Skip to content

Not using anon consts for uses of const parameters caused us to accept more code for free type aliases #149774

Closed
#152931
Closed
Not using anon consts for uses of const parameters caused us to accept more code for free type aliases#149774
#152931
Assignees
BoxyUwU
Labels
C-bugCategory: This is a bug.P-highHigh priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-typesRelevant to the types team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.

Description

@BoxyUwU
BoxyUwU
opened on Dec 8, 2025
type Alias<const B: bool> = [u8; B];

This previously didn't compile but started to in 1.83, not sure which PR but probably something like #129137.

This is because the RHS of free type aliases are not well formed checked. Previously we caught this anyway by implicitly having an anon const with type usize that wrapped the usage of B which caused a separate type checker error when type checking the anon const.

Nowadays we just directly represent it without an anon const and rely on WellFormed([u8; B]) checking that B is of type usize. It might be possible to generate the list of wf predicates for the RHS of free type aliases and then only prove the ConstArgHasType clauses to fix this?

cc @rust-lang/types

Activity

  1. BoxyUwU
    added
    P-highHigh priority
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.
    C-bugCategory: This is a bug.
    I-types-nominatedNominated for discussion during a types team meeting.
    T-typesRelevant to the types team, which will review and decide on the PR/issue.
    on Dec 8, 2025
  2. rustbot
    added
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    on Dec 8, 2025
  3. BoxyUwU
    removed
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    on Dec 8, 2025
  4. BoxyUwU
    self-assigned this
    on Dec 23, 2025
  5. rust-bors
    added a commit that references this issue on Mar 23, 2026

    Auto merge of #152931 - khyperia:always-check-constarghastype, r=BoxyUwU

    f66622c
  6. rust-bors
    closed this as completedin f66622con Mar 23, 2026
  7. RalfJung
    added a commit that references this issue on Mar 24, 2026

    Auto merge of #152931 - khyperia:always-check-constarghastype, r=BoxyUwU

    e8897fa
  8. github-actions
    added 2 commits that reference this issue on Mar 26, 2026

    Auto merge of #152931 - khyperia:always-check-constarghastype, r=BoxyUwU

    395ad2b

    Auto merge of #152931 - khyperia:always-check-constarghastype, r=BoxyUwU

    854de41
  9. makai410
    added a commit that references this issue on Mar 27, 2026

    Auto merge of #152931 - khyperia:always-check-constarghastype, r=BoxyUwU

    9af321f
  10. github-actions
    added a commit that references this issue on Apr 6, 2026

    Auto merge of #152931 - khyperia:always-check-constarghastype, r=BoxyUwU

    3fdff5b
  11. jackh726
    removed
    I-types-nominatedNominated for discussion during a types team meeting.
    on Jun 2, 2026
  12. ghaaj
    added a commit that references this issue on Aug 7, 2026

    Auto merge of #152931 - khyperia:always-check-constarghastype, r=BoxyUwU

    4f51cb4
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Metadata

Metadata

Assignees

Labels

C-bugCategory: This is a bug.P-highHigh priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-typesRelevant to the types team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.

Type

No type

Projects

No projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions