Add very basic "comptime" fn implementation #148820

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

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

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rustbot · 2025-11-11T12:37:13Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

compiler/rustc_hir/src/hir.rs

compiler/rustc_parse/src/parser/item.rs

compiler/rustc_hir/src/hir.rs

compiler/rustc_feature/src/builtin_attrs.rs

compiler/rustc_hir_typeck/src/callee.rs

compiler/rustc_ast/src/ast.rs

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

compiler/rustc_hir_pretty/src/lib.rs

compiler/rustc_middle/src/ty/mod.rs

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 · 2025-11-25T11:36:02Z

All reviews have been addressed

bors · 2025-11-26T16:23:18Z

☔ The latest upstream changes (presumably #149140) made this pull request unmergeable. Please resolve the merge conflicts.

…d in comptime fn

…rgetted at what it does

rustbot · 2025-11-27T11:00:27Z

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.

fee1-dead · 2025-11-28T15:46:24Z

compiler/rustc_const_eval/src/check_consts/ops.rs

                    }
                }
-            } 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 · 2025-11-28T15:46:59Z

compiler/rustc_hir/src/hir.rs

    pub fn is_const(&self) -> bool {
-        matches!(self.constness, Constness::Const)
+        matches!(self.constness, Constness::Maybe)
    }


I think we might want to audit usages of this. Leave a FIXME?

fee1-dead · 2025-11-28T15:48:49Z

compiler/rustc_metadata/src/rmeta/encoder.rs

            if should_encode_constness(def_kind) {
                let constness = self.tcx.constness(def_id);
-                self.tables.constness.set(def_id.index, constness);
+                self.tables.constness.set(def_id.index, constness)


? (please revert)

Rebase fallout

fee1-dead · 2025-11-28T15:49:09Z

compiler/rustc_metadata/src/rmeta/table.rs

+        ( Maybe    )
+        ( Never )
+        ( Always )


Suggested change

( Maybe )

( Never )

( Always )

( Maybe )

( Never )

( Always )

fee1-dead · 2025-11-28T15:57:13Z

compiler/rustc_mir_transform/src/lib.rs

                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.

rustbot · 2025-11-28T15:57:30Z

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

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

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

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oli-obk force-pushed the comptime branch from 82c5924 to 1e2bd3f Compare November 24, 2025 17:45

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oli-obk force-pushed the comptime branch 2 times, most recently from da63f2f to 785eebe Compare November 25, 2025 11:35

oli-obk and others added 7 commits November 27, 2025 09:42

Add Constness::Comptime

d843820

Check comptime fns

6ef6b1a

Prevent comptime fns from being called at runtime

012ac6d

Allow comptime fns to call other comptime fns

ea457c0

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

c77c2dc

…d in comptime fn

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

af9bc67

…rgetted at what it does

Rename Constness variants to match BoundConstness

3d02bbc

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

fee1-dead requested changes Nov 28, 2025

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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 Nov 28, 2025

Add very basic "comptime" fn implementation #148820

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oli-obk commented Nov 11, 2025 •

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