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

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

fee1-dead · 2025-11-28T15:46:24Z

                    }
                }
-            } else if ccx.tcx.constness(callee) != hir::Constness::Const {
+            } else if ccx.tcx.constness(callee) != hir::Constness::Maybe {


This is a bit sus

What part? The rename or the inequality?

The inequality. I feel like this should be if == hir::Constness::NotConst?

fee1-dead · 2025-11-28T15:57:13Z

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                let num_args = fields.len();
                let func =
-                    if context == hir::Constness::Const { called_in_const } else { called_at_rt };
+                    if context == hir::Constness::Maybe { called_in_const } else { called_at_rt };


I feel like we shouldn't add another variant to hir::Constness given how this changes a lot of other things which might be a headache to update if we were to add comptime impls in the future, etc. Would it make sense to keep the hir:Constness::{,Not}Const, and extract the comptime enum specifically to a new enum? Might be a bit more work to do but worth it IMO, and might be better than doing it in a followup.

I can draft up a version of that, but mostly as a counterexample to why we shouldn't do that. This enum mirrors the BoundConstness perfectly (and those should probably be merged), because there are only those 3 options. Having two separate enums creates an odd space where we need to reject an illegal state in the type system instead of just in the parser or ast lowering

x not const const

not comptime fn const fn

comptime comptime fn ???

If you're worried about footguns I think we could try to remove impl PartialEq for Constness and require exhaustive handling at all sites.

Alternatively I can make the "alwaysness" of the Comptime variant be a field on the Const/Maybe variant. This way you can't accidentally forget to handle it, but Const and Maybe end up generally handled together unless explicitly differentiated

I made this change, which made the "renaming Constness variants to match BoundConstness" change a little weird, so I removed that and will look into it in another PR. Maybe by changing BoundConstness to match Constness instead of the other way around

rustbot · 2025-11-28T15:57:30Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

bors · 2025-11-30T18:14:18Z

☔ The latest upstream changes (presumably #149478) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2025-12-16T17:14:32Z

☔ The latest upstream changes (presumably #143924) made this pull request unmergeable. Please resolve the merge conflicts.

izagawd · 2026-03-18T22:48:30Z

Is this PR still intended to move forward, or has the work been superseded elsewhere?

oli-obk · 2026-03-19T10:26:27Z

I have some review comments to address and some heavy rebasing to do :D

bushrat011899 · 2026-05-01T00:12:42Z

+#[rustc_comptime]
+fn always_const<T: const Trait>() {
+    T::method()
+}


No Action Required: Makes me wish the current const modifier was actually named const?, to imply it can be called in comptime, and can be called at runtime. That way, const would mean must be called in comptime, and the absence of const means can't be called in comptime. It doesn't really matter, since this is a compiler feature, but it'd be nicer for users to understand what's going on.

View changes since the review

oli-obk · 2026-05-01T08:01:12Z

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+#[rustc_comptime]
+const fn foo() {}
+//~^ ERROR a function cannot be both `comptime` and `const`


It's actually easier this way for the compiler, because it's now trivial to reject [const] bounds

It's also nowhere near the final syntax, just sth to enable the feature

Ahh ok, I assumed this was mostly a stylistic choice. If it's more consistent with the rest of the compiler that makes sense.

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                let num_args = fields.len();
                let func =
-                    if context == hir::Constness::Const { called_in_const } else { called_at_rt };
+                    if context == hir::Constness::Maybe { called_in_const } else { called_at_rt };


rustbot · 2026-05-03T09:50:19Z

This PR changes rustc_public

cc @celinval, @ouz-a, @makai410

…d in comptime fn

…rgetted at what it does

rustbot · 2026-05-03T09:58:22Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rust-log-analyzer · 2026-05-03T10:07:06Z

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)


error[E0433]: cannot find type `Constness` in this scope
   --> compiler/rustc_passes/src/stability.rs:209:54
    |
209 |                 && matches!(fn_sig.header.constness, Constness::Const { .. })
    |                                                      ^^^^^^^^^ use of undeclared type `Constness`
    |
help: consider importing this enum
    |
  4 + use rustc_hir::Constness;
    |

error[E0433]: cannot find type `Constness` in this scope
   --> compiler/rustc_passes/src/stability.rs:231:46
    |
231 |         && matches!(fn_sig.header.constness, Constness::Const { .. })
    |                                              ^^^^^^^^^ use of undeclared type `Constness`
    |
help: consider importing this enum
    |
  4 + use rustc_hir::Constness;
    |

error[E0433]: cannot find type `Constness` in this scope
   --> compiler/rustc_passes/src/stability.rs:387:51
    |
387 |             && !matches!(fn_sig.header.constness, Constness::Const { .. })
    |                                                   ^^^^^^^^^ use of undeclared type `Constness`
    |
help: consider importing this enum
    |
  4 + use rustc_hir::Constness;

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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nikomatsakis mentioned this pull request Nov 12, 2025

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

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

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oli-obk force-pushed the comptime branch from 785eebe to 3d02bbc Compare November 27, 2025 11:00

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Manishearth mentioned this pull request Jan 26, 2026

Add FixedLengthVarULE and user in icu_plurals unicode-org/icu4x#7394

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Dispatch MIR pipeline by BodyOwnerKind instead of const context #155808

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oli-obk force-pushed the comptime branch from 8cd87b9 to 21a6815 Compare May 3, 2026 09:53

oli-obk and others added 7 commits May 3, 2026 11:56

Add field to distinguish const fns from always-const fns

c269d54

Check comptime fns

61ba7f5

Prevent comptime fns from being called at runtime

fbce626

Allow comptime fns to call other comptime fns

bb3d6c5

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

beed5c7

…d in comptime fn

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

ddb9e59

…rgetted at what it does

Remove possibly footgunny is_const method

d092d4e

oli-obk force-pushed the comptime branch 2 times, most recently from 01088b9 to d092d4e Compare May 3, 2026 09:58

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