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Add very basic "comptime" fn implementation#148820

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oli-obk wants to merge 7 commits intorust-lang:mainfrom
oli-obk:comptime
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Add very basic "comptime" fn implementation#148820
oli-obk wants to merge 7 commits intorust-lang:mainfrom
oli-obk:comptime

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@oli-obk oli-obk commented Nov 11, 2025 •
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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

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rustbot commented Nov 11, 2025

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 rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) 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. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. T-rustfmt Relevant to the rustfmt team, which will review and decide on the PR/issue. labels Nov 11, 2025
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rustbot commented Nov 11, 2025

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.

Use r? to explicitly pick a reviewer

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rustbot commented Nov 11, 2025

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

RalfJung
RalfJung reviewed Nov 11, 2025
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Comment thread compiler/rustc_hir/src/hir.rs Outdated
fmease
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Comment thread compiler/rustc_parse/src/parser/item.rs Outdated
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fmease
fmease reviewed Nov 11, 2025
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Comment thread compiler/rustc_hir/src/hir.rs
@jdonszelmann jdonszelmann self-assigned this Nov 11, 2025
@fee1-dead fee1-dead self-assigned this Nov 11, 2025
fee1-dead
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Comment thread compiler/rustc_feature/src/builtin_attrs.rs Outdated
Comment thread compiler/rustc_hir_typeck/src/callee.rs Outdated
@oli-obk oli-obk mentioned this pull request Nov 11, 2025
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jdonszelmann
jdonszelmann reviewed Nov 12, 2025
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Comment thread compiler/rustc_ast/src/ast.rs
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Comment thread compiler/rustc_hir/src/attrs/encode_cross_crate.rs Outdated
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Comment thread compiler/rustc_middle/src/ty/mod.rs Outdated
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@JonathanBrouwer JonathanBrouwer removed their assignment Nov 15, 2025
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rustbot commented Nov 21, 2025

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to constck

cc @fee1-dead

@rustbot rustbot added the T-clippy Relevant to the Clippy team. label Nov 21, 2025
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rustbot commented May 3, 2026

This PR changes rustc_public

cc @celinval, @ouz-a, @makai410

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@oli-obk oli-obk force-pushed the comptime branch 3 times, most recently from 01088b9 to d092d4e Compare May 3, 2026 09:58
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oli-obk commented May 6, 2026

@rustbot ready

@rustbot 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 May 6, 2026
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@rustbot rustbot added the A-rustdoc-json Area: Rustdoc JSON backend label May 6, 2026
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@oli-obk oli-obk force-pushed the comptime branch 2 times, most recently from 04bdee9 to ab17130 Compare May 7, 2026 06:36
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rustbot commented May 7, 2026

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.

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@RalfJung RalfJung RalfJung left review comments

@fmease fmease fmease left review comments

@jdonszelmann jdonszelmann jdonszelmann left review comments

@fee1-dead fee1-dead fee1-dead requested changes

+1 more reviewer

@bushrat011899 bushrat011899 bushrat011899 left review comments

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A-attributes Area: Attributes (`#[…]`, `#![…]`) A-rustdoc-json Area: Rustdoc JSON backend S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-clippy Relevant to the Clippy team. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. T-rustfmt Relevant to the rustfmt team, which will review and decide on the PR/issue.

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