Fix performance regression in LintLevelsBuilder

Author: michaelwoerister

Cargo.lock

@@ -4069,6 +4069,7 @@ dependencies = [
  "rustc_target",
  "rustc_trait_selection",
  "rustc_type_ir",
+ "smallvec",
  "tracing",
  "unicode-security",
 ]

compiler/rustc_lint/Cargo.toml

@@ -23,6 +23,7 @@ rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
 rustc_type_ir = { path = "../rustc_type_ir" }
+smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 tracing = "0.1"
 unicode-security = "0.1.0"
 # tidy-alphabetical-end

compiler/rustc_lint/src/levels.rs

@@ -40,6 +40,7 @@ use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
+use smallvec::SmallVec;
 
 use crate::errors::{
     MalformedAttribute, MalformedAttributeSub, OverruledAttribute, OverruledAttributeSub,
@@ -1028,16 +1029,27 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
         }
 
         if !is_crate_node {
-            for (id, &(level, ref src)) in self.current_specs().to_sorted_stable_ord() {
-                if !id.lint.crate_level_only {
-                    continue;
-                }
-
-                let LintLevelSource::Node { name: lint_attr_name, span: lint_attr_span, .. } = *src
-                else {
-                    continue;
-                };
+            // Make sure we filter before collecting and sorting, so we don't
+            // waste time stabilizing the iteration order only to throw it away
+            let relevant_specs: SmallVec<[_; 8]> = self
+                .current_specs()
+                .items()
+                .filter(|(id, _)| id.lint.crate_level_only)
+                .filter_map(|(id, &(level, ref src))| {
+                    if let LintLevelSource::Node {
+                        name: lint_attr_name,
+                        span: lint_attr_span,
+                        ..
+                    } = *src
+                    {
+                        Some((id, level, lint_attr_name, lint_attr_span))
+                    } else {
+                        None
+                    }
+                })
+                .collect_stable_ord_by_key(|&(id, ..)| id);
 
+            for (_, level, lint_attr_name, lint_attr_span) in relevant_specs {
                 self.emit_spanned_lint(
                     UNUSED_ATTRIBUTES,
                     lint_attr_span.into(),