From b717c74823256d439855bad453a08710e5731a31 Mon Sep 17 00:00:00 2001
From: Lukas Wirth <me@lukaswirth.dev>
Date: Sun, 28 Dec 2025 11:18:43 +0100
Subject: [PATCH] perf: Pre-allocate some buffers in parsing

---
 src/tools/rust-analyzer/crates/parser/src/input.rs     | 2 +-
 src/tools/rust-analyzer/crates/parser/src/lexed_str.rs | 7 ++++++-
 src/tools/rust-analyzer/crates/parser/src/parser.rs    | 2 +-
 src/tools/rust-analyzer/crates/span/src/ast_id.rs      | 5 +++--
 4 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/tools/rust-analyzer/crates/parser/src/input.rs b/src/tools/rust-analyzer/crates/parser/src/input.rs
index 57eeb431cdc2..42e8a400edd1 100644
--- a/src/tools/rust-analyzer/crates/parser/src/input.rs
+++ b/src/tools/rust-analyzer/crates/parser/src/input.rs
@@ -97,7 +97,7 @@ fn bit_index(&self, n: usize) -> (usize, usize) {
         let b_idx = n % (bits::BITS as usize);
         (idx, b_idx)
     }
-    fn len(&self) -> usize {
+    pub fn len(&self) -> usize {
         self.kind.len()
     }
 }
diff --git a/src/tools/rust-analyzer/crates/parser/src/lexed_str.rs b/src/tools/rust-analyzer/crates/parser/src/lexed_str.rs
index 7c78ba8faf5f..d7eec6cde8c0 100644
--- a/src/tools/rust-analyzer/crates/parser/src/lexed_str.rs
+++ b/src/tools/rust-analyzer/crates/parser/src/lexed_str.rs
@@ -150,7 +150,12 @@ struct Converter<'a> {
 impl<'a> Converter<'a> {
     fn new(edition: Edition, text: &'a str) -> Self {
         Self {
-            res: LexedStr { text, kind: Vec::new(), start: Vec::new(), error: Vec::new() },
+            res: LexedStr {
+                text,
+                kind: Vec::with_capacity(text.len() / 3),
+                start: Vec::with_capacity(text.len() / 3),
+                error: Vec::new(),
+            },
             offset: 0,
             edition,
         }
diff --git a/src/tools/rust-analyzer/crates/parser/src/parser.rs b/src/tools/rust-analyzer/crates/parser/src/parser.rs
index c41bd593c6a2..4557078de991 100644
--- a/src/tools/rust-analyzer/crates/parser/src/parser.rs
+++ b/src/tools/rust-analyzer/crates/parser/src/parser.rs
@@ -32,7 +32,7 @@ pub(crate) struct Parser<'t> {
 
 impl<'t> Parser<'t> {
     pub(super) fn new(inp: &'t Input) -> Parser<'t> {
-        Parser { inp, pos: 0, events: Vec::new(), steps: Cell::new(0) }
+        Parser { inp, pos: 0, events: Vec::with_capacity(2 * inp.len()), steps: Cell::new(0) }
     }
 
     pub(crate) fn finish(self) -> Vec<Event> {
diff --git a/src/tools/rust-analyzer/crates/span/src/ast_id.rs b/src/tools/rust-analyzer/crates/span/src/ast_id.rs
index e54e0bd2fcc6..3bf14dea75b3 100644
--- a/src/tools/rust-analyzer/crates/span/src/ast_id.rs
+++ b/src/tools/rust-analyzer/crates/span/src/ast_id.rs
@@ -603,8 +603,9 @@ pub fn from_source(node: &SyntaxNode) -> AstIdMap {
         // After all, the block will then contain the *outer* item, so we allocate
         // an ID for it anyway.
         let mut blocks = Vec::new();
-        let mut curr_layer = vec![(node.clone(), None)];
-        let mut next_layer = vec![];
+        let mut curr_layer = Vec::with_capacity(32);
+        curr_layer.push((node.clone(), None));
+        let mut next_layer = Vec::with_capacity(32);
         while !curr_layer.is_empty() {
             curr_layer.drain(..).for_each(|(node, parent_idx)| {
                 let mut preorder = node.preorder();