Add very basic "comptime" fn implementation by oli-obk · Pull Request #148820 · rust-lang/rust

oli-obk · 2025-11-11T12:25:32Z

Implements functions that cannot be called at runtime (and thus also not from normal const functions, as those could be called from runtime).

This is done via the internal attribute rustc_comptime that can be added to normal functions, turning them into compile-time-only functions.

Because @fee1-dead and @compiler-errors did amazing work, we even get trait bounds that work inside comptime fns: via unconditionally-const const Trait bounds.

Use cases are

Reflection MVP #146923
const heap intrinsics
and the other various intrinsics (e.g. size_of 😆) that will just ICE in codegen or panic at runtime if they actually end up in runtime code

project goal issue: rust-lang/rust-project-goals#406

no tracking issue until we have a feature gate and some sort of syntax

cc @scottmcm as the T-lang goal champion

rustbot · 2025-11-11T12:25:35Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

rustbot · 2025-11-11T12:25:38Z

r? @JonathanBrouwer

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rustbot · 2025-11-11T12:37:13Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

fmease · 2025-11-11T13:05:39Z

@@ -4225,13 +4227,15 @@ impl fmt::Display for Safety {

 #[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable, Decodable, HashStable_Generic)]
 pub enum Constness {


Personally speaking I would rename the variants like so:

pub enum Constness { Always, // `#[rustc_comptime]` Maybe, // `const` Never, }

to mirror BoundConstness which is Always (for const), Maybe (for [const]) and Never. This makes it crystal clear what they represent and leaves no room for ambiguity.

If it were up to me, I would even rename Constness to ItemConstness if traits will be allowed to be compiletime, too. Otherwise, I would split it into FnConstness and TraitConstness.

I prefer this naming over the suggestion I made in https://github.com/rust-lang/rust/pull/148820/files#r2517464261 actually

Apparently this wasn't addressed? I would personally prefer the naming to be Const/Comptime/NotConst but Always is okay too (I don't prefer it though)

#148820 (comment)

rustbot · 2025-11-21T11:27:05Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to constck

cc @fee1-dead

oli-obk · 2026-06-08T10:20:02Z

@bors r=fee1-dead

rust-bors · 2026-06-08T10:20:04Z

📌 Commit d129cd0 has been approved by fee1-dead

It is now in the queue for this repository.

…d in comptime fn

…rgetted at what it does

rust-bors · 2026-06-08T10:37:30Z

⚠️ A new commit 4089c2afc62524aa8bb2ecf17314bc39c43585c1 was pushed.

This pull request was unapproved.

oli-obk · 2026-06-08T10:38:14Z

@bors r=fee1-dead

I edited the attribute name to match how the rustc_ attributes are represented internally

rust-bors · 2026-06-08T10:38:17Z

📌 Commit 4089c2a has been approved by fee1-dead

It is now in the queue for this repository.

@fee1-dead

Add very basic "comptime" fn implementation Implements functions that cannot be called at runtime (and thus also not from normal const functions, as those could be called from runtime). This is done via the internal attribute `rustc_comptime` that can be added to normal functions, turning them into compile-time-only functions. Because @fee1-dead and @compiler-errors did amazing work, we even get trait bounds that work inside comptime fns: via unconditionally-const `const Trait` bounds. Use cases are * rust-lang#146923 * const heap intrinsics * and the other various intrinsics (e.g. size_of 😆) that will just ICE in codegen or panic at runtime if they actually end up in runtime code project goal issue: rust-lang/rust-project-goals#406 no tracking issue until we have a feature gate and some sort of syntax cc @scottmcm as the T-lang goal champion

…uwer Rollup of 13 pull requests Successful merges: - #147302 (asm! support for the Xtensa architecture) - #148820 (Add very basic "comptime" fn implementation) - #157299 (Fix unstable diagnostics in tests) - #143511 (Improve TLS codegen by marking the panic/init path as cold) - #154608 (Add `_value` API for number literals in proc-macro) - #156762 (xfs support in `test_rename_directory_to_non_empty_directory`) - #157300 (Relax test requirements for consistency) - #157383 (tests: codegen-llvm: Ignore BPF targets in c-variadic-opt) - #157413 (fix: don't suggest .into_iter() for .cloned()/.copied() on non-reference Option) - #157578 (Fix diagnostics for non-exhaustive destructuring assignments (#157553)) - #157587 (explain that the size_of constant also serves to avoid optimizing away 'unused' size_of calls) - #157596 (test: remove ineffective link-extern-crate-with-drop-type test) - #157602 (rustdoc: Remove unnecessary fast path)

@fee1-dead

Rollup merge of #148820 - oli-obk:comptime, r=fee1-dead Add very basic "comptime" fn implementation Implements functions that cannot be called at runtime (and thus also not from normal const functions, as those could be called from runtime). This is done via the internal attribute `rustc_comptime` that can be added to normal functions, turning them into compile-time-only functions. Because @fee1-dead and @compiler-errors did amazing work, we even get trait bounds that work inside comptime fns: via unconditionally-const `const Trait` bounds. Use cases are * #146923 * const heap intrinsics * and the other various intrinsics (e.g. size_of 😆) that will just ICE in codegen or panic at runtime if they actually end up in runtime code project goal issue: rust-lang/rust-project-goals#406 no tracking issue until we have a feature gate and some sort of syntax cc @scottmcm as the T-lang goal champion

Mark-Simulacrum · 2026-06-09T02:23:55Z

@rust-timer build 370b172

cc #157616 (comment)

rust-timer · 2026-06-09T03:08:32Z

Finished benchmarking commit (370b172): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.3%	[0.3%, 0.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.1%, secondary 6.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	6.6%	[6.6%, 6.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Cycles

Results (primary -2.3%, secondary 15.7%)

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)	15.7%	[2.6%, 41.4%]	3
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 517.572s -> 540.149s (4.36%)
Artifact size: 400.85 MiB -> 400.78 MiB (-0.02%)

rustbot assigned JonathanBrouwer Nov 11, 2025

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oli-obk force-pushed the comptime branch from 9623203 to 953a740 Compare November 11, 2025 12:37

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oli-obk mentioned this pull request Nov 11, 2025

const_allocate intrinsic: what should it do at runtime? #93935

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jdonszelmann reviewed Nov 12, 2025

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Comment thread compiler/rustc_ast/src/ast.rs

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reflection and comptime rust-lang/rust-project-goals#406

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JonathanBrouwer removed their assignment Nov 15, 2025

oli-obk force-pushed the comptime branch from 953a740 to 82c5924 Compare November 21, 2025 11:27

rustbot added the T-clippy Relevant to the Clippy team. label Nov 21, 2025

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rust-bors Bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 8, 2026

oli-obk and others added 7 commits June 8, 2026 12:32

Add field to distinguish const fns from always-const fns

bde4379

Check comptime fns

322a540

Prevent comptime fns from being called at runtime

2a77be2

Allow comptime fns to call other comptime fns

68890b7

Demonstrate that const Trait bounds allow trait methods to be calle…

406b8ef

…d in comptime fn

Make the inline field on the Const ConstContext more clearly ta…

94e9c0b

…rgetted at what it does

Remove possibly footgunny is_const method

4089c2a

oli-obk force-pushed the comptime branch from d129cd0 to 4089c2a Compare June 8, 2026 10:37

rust-bors Bot 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-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jun 8, 2026

rust-bors Bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 8, 2026

This was referenced Jun 8, 2026

Rollup of 9 pull requests #157610

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Rollup of 13 pull requests #157616

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rust-bors Bot merged commit 1419d3c into rust-lang:main Jun 8, 2026
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oli-obk mentioned this pull request Jun 9, 2026

Start using comptime for reflection intrinsics and their wrapper functions #157647

Open

		@@ -4225,13 +4227,15 @@ impl fmt::Display for Safety {

		#[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable, Decodable, HashStable_Generic)]
		pub enum Constness {

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rust-timer commented Jun 9, 2026

Overall result: ❌ regressions - no action needed

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