Make TypeId const comparable #142789

oli-obk · 2025-06-20T16:13:56Z

This should unblock stabilizing const TypeId::of and allow us to progress into any possible future we want to take TypeId to.

To achieve that TypeId now contains 16 / size_of<usize>() pointers which each are actually just size_of<usize>() bytes of the stable hash. At compile-time the first of these pointers cannot be dereferenced or otherwise inspected (at present doing so might ICE the compiler). Preventing inspection of this data allows us to refactor TypeId to any other scheme in the future without breaking anyone who was tempted to transmute TypeId to obtain the hash at compile-time.

cc @eddyb for their previous work on #95845 (which we still can do in the future if we want to get rid of the hash as the final thing that declares two TypeIds as equal).

Tracking Issue for const fn type_id #77125

r? @RalfJung

rustbot · 2025-06-20T16:14:02Z

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

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

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE / Miri interpreter

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

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

library/core/src/intrinsics/mod.rs

library/core/src/any.rs

RalfJung · 2025-06-21T10:37:29Z

It will be a while until I have the capacity to review a PR of this scale.

Meanwhile, could you say a bit more about the architecture of the change? It seems you want for the "new kind of allocation" approach, but it's not clear from the PR description how exactly that shows up in TypeId.

Also, I am definitely not comfortable landing this by myself, I can only review the const-eval parts. Changing the representation of TypeId has ramifications well beyond that that I do not feel qualified to evaluate -- I think an MCP would be justified.

rustbot · 2025-06-21T11:17:11Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

oli-obk · 2025-06-21T11:18:41Z

Well, I got private feedback yesterday that instead of encoding a 16 byte value as an 8 byte pointer to the 16 byte value and an 8 byte hash, I should just do the thing where we split up type id internally into pointer sized chunks and codegen will make a hash out of it again.

TLDR: no changes to runtime type id anymore in the latest revision of this PR. Only compile-time type id is now a bit funny

oli-obk · 2025-06-21T11:22:11Z

It will be a while until I have the capacity to review a PR of this scale.

I'm splitting unrelated parts out, so the high level feedback is already useful and I'll look for libs and codegen ppl to review the appropriate parts

compiler/rustc_codegen_gcc/src/common.rs

compiler/rustc_codegen_cranelift/src/constant.rs

Make `PartialEq` a `const_trait` r? `@fee1-dead` or `@compiler-errors` something generally useful but also required for rust-lang#142789

Make `PartialEq` a `const_trait` r? ``@fee1-dead`` or ``@compiler-errors`` something generally useful but also required for rust-lang#142789

Rollup merge of #142822 - oli-obk:const-partial-eq, r=fee1-dead Make `PartialEq` a `const_trait` r? ``@fee1-dead`` or ``@compiler-errors`` something generally useful but also required for #142789

bors · 2025-06-23T16:57:20Z

☔ The latest upstream changes (presumably #142906) made this pull request unmergeable. Please resolve the merge conflicts.

compiler/rustc_codegen_cranelift/src/constant.rs

Make TypeId const comparable This should unblock stabilizing const `TypeId::of` and allow us to progress into any possible future we want to take `TypeId` to. To achieve that `TypeId` now contains `16 / size_of<usize>()` pointers which each are actually just `size_of<usize>()` bytes of the stable hash. At compile-time the first of these pointers cannot be dereferenced or otherwise inspected (at present doing so might ICE the compiler). Preventing inspection of this data allows us to refactor `TypeId` to any other scheme in the future without breaking anyone who was tempted to transmute `TypeId` to obtain the hash at compile-time. cc `@eddyb` for their previous work on #95845 (which we still can do in the future if we want to get rid of the hash as the final thing that declares two TypeIds as equal). * #77125 r? `@RalfJung`

rust-bors · 2025-07-04T13:02:42Z

⌛ Trying commit a7888a7 with merge 448540b…

To cancel the try build, run the command @bors2 try cancel.

compiler/rustc_const_eval/src/interpret/memory.rs

tests/ui/consts/const_transmute_type_id2.rs

…insic

…-id provenance

rust-bors · 2025-07-04T15:20:06Z

☀️ Try build successful (CI)
Build commit: 448540b (448540b0e20d48e1a38b06460a73ebec5ee805f0, parent: 1b61d43bdbf875183b1f436302d62ff93f9a6bba)

rust-timer · 2025-07-04T17:36:47Z

Finished benchmarking commit (448540b): 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)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	2
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	2
All ❌✅ (primary)	-0.1%	[-0.4%, 0.4%]	3

Max RSS (memory usage)

Results (primary 1.7%, secondary 1.2%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[1.7%, 4.9%]	5
Regressions ❌ (secondary)	4.0%	[0.5%, 9.0%]	5
Improvements ✅ (primary)	-5.8%	[-5.8%, -5.8%]	1
Improvements ✅ (secondary)	-2.3%	[-3.3%, -0.4%]	4
All ❌✅ (primary)	1.7%	[-5.8%, 4.9%]	6

Cycles

Results (primary 0.8%, secondary 1.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	2
Regressions ❌ (secondary)	3.3%	[1.4%, 4.5%]	4
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.1%]	2
All ❌✅ (primary)	0.8%	[-2.3%, 2.3%]	3

Binary size

Results (primary 0.1%, secondary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.3%]	23
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.4%	[-0.5%, -0.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.5%, 0.3%]	25

Bootstrap: 462.82s -> 460.745s (-0.45%)
Artifact size: 372.13 MiB -> 372.22 MiB (0.03%)

RalfJung · 2025-07-04T17:55:58Z

Lol, seems it speeds up more benchmarks than it slows down. ;) I assume Any isn't used that much in rustc... but if it's a problem we can always keep adjusting the representation later.

bors · 2025-07-04T18:55:43Z

☔ The latest upstream changes (presumably #143434) made this pull request unmergeable. Please resolve the merge conflicts.

oli-obk · 2025-07-04T19:06:07Z

ctfe improvements are just the opt of not creating an allocation anymore just to copy it and throw it away.

The others are llvm opt changes that are probably unrelated. This is not used enough to cause compile time changes and we have no dedicated runtime benchmarks for it

RalfJung · 2025-07-04T19:16:36Z

Makes sense.

There is one unresolved comment still up, and the one I just commented on.
Also, needs a rebase.
Also, this is blocked on #143444.
@rustbot author

I am not 100% confident in the GCC and cranelift changes; @bjorn3 @rust-lang/wg-gcc-backend would be good if you could take a look. This is a new "magic" kind of pointer where codegen should just ignore the AllocId and emit a regular integer with the pointer's offset.

clean up GVN TypeId test addresses rust-lang#142789 (comment) This is an attempt to clarify what this test is actually supposed to test and make it less dependent on `TypeId` internals (it now depends on the output of `type_name` instead). I verified that this version still miscompiles on `nightly-2025-02-11`. r? `@oli-obk` `@RalfJung`

clean up GVN TypeId test addresses rust-lang#142789 (comment) This is an attempt to clarify what this test is actually supposed to test and make it less dependent on `TypeId` internals (it now depends on the output of `type_name` instead). I verified that this version still miscompiles on `nightly-2025-02-11`. r? ``@oli-obk`` ``@RalfJung``

rustbot assigned RalfJung Jun 20, 2025

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oli-obk force-pushed the constable-type-id branch from 3cddd21 to 1fd7b66 Compare June 21, 2025 10:20

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oli-obk mentioned this pull request Jun 21, 2025

Make PartialEq a const_trait #142822

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antoyo reviewed Jun 21, 2025

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oli-obk force-pushed the constable-type-id branch 2 times, most recently from b8a7a10 to 1c47a64 Compare June 24, 2025 09:25

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oli-obk force-pushed the constable-type-id branch from 1c47a64 to bcb4aa2 Compare June 24, 2025 13:02

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 4, 2025

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oli-obk added 3 commits July 4, 2025 13:38

Explain mad type_id_of_val

29bb524

Unify the miri and ctfe type id equality checks

40f3045

Document TypeId internals

045c2f1

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oli-obk added 5 commits July 4, 2025 13:57

Use const_eval_select macro instead of handrolling a call to the intr…

47de1f8

…insic

Expose provenance instead of transmuting provenance-ful memory to u128

47df5ea

Fix ICE in case of TypeId provenance being overridden with a non-type…

95e10fb

…-id provenance

Rename GlobalAlloc::Type to TypeId

803b6ce

Comment updates

58d86ec

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lukas-code mentioned this pull request Jul 4, 2025

clean up GVN TypeId test #143444

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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 Jul 4, 2025

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 4, 2025

oli-obk force-pushed the constable-type-id branch from a7888a7 to 58d86ec Compare July 4, 2025 19:17

Make TypeId const comparable #142789

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Make TypeId const comparable #142789

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rust-timer commented Jul 4, 2025

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

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Max RSS (memory usage)

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Binary size

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