Reduce memory used by diagnostics

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);
     }