From 73f9f3c00809ce793f34bbd4a367e02b8d46d304 Mon Sep 17 00:00:00 2001
From: Charlie Marsh <charlie.r.marsh@gmail.com>
Date: Sun, 27 Sep 2026 11:02:17 -0400
Subject: [PATCH] Reduce memory used by diagnostics

---
 crates/ruff_db/src/diagnostic/mod.rs | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/crates/ruff_db/src/diagnostic/mod.rs b/crates/ruff_db/src/diagnostic/mod.rs
index a39fb593c6cf35..2acd785ad1c73d 100644
--- a/crates/ruff_db/src/diagnostic/mod.rs
+++ b/crates/ruff_db/src/diagnostic/mod.rs
@@ -145,7 +145,13 @@ impl Diagnostic {
     /// should be constructed via [`Annotation::primary`]. A diagnostic with no
     /// primary annotations is allowed, but its rendering may be sub-optimal.
     pub fn annotate(&mut self, ann: Annotation) {
-        Arc::make_mut(&mut self.inner).annotations.push(ann);
+        let annotations = &mut Arc::make_mut(&mut self.inner).annotations;
+        // Most diagnostics have exactly one annotation. Avoid the default allocation of four
+        // annotations on the first push, while retaining normal amortized growth thereafter.
+        if annotations.is_empty() {
+            annotations.reserve_exact(1);
+        }
+        annotations.push(ann);
     }
 
     /// Adds an "info" sub-diagnostic with the given message.
@@ -172,9 +178,11 @@ impl Diagnostic {
 
     /// Adds an "info" sub-diagnostic before any existing sub-diagnostics.
     pub fn prepend_info<'a>(&mut self, message: impl IntoDiagnosticMessage + 'a) {
-        Arc::make_mut(&mut self.inner)
-            .subs
-            .insert(0, SubDiagnostic::new(SubDiagnosticSeverity::Info, message));
+        let subs = &mut Arc::make_mut(&mut self.inner).subs;
+        if subs.is_empty() {
+            subs.reserve_exact(1);
+        }
+        subs.insert(0, SubDiagnostic::new(SubDiagnosticSeverity::Info, message));
     }
 
     /// Adds a "help" sub-diagnostic with the given message.
@@ -190,7 +198,11 @@ impl Diagnostic {
     /// to it. For the simpler case of a sub-diagnostic with only a message,
     /// using a method like [`Diagnostic::info`] may be more convenient.
     pub fn sub(&mut self, sub: SubDiagnostic) {
-        Arc::make_mut(&mut self.inner).subs.push(sub);
+        let subs = &mut Arc::make_mut(&mut self.inner).subs;
+        if subs.is_empty() {
+            subs.reserve_exact(1);
+        }
+        subs.push(sub);
     }
 
     /// Return a `std::fmt::Display` implementation that renders this
@@ -685,6 +697,9 @@ impl SubDiagnostic {
     /// to have no annotations (e.g., a simple note) than for a diagnostic to
     /// have no annotations.
     pub fn annotate(&mut self, ann: Annotation) {
+        if self.inner.annotations.is_empty() {
+            self.inner.annotations.reserve_exact(1);
+        }
         self.inner.annotations.push(ann);
     }