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mGCA: Make trait object types with type-level associated consts dyn compatible if all of the latter are specified via bindings #150843

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mGCA: Make trait object types with type-level associated consts dyn compatible if all of the latter are specified via bindings #150843

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@fmease fmease commented Jan 8, 2026 •
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Under feature min_generic_const_args (mGCA) (#132980), render traits with non-parametrized type-level associated constants (i.e., #[type_const] ones) dyn compatible but force the user to specify all type-level associated consts in the trait object type via bindings (either directly, via supertrait bounds and/or behind trait aliases).

Fixes #130300 (feature request).
Fixes #136063 (bug).
Fixes #137260 (bug).
Fixes #137514 (bug).

Blockers

  1. Merge RUST-150844 (split out of this PR)
  2. This PR basically sits atop PR RUST-150799 (but squashed)
    • I "need" some of the fixes it contains for a single test.
    • I'm going to drop that commit once the linked PR gets merged.
  3. Crate rustc-demangle needs to be updated first, mainly for v0 mangling
  4. Apart from v0 mangling, legacy mangling (no longer the default) ICEs on such dyn traits
    • we might want to address that
  5. Perf test
  6. Need to audit HIR ty lowering (see FIXMEs)
    • I'm certain there are still cases of Self references I need to forbid
    • (I'm intentionally excluding RUST-123140 here which is to be tackled some other time)
  7. There are some minor diagnostic tweaks I'd still like to perform (see FIXMEs)

@fmease fmease added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. S-blocked Status: Blocked on something else such as an RFC or other implementation work. T-types Relevant to the types team, which will review and decide on the PR/issue. F-associated_const_equality `#![feature(associated_const_equality)]` labels Jan 8, 2026
@fmease fmease added the F-min_generic_const_args `#![feature(min_generic_const_args)]` label Jan 8, 2026
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rustbot commented Jan 8, 2026

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

@rustbot rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-tidy Area: The tidy tool S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) labels Jan 8, 2026
@fmease fmease moved this to In Progress in Associated Const Equality (ACE) Jan 8, 2026
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rust-bors bot added a commit that referenced this pull request Jan 8, 2026
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Auto merge of #150843 - fmease:dyn-ace, r=<try>
ed12841 
mGCA: Make trait object types with type associated consts dyn compatible if the latter are specified via bindings
@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 8, 2026
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rust-bors bot added a commit that referenced this pull request Jan 8, 2026
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Auto merge of #150843 - fmease:dyn-ace, r=<try>
c1345e1 
mGCA: Make trait object types with type associated consts dyn compatible if the latter are specified via bindings
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rustbot commented Jan 8, 2026

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

@rustbot rustbot added the T-clippy Relevant to the Clippy team. label Jan 8, 2026
@fmease fmease changed the title mGCA: Make trait object types with type associated consts dyn compatible if the latter are specified via bindings mGCA: Make trait object types with type-level associated consts dyn compatible if the latter are specified via bindings Jan 8, 2026
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rust-bors bot added a commit that referenced this pull request Jan 9, 2026
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Auto merge of #150843 - fmease:dyn-ace, r=<try>
765f962 
mGCA: Make trait object types with type-level associated consts dyn compatible if all of the latter are specified via bindings
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Keith-Cancel and others added 12 commits January 9, 2026 21:50
@Keith-Cancel @fmease
Fix ICE: can't type-check body of DefId, since type_consts don't have…
ef2ed45 
… a body.
@fmease
Replace more mentions of associated_const_equality
7d0f3e9
@fmease
Move tests for assoc const bindings into dedicated directory
3cb7c13
@fmease
Under mGCA make traits with assoc consts dyn compatible but require a…
357375b 
…ll assoc consts to be specified via bindings
@fmease
[HAS TMP GIT DEP] Fix v0 symbol mangling for assoc const bindings
fa950ab
@fmease
Under mGCA permit certain const projections in dyn-compatible traits
5978248
@fmease
Generalize diag for missing assoc types to account for assoc consts
2497868
@fmease
Generalize diag for conflicting assoc type bindings to account for as…
26e0231 
…soc consts
@fmease
Introduce AssocTag::descr & refactor in the vicinity
7eb3f33
@fmease
Fix handling of const params defaults that ref Self & generalize diag
f432683 
We used to lower such bad defaulted const args in trait object types to
`{type error}`; now correctly lower them to `{const error}`.
@fmease
Force users to specify assoc consts that would otherwise ref Self
e21d6a2
@fmease
[TMP] Add some FIXMEs
2c1bb00
@fmease fmease added the rla-silenced Silences rust-log-analyzer postings to the PR it's added on. label Jan 9, 2026
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fmease commented Jan 10, 2026

@bors try @rust-timer queue

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rust-timer commented Jan 10, 2026

This pull request is already queued and waiting for a try build to finish.

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rust-bors bot added a commit that referenced this pull request Jan 10, 2026
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Auto merge of #150843 - fmease:dyn-ace, r=<try>
e81452f 
mGCA: Make trait object types with type-level associated consts dyn compatible if all of the latter are specified via bindings
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rust-bors bot commented Jan 10, 2026

☀️ Try build successful (CI)
Build commit: e81452f (e81452fec4e0e95e06a3bb9353fe1b32508854f2, parent: 1191620b8c4a93074796408afa538e086acf38c7)

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rust-timer commented Jan 10, 2026

Queued e81452f with parent 1191620, future comparison URL.
There is currently 1 preceding artifact in the queue.
It will probably take at least ~1.7 hours until the benchmark run finishes.

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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-tidy Area: The tidy tool F-associated_const_equality `#![feature(associated_const_equality)]` F-min_generic_const_args `#![feature(min_generic_const_args)]` rla-silenced Silences rust-log-analyzer postings to the PR it's added on. S-blocked Status: Blocked on something else such as an RFC or other implementation work. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. S-waiting-on-perf Status: Waiting on a perf run to be completed. T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) T-clippy Relevant to the Clippy team. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.

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Associated Const Equality (ACE)
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