Accept trivial consts based on trivial consts #148182

saethlin · 2025-10-27T17:37:27Z

This is an expansion of #148040.

The previous implementation only accepted trivial consts that assign a literal. For example:

const A: usize = 0;
const B: usize = A;

Before this PR, only A was a trivial const. Now B is too.

saethlin · 2025-10-27T17:37:38Z

@bors try @rust-timer queue

Accept trivial consts based on trivial consts

rust-bors · 2025-10-27T19:53:31Z

☀️ Try build successful (CI)
Build commit: 2f43088 (2f430888c54e6073a1742439ac5020b07745922b, parent: 9ea8d67cc60e88ad6fffbf299a454c44227e001c)

rust-timer · 2025-10-27T23:20:15Z

Finished benchmarking commit (2f43088): 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
Regressions ❌ (secondary)	0.4%	[0.1%, 0.9%]	3
Improvements ✅ (primary)	-0.7%	[-1.1%, -0.2%]	13
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.7%	[-1.1%, -0.2%]	13

Max RSS (memory usage)

Results (primary 0.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.0%	[0.6%, 3.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-2.4%, 3.3%]	3

Cycles

Results (primary -4.4%, secondary -2.1%)

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)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-4.4%	[-5.1%, -3.6%]	2
Improvements ✅ (secondary)	-5.8%	[-5.8%, -5.8%]	1
All ❌✅ (primary)	-4.4%	[-5.1%, -3.6%]	2

Binary size

Results (primary -0.3%, secondary -0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	2
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.0%]	16
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	2
All ❌✅ (primary)	-0.3%	[-0.6%, 0.0%]	18

Bootstrap: 474.628s -> 474.127s (-0.11%)
Artifact size: 390.61 MiB -> 390.58 MiB (-0.01%)

cjgillot · 2025-10-28T01:42:25Z

compiler/rustc_mir_transform/src/trivial_const.rs

+            tcx.trivial_const(def)
        }
+        Const::Val(v, ty) => Some((v, ty)),
    }


Should the query just return c.const_ in general?

What would callers do with Const::Ty and Const::Unevaluated?

rustbot · 2025-10-28T03:46:13Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

rustbot · 2025-10-28T03:46:15Z

r? @eholk

rustbot has assigned @eholk.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

eholk · 2025-10-28T19:15:14Z

@bors r+

bors · 2025-10-28T19:15:17Z

📌 Commit 9cbfbb1 has been approved by eholk

It is now in the queue for this repository.

bors · 2025-10-28T22:52:11Z

⌛ Testing commit 9cbfbb1 with merge 044d68c...

bors · 2025-10-29T01:58:45Z

☀️ Test successful - checks-actions
Approved by: eholk
Pushing 044d68c to master...

github-actions · 2025-10-29T02:01:36Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 278a909 (parent) -> 044d68c (this PR)

Test differences

Show 8 test diffs

8 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 044d68c3cb6a0b893b18293fa7f5719119403215 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-apple-various: 4159.4s -> 2986.2s (-28.2%)
pr-check-1: 1934.5s -> 1564.7s (-19.1%)
dist-aarch64-apple: 7564.8s -> 6393.5s (-15.5%)
aarch64-gnu-debug: 4142.3s -> 4613.0s (11.4%)
dist-powerpc-linux: 5524.7s -> 5049.2s (-8.6%)
aarch64-apple: 9692.6s -> 8900.4s (-8.2%)
dist-i686-mingw: 10055.3s -> 9271.7s (-7.8%)
x86_64-msvc-2: 7028.8s -> 6482.2s (-7.8%)
dist-aarch64-msvc: 6239.4s -> 5783.6s (-7.3%)
x86_64-gnu-gcc: 3174.4s -> 3405.6s (7.3%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-10-29T03:09:55Z

Finished benchmarking commit (044d68c): comparison URL.

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

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.7%	[-1.1%, -0.2%]	13
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	-0.7%	[-1.1%, -0.2%]	13

Max RSS (memory usage)

Results (secondary -3.9%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.9%	[-3.9%, -3.8%]	2
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.6%, secondary -5.7%)

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)	-	-	0
Improvements ✅ (primary)	-2.6%	[-3.1%, -2.1%]	2
Improvements ✅ (secondary)	-5.7%	[-5.7%, -5.7%]	1
All ❌✅ (primary)	-2.6%	[-3.1%, -2.1%]	2

Binary size

Results (primary -0.3%, secondary -0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	2
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.0%]	16
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-0.3%	[-0.6%, 0.0%]	18

Bootstrap: 474.493s -> 474.307s (-0.04%)
Artifact size: 390.04 MiB -> 390.07 MiB (0.01%)

Kobzol · 2025-11-03T20:54:26Z

Tiny regression on a secondary benchmark, improvements outweigh regressions.

@rustbot label: +perf-regression-triaged

Accept trivial consts based on trivial consts

9cbfbb1

rustbot added 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 Oct 27, 2025

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rust-bors bot added a commit that referenced this pull request Oct 27, 2025

Auto merge of #148182 - saethlin:trivial-consts-recursive, r=<try>

2f43088

Accept trivial consts based on trivial consts

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 27, 2025

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Oct 27, 2025

cjgillot reviewed Oct 28, 2025

View reviewed changes

saethlin marked this pull request as ready for review October 28, 2025 03:46

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 28, 2025

rustbot assigned eholk Oct 28, 2025

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 28, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Oct 29, 2025

bors merged commit 044d68c into rust-lang:master Oct 29, 2025
13 checks passed

rustbot added this to the 1.93.0 milestone Oct 29, 2025

bors mentioned this pull request Oct 29, 2025

Mention crate being analyzed in query description #147137

Merged

saethlin deleted the trivial-consts-recursive branch October 29, 2025 05:21

rustbot added the perf-regression-triaged The performance regression has been triaged. label Nov 3, 2025

Accept trivial consts based on trivial consts #148182

Accept trivial consts based on trivial consts #148182

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

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Test differences

Job duration changes

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

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saethlin commented Oct 27, 2025 •

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