From f38bdb42581d4cc7c50845f9f43fc79119e6f173 Mon Sep 17 00:00:00 2001
From: Makro <mnlkroiss@gmail.com>
Date: Mon, 13 Jul 2026 19:31:38 +0000
Subject: [PATCH] perf: skip span hashing in non-incremental builds

---
 compiler/rustc_middle/src/ich.rs          | 22 +++++++++++++---------
 compiler/rustc_query_impl/src/plumbing.rs |  7 ++++++-
 compiler/rustc_span/src/lib.rs            |  5 +++--
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/compiler/rustc_middle/src/ich.rs b/compiler/rustc_middle/src/ich.rs
index 80db9a95c918a..fb2ed2b6d1312 100644
--- a/compiler/rustc_middle/src/ich.rs
+++ b/compiler/rustc_middle/src/ich.rs
@@ -22,9 +22,9 @@ enum CachingSourceMap<'a> {
 /// things (e.g., each `DefId`/`DefPath` is only hashed once).
 pub struct StableHashState<'a> {
     untracked: &'a Untracked,
-    // The value of `-Z incremental-ignore-spans`.
-    // This field should only be used by `unstable_opts_incremental_ignore_span`
-    incremental_ignore_spans: bool,
+    // The session-wide default for `hash_spans`, computed in `Self::new`. Only
+    // used by `assert_default_stable_hash_controls`.
+    default_hash_spans: bool,
     caching_source_map: CachingSourceMap<'a>,
     stable_hash_controls: StableHashControls,
 }
@@ -32,11 +32,15 @@ pub struct StableHashState<'a> {
 impl<'a> StableHashState<'a> {
     #[inline]
     pub fn new(sess: &'a Session, untracked: &'a Untracked) -> Self {
-        let hash_spans_initial = !sess.opts.unstable_opts.incremental_ignore_spans;
+        // Only hash spans for incremental and coverage, where hashes must react to
+        // code moving around; otherwise skip them and their source map lookups.
+        // Spans are also skipped if `-Z incremental-ignore-spans` is set.
+        let hash_spans_initial = (sess.opts.incremental.is_some() || sess.instrument_coverage())
+            && !sess.opts.unstable_opts.incremental_ignore_spans;
 
         StableHashState {
             untracked,
-            incremental_ignore_spans: sess.opts.unstable_opts.incremental_ignore_spans,
+            default_hash_spans: hash_spans_initial,
             caching_source_map: CachingSourceMap::Unused(sess.source_map()),
             stable_hash_controls: StableHashControls { hash_spans: hash_spans_initial },
         }
@@ -178,15 +182,15 @@ impl<'a> StableHashCtxt for StableHashState<'a> {
         let stable_hash_controls = self.stable_hash_controls;
         let StableHashControls { hash_spans } = stable_hash_controls;
 
-        // Note that we require that `hash_spans` be the inverse of the global `-Z
-        // incremental-ignore-spans` option. Normally, this option is disabled, in which case
-        // `hash_spans` must be true.
+        // Note that we require that `hash_spans` matches the session-wide default computed in
+        // `StableHashState::new` from the compilation mode and the global `-Z
+        // incremental-ignore-spans` option.
         //
         // Span hashing can also be disabled without `-Z incremental-ignore-spans`. This is the
         // case for instance when building a hash for name mangling. Such configuration must not be
         // used for metadata.
         assert_eq!(
-            hash_spans, !self.incremental_ignore_spans,
+            hash_spans, self.default_hash_spans,
             "Attempted hashing of {msg} with non-default StableHashControls: {stable_hash_controls:?}"
         );
     }
diff --git a/compiler/rustc_query_impl/src/plumbing.rs b/compiler/rustc_query_impl/src/plumbing.rs
index 8b7a2ccb232d0..4e21e4e14b8f5 100644
--- a/compiler/rustc_query_impl/src/plumbing.rs
+++ b/compiler/rustc_query_impl/src/plumbing.rs
@@ -100,7 +100,12 @@ fn encode_query_values_inner<'a, 'tcx, C, V>(
 }
 
 pub(crate) fn verify_query_key_hashes<'tcx>(tcx: TyCtxt<'tcx>) {
-    if tcx.sess.opts.unstable_opts.incremental_verify_ich || cfg!(debug_assertions) {
+    // Only run this if the dep graph is enabled. Otherwise there are no `DepNode`s
+    // to collide, and non-incremental builds skip span hashing, so keys differing
+    // only in spans collide by design.
+    if tcx.dep_graph.is_fully_enabled()
+        && (tcx.sess.opts.unstable_opts.incremental_verify_ich || cfg!(debug_assertions))
+    {
         tcx.sess.time("verify_query_key_hashes", || {
             for_each_query_vtable!(ALL, tcx, |query| {
                 verify_query_key_hashes_inner(query, tcx);
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 5015741f10c5f..05095a0d99404 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -1983,8 +1983,9 @@ impl<S: SpanEncoder> Encodable<S> for SourceFile {
         self.normalized_source_len.encode(s);
         self.unnormalized_source_len.encode(s);
 
-        // We are always in `Lines` form by the time we reach here.
-        assert!(self.lines.read().is_lines());
+        // Imported files may still be in compressed `Diffs` form if nothing needed
+        // their line table (non-incremental builds do not force the conversion by
+        // hashing spans). `lines()` converts on demand.
         let lines = self.lines();
         // Store the length.
         s.emit_u32(lines.len() as u32);