From 54ff9a2290cca64550d8fa5692f9d3d91ef174ba Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov@redhat.com>
Date: Thu, 23 Jul 2026 12:26:19 +0200
Subject: [PATCH] Optimize crate resolution

By keeping a map from hash to crate number.
---
 compiler/rustc_metadata/src/creader.rs | 25 +++++++++----------------
 1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/compiler/rustc_metadata/src/creader.rs b/compiler/rustc_metadata/src/creader.rs
index bb91d855feaeb..8879c175da2f9 100644
--- a/compiler/rustc_metadata/src/creader.rs
+++ b/compiler/rustc_metadata/src/creader.rs
@@ -34,7 +34,7 @@ use rustc_span::def_id::DefId;
 use rustc_span::edition::Edition;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, sym};
 use rustc_target::spec::{PanicStrategy, Target};
-use tracing::{debug, info, trace};
+use tracing::{debug, info};
 
 use crate::diagnostics;
 use crate::locator::{CrateError, CrateLocator, CratePaths, CrateRejections};
@@ -69,6 +69,9 @@ pub struct CStore {
     /// This crate has a `#[alloc_error_handler]` item.
     has_alloc_error_handler: bool,
 
+    /// Cached map from hash to CrateNum, to avoid scanning metas during crate resolution.
+    hash_to_cnum: UnordMap<Svh, CrateNum>,
+
     /// Names that were used to load the crates via `extern crate` or paths.
     resolved_externs: UnordMap<Symbol, CrateNum>,
 
@@ -237,6 +240,7 @@ impl CStore {
 
     fn set_crate_data(&mut self, cnum: CrateNum, data: CrateMetadata) {
         assert!(self.metas[cnum].is_none(), "Overwriting crate metadata entry");
+        self.hash_to_cnum.insert(data.hash(), cnum);
         self.metas[cnum] = Some(Box::new(data));
     }
 
@@ -546,6 +550,7 @@ impl CStore {
             alloc_error_handler_kind: None,
             has_global_allocator: false,
             has_alloc_error_handler: false,
+            hash_to_cnum: UnordMap::default(),
             resolved_externs: UnordMap::default(),
             unused_externs: Vec::new(),
             used_extern_options: Default::default(),
@@ -555,21 +560,9 @@ impl CStore {
 
     fn existing_match(&self, name: Symbol, hash: Option<Svh>) -> Option<CrateNum> {
         let hash = hash?;
-
-        for (cnum, data) in self.iter_crate_data() {
-            if data.name() != name {
-                trace!("{} did not match {}", data.name(), name);
-                continue;
-            }
-
-            if hash == data.hash() {
-                return Some(cnum);
-            } else {
-                debug!("actual hash {} did not match expected {}", hash, data.hash());
-            }
-        }
-
-        None
+        let cnum = *self.hash_to_cnum.get(&hash)?;
+        debug_assert_eq!(self.get_crate_data(cnum).name(), name);
+        Some(cnum)
     }
 
     /// Determine whether a dependency should be considered private.