codex was used. The contributor remains responsible for
this change.

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crates/oxc_diagnostics/src/handlers/graphical/label.rs

@@ -13,6 +13,7 @@ use std::{
 };
 
 use owo_colors::{OwoColorize, Style};
+use smallvec::SmallVec;
 
 use super::{
     handler::GraphicalReportHandler,
@@ -133,7 +134,7 @@ impl GraphicalReportHandler {
         let mut highest = 0;
 
         let chars = &self.theme.characters;
-        let mut vbar_offsets = Vec::with_capacity(single_liners.len());
+        let mut vbar_offsets = SmallVec::<[_; 2]>::with_capacity(single_liners.len());
         for &hl in single_liners {
             let byte_start = hl.offset();
             let byte_end = hl.offset() + hl.len();

crates/oxc_diagnostics/src/handlers/graphical/line.rs

@@ -12,6 +12,7 @@
 
 use std::str::{CharIndices, from_utf8};
 
+use smallvec::SmallVec;
 use unicode_segmentation::UnicodeSegmentation;
 use unicode_width::UnicodeWidthStr;
 
@@ -203,7 +204,7 @@ impl GraphicalReportHandler {
     }
 
     /// Splits already-scanned span contents into [`Line`]s.
-    pub(super) fn get_lines<'a>(context_data: &SpanContents<'a>) -> Vec<Line<'a>> {
+    pub(super) fn get_lines<'a>(context_data: &SpanContents<'a>) -> SmallVec<[Line<'a>; 3]> {
         let context = from_utf8(context_data.data()).expect("Bad utf8 detected");
         let mut line = context_data.line();
         let base = context_data.span().start as usize;
@@ -213,7 +214,7 @@ impl GraphicalReportHandler {
         // Cap the hint by byte length because custom sources own this metadata.
         let capacity =
             context_data.line_count().saturating_sub(context_data.line()).max(1).min(bytes.len());
-        let mut lines = Vec::with_capacity(capacity);
+        let mut lines = SmallVec::with_capacity(capacity);
         let mut start = 0;
         for newline in memchr::memchr_iter(b'\n', bytes) {
             let end = newline + 1;

crates/oxc_diagnostics/src/handlers/graphical/snippet.rs

@@ -11,6 +11,7 @@
 use std::{borrow::Cow, cmp::max, fmt};
 
 use owo_colors::OwoColorize;
+use smallvec::SmallVec;
 
 use super::{
     handler::GraphicalReportHandler,
@@ -112,7 +113,7 @@ impl GraphicalReportHandler {
             .copied()
             .zip(self.theme.styles.highlights.iter().copied().cycle())
             .map(|(label, style)| FancySpan::new(label.label(), label.span(), style))
-            .collect::<Vec<_>>();
+            .collect::<SmallVec<[_; 2]>>();
 
         // Find the maximum number of active gutter lines to determine indentation.
         let mut max_gutter = 0usize;
@@ -166,7 +167,7 @@ impl GraphicalReportHandler {
             self.render_line_gutter(f, max_gutter, line, &labels)?;
             Self::render_line_text(f, line.text)?;
 
-            let (single_line, multi_line): (Vec<_>, Vec<_>) = labels
+            let (single_line, multi_line): (SmallVec<[_; 2]>, SmallVec<[_; 2]>) = labels
                 .iter()
                 .filter(|hl| line.span_applies(hl))
                 .partition(|hl| line.span_line_only(hl));