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Use interior mutability for StableHashingContext::caching_source_map.#154735

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Use interior mutability for StableHashingContext::caching_source_map.#154735
nnethercote wants to merge 3 commits intorust-lang:mainfrom
nnethercote:ref-Hcx

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@nnethercote nnethercote commented Apr 2, 2026 •
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This lets a lot of &mut hcx parameters change to &hcx, for symmetry with to_hash_stable_key, and avoids some unnecessary cloning of StableHashingContext. Details in individual commits.

@nnethercote
Use interior mutability for StableHashingContext::caching_source_map.
516ac2a 
`StableHashingContext::caching_source_map` is currently externally
mutated by `span_hash_stable`, which is reachable from many
`hash_stable` methods. But the mutation that happens is just some
internal caching, via `CachingSourceMap`. Conceptually it feels like a
read-only lookup of source map data.

This commit changes the field to interior mutability. This will let
`hash_stable` use `&Hcx` instead of `&mut Hcx`. The switch involves
replacing the existing `StableHashingContext::source_map` method with a
new `StableHashingContxt::span_data_to_lines_and_cols` method, because
returning a `&mut` was problematic with the use of `RefCell`. The
existing `CachingSourceMapView::span_data_to_lines_and_cols` now returns
an `Arc`, but that seems fine because the nearby
`CachingSourceMapView::byte_pos_to_line_and_col` already does that.
@nnethercote
Pass &hcx instead of &mut Hcx to hash_stable.
bd9b5ae 
This was enabled by the previous commit. A few related functions also
get a similar change.
@nnethercote
Avoid cloning of StableHashingContext.
79b99e6 
It's no longer needed now that `hash_stable` takes a `&hcx` instead of
`&mut hcx`.
@rustbot rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-query-system Area: The rustc query system (https://rustc-dev-guide.rust-lang.org/query.html) S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 2, 2026
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nnethercote commented Apr 2, 2026

@bors try @rust-timer queue

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rust-bors bot pushed a commit that referenced this pull request Apr 2, 2026
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Auto merge of #154735 - nnethercote:ref-Hcx, r=<try>
f43d2c0 
Use interior mutability for `StableHashingContext::caching_source_map`.
@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 2, 2026
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rust-bors bot commented Apr 3, 2026

☔ The latest upstream changes (presumably #154727) made this pull request unmergeable. Please resolve the merge conflicts.

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rust-bors bot commented Apr 3, 2026

☀️ Try build successful (CI)
Build commit: f43d2c0 (f43d2c0d111676f3fea7786460af17e466d1455b, parent: e6b64a2f4c696b840f8a384ec28690eed6a5d267)

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rust-timer commented Apr 3, 2026

Finished benchmarking commit (f43d2c0): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

mean range count
Regressions ❌
(primary)		 0.2% [0.2%, 0.3%] 10
Regressions ❌
(secondary)		 0.2% [0.1%, 0.3%] 23
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -0.1% [-0.1%, -0.0%] 3
All ❌✅ (primary) 0.2% [0.2%, 0.3%] 10

Max RSS (memory usage)

Results (primary -2.2%, secondary 0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary)		 - - 0
Regressions ❌
(secondary)		 3.4% [2.2%, 4.6%] 2
Improvements ✅
(primary)		 -2.2% [-2.2%, -2.2%] 1
Improvements ✅
(secondary)		 -2.7% [-3.6%, -1.8%] 2
All ❌✅ (primary) -2.2% [-2.2%, -2.2%] 1

Cycles

Results (secondary 0.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary)		 - - 0
Regressions ❌
(secondary)		 6.3% [3.9%, 8.5%] 3
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -4.5% [-4.8%, -4.2%] 3
All ❌✅ (primary) - - 0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 487.024s -> 488.913s (0.39%)
Artifact size: 395.12 MiB -> 395.23 MiB (0.03%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Apr 3, 2026
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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-query-system Area: The rustc query system (https://rustc-dev-guide.rust-lang.org/query.html) perf-regression Performance regression. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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