Patterns: represent constants as valtrees by RalfJung · Pull Request #144591 · rust-lang/rust

RalfJung · 2025-07-28T16:23:45Z

Const patterns are always valtrees now. Let's represent that in the types. We use ty::Value for this since it nicely packages value and type, and has some convenient methods.

Cc @Nadrieril @BoxyUwU

rustbot · 2025-07-28T16:23:49Z

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rustbot · 2025-07-28T16:23:51Z

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred in match lowering

cc @Nadrieril

Some changes occurred in exhaustiveness checking

cc @Nadrieril

Some changes occurred in match checking

cc @Nadrieril

RalfJung · 2025-07-28T16:25:07Z

compiler/rustc_mir_build/src/builder/matches/test.rs


-                let mut expect_ty = value.ty();
+                let mut expect_ty = value_ty;
+                let value = Const::Ty(value_ty, ty::Const::new_value(tcx, value, value_ty));


Oddly, constructing a mir::Const from a valtree requires the type twice...

Const::Ty storing more than just a ty::Const is kinda meh yeah. This came about when we stopped storing a Ty alongside all ty::Consts. Removing the ty field from Const::Ty would require updating the function mir::Const::ty( to take either a TypingEnv so we can lookup the type of const parameters, and so that we can normalize the resulting type from type_of unevaluated constants and const generic parameter declarations

It is odd that mir::Const and ty::Value know their type but ty::Const does not... but I won't touch this now, there's probably a reason we ended up here.^^

you can likely find a bit of context in #125958 as to how ty::Const wound up the way it did. For ty::Const only ty::Value needs a Ty because everything else we can "figure out" the type from the environment we're in or from the ty::Const itself.

I would expect that mir::Const::Unevaluated doesnt really need to store a Ty either as it should just be equivalent to type_of(uv.def).instantiate(uv.args) and then a normalization call 🤔

tests/ui/thir-print/thir-tree-loop-match.stdout

RalfJung · 2025-07-28T16:31:39Z

compiler/rustc_pattern_analysis/src/rustc.rs

+                let max = ty.numeric_max_val(cx.tcx).unwrap();
+                let max = ty::ValTree::from_scalar_int(cx.tcx, max.try_to_scalar_int().unwrap());


This is a kind of silly dance. Maybe numeric_max_val should just return a ScalarInt?

RalfJung · 2025-07-28T21:18:04Z

@rust-timer queue
@bors try

rust-bors · 2025-07-28T21:18:07Z

⌛ Trying commit b39c019 with merge e111586…

To cancel the try build, run the command @bors try cancel.

Patterns: represent constants as valtrees

rust-bors · 2025-07-28T23:33:17Z

☀️ Try build successful (CI)
Build commit: e111586 (e1115864e9f2387f19e755d0a8a30df5a8933f7c, parent: e3514bde96d2d13586337a48db77fa64b850d249)

BoxyUwU · 2025-07-29T00:41:21Z

We could use ty::Value which nicely pairs up a valtree and a value -- however, a const pattern can be a raw pointer, which is not a valid "type system value".

Given that we evaluate all const patterns by going through a ty::Const or ty::UnevaluatedConst we already don't get the property that ty::Const s only have to deal with stuff in const generics. I'm not sure if this is desirable or not and I think I need to finish adt_const_params before I'll be able to tell for sure 🤔

Regardless, with this in mind I think using ty::Value is fine, we can always swap it out for a different Ty/ValTree tuple down the road if it winds up being a problem. It doesn't seem worth making code-quality worse in order to try and maintain a property that doesn't even hold rn and I don't know how we even would do so nicely :D

rust-timer · 2025-07-29T01:02:57Z

Finished benchmarking commit (e111586): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Improvements ✅ (primary)	-2.0%	[-2.8%, -0.1%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-2.8%, -0.1%]	7

Max RSS (memory usage)

Results (secondary -0.8%)

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)	2.3%	[2.3%, 2.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.9%	[-3.1%, -2.7%]	3
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -3.3%, secondary 2.3%)

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)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-3.3%	[-3.3%, -3.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.3%	[-3.3%, -3.3%]	1

Binary size

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

Bootstrap: 468.526s -> 467.78s (-0.16%)
Artifact size: 376.81 MiB -> 376.82 MiB (0.00%)

RalfJung · 2025-07-29T07:01:42Z

compiler/rustc_pattern_analysis/src/rustc.rs

    type Error = ErrorGuaranteed;
    type VariantIdx = VariantIdx;
-    type StrLit = Const<'tcx>;
+    type StrLit = ty::Value<'tcx>;


Oddly it seems like I can just change this type and nobody cares...

RalfJung · 2025-07-29T07:03:57Z

compiler/rustc_mir_build/src/builder/matches/mod.rs

    Success,
    /// Branch corresponding to this constant.
-    Constant(Const<'tcx>, u128),
+    Constant(ty::Value<'tcx>, u128),


This is the one place where dropping the type and just using a ValTree would work... but for consistency I kept it here, too.

RalfJung · 2025-07-29T07:04:33Z

@bors try
@rust-timer queue

Patterns: represent constants as valtrees

rust-bors · 2025-07-29T07:04:37Z

⌛ Trying commit 9f3f015 with merge 13cfa7a…

To cancel the try build, run the command @bors try cancel.

rust-bors · 2025-07-29T09:20:47Z

☀️ Try build successful (CI)
Build commit: 13cfa7a (13cfa7a8f9f23454b856e7ac703311674fe94229, parent: cb6785f73df1aa3f558796a22a4ab9652cf38e26)

Co-authored-by: Boxy <rust@boxyuwu.dev>

RalfJung · 2025-08-14T07:45:00Z

@rustbot ready

BoxyUwU · 2025-08-14T12:50:22Z

@bors r+ rollup=never

I don't know why the perf regression is there 🤔 I have to assume its just noise from boring code movement changes.

bors · 2025-08-14T12:50:25Z

📌 Commit 2330afa has been approved by BoxyUwU

It is now in the queue for this repository.

BoxyUwU · 2025-08-14T13:25:42Z

ah actually @RalfJung can you update the PR description? its a bit out of date now as we're using ty::Value

bors · 2025-08-15T05:49:58Z

⌛ Testing commit 2330afa with merge 8800ec1...

bors · 2025-08-15T09:12:01Z

☀️ Test successful - checks-actions
Approved by: BoxyUwU
Pushing 8800ec1 to master...

github-actions · 2025-08-15T09:15:08Z

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 3507a74 (parent) -> 8800ec1 (this PR)

Test differences

Show 2 test diffs

2 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 8800ec16657b24ad8a2f443c133bf0b56ae76033 --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

x86_64-apple-1: 6428.2s -> 9301.0s (44.7%)
dist-aarch64-apple: 5388.6s -> 6702.5s (24.4%)
pr-check-1: 1448.5s -> 1717.3s (18.6%)
i686-gnu-2: 5392.1s -> 6201.2s (15.0%)
aarch64-apple: 6410.0s -> 5535.6s (-13.6%)
dist-aarch64-windows-gnullvm: 4415.5s -> 4868.2s (10.3%)
dist-x86_64-apple: 9657.9s -> 10625.3s (10.0%)
pr-check-2: 2983.1s -> 2692.9s (-9.7%)
dist-apple-various: 5505.5s -> 4982.4s (-9.5%)
x86_64-gnu-aux: 7397.4s -> 6712.7s (-9.3%)

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 · 2025-08-15T10:21:38Z

Finished benchmarking commit (8800ec1): 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)	5.3%	[5.3%, 5.3%]	1
Regressions ❌ (secondary)	0.7%	[0.2%, 1.0%]	9
Improvements ✅ (primary)	-2.7%	[-3.2%, -2.2%]	6
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-3.2%, 5.3%]	7

Max RSS (memory usage)

Results (primary 2.0%, secondary -1.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.4%, -2.9%]	3
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

Cycles

Results (primary 4.8%, secondary -1.7%)

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

	mean	range	count
Regressions ❌ (primary)	4.8%	[4.8%, 4.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.7%	[-1.7%, -1.7%]	1
All ❌✅ (primary)	4.8%	[4.8%, 4.8%]	1

Binary size

Results (primary 0.5%, 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)	0.5%	[0.5%, 0.5%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.1%]	6
All ❌✅ (primary)	0.5%	[0.5%, 0.5%]	3

Bootstrap: 469.244s -> 470.592s (0.29%)
Artifact size: 377.39 MiB -> 377.42 MiB (0.01%)

RalfJung · 2025-08-15T17:55:32Z

That clap_derive regression is new (didn't show up in the prior bench runs here)... but that benchmark seems to have fluctuated with the last few PRs so I assume it is spurious.

Kobzol · 2025-08-15T18:08:47Z

Yes, it's indeed noise.

@rustbot label: +perf-regression-triaged

Mark-Simulacrum · 2025-08-19T02:20:34Z

The match-stress regression seems real, but probably not important to look into given the context of this PR.

RalfJung · 2025-08-19T05:58:13Z

I tried various things in this PR to avoid it and nothing helped 🤷 .

Nadrieril · 2025-10-03T12:08:51Z

Oh yay, constants is one of the uglier parts of handling patterns, this is a nice improvement.

rustbot assigned fee1-dead Jul 28, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 28, 2025

RalfJung changed the title ~~Pattern valtrees~~ Patterns: represent constants as valtrees Jul 28, 2025

RalfJung commented Jul 28, 2025

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RalfJung force-pushed the pattern-valtrees branch from ee610cc to b39c019 Compare July 28, 2025 16:33

RalfJung mentioned this pull request Jul 28, 2025

Raw Pointers are Constant PatKinds too #144573

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Auto merge of #144591 - RalfJung:pattern-valtrees, r=<try>

e111586

Patterns: represent constants as valtrees

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RalfJung force-pushed the pattern-valtrees branch from 52c6e8f to 1870bd2 Compare July 29, 2025 06:58

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Auto merge of #144591 - RalfJung:pattern-valtrees, r=<try>

13cfa7a

Patterns: represent constants as valtrees

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 29, 2025

RalfJung and others added 6 commits August 14, 2025 09:44

pattern testing: store constants as valtrees

d61fdbf

use ty::Value instead of manual pairs of types and valtrees

a171eaa

change StrLit type to ty::Value as well

a1acbfb

shrink TestBranch::Constant and PatRangeBoundary::Finite

dbc030e

avoid unnecessary type sanity checks

e2cc775

Apply suggestions from code review

2330afa

Co-authored-by: Boxy <rust@boxyuwu.dev>

RalfJung force-pushed the pattern-valtrees branch from eacd56a to 2330afa Compare August 14, 2025 07:44

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 Aug 14, 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 Aug 14, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Aug 15, 2025

bors merged commit 8800ec1 into rust-lang:master Aug 15, 2025
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rustbot added this to the 1.91.0 milestone Aug 15, 2025

rustbot added the perf-regression-triaged The performance regression has been triaged. label Aug 15, 2025

github-actions bot mentioned this pull request Aug 18, 2025

Toolchain upgrade to nightly-2025-08-18 flux-rs/flux#1249

Merged

RalfJung deleted the pattern-valtrees branch August 18, 2025 16:49

		let max = ty.numeric_max_val(cx.tcx).unwrap();
		let max = ty::ValTree::from_scalar_int(cx.tcx, max.try_to_scalar_int().unwrap());

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Overall result: ✅ improvements - no action needed

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Test differences

Job duration changes

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rust-timer commented Aug 15, 2025

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

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