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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) just like associated types, their sibling.

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

While I'm accounting for most illegal Self references via const projections & params, I'm intentionally ignoring RUST-123140 (and duplicates) in this PR which is to be tackled some other time.

Additional context: Crate rustc-demangle had to be updated to fix v0 demangling. I've patched it in PR rust-lang/rustc-demangle#87 which was was released in version 0.1.27 via PR rust-lang/rustc-demangle#88.

@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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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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@fmease fmease mentioned this pull request Jan 11, 2026
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@fmease fmease force-pushed the dyn-ace branch 3 times, most recently from 19c584d to fb5b179 Compare January 11, 2026 18:44
rust-bors bot pushed a commit that referenced this pull request Jan 12, 2026
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Auto merge of #150844 - fmease:mv-ace-tests, r=BoxyUwU
2f1bd3f 
mGCA: Move tests for assoc const bindings (formerly ACE) into dedicated directory & replace more mentions of ACE

Split out of PR #150843.
As discussed.

Somewhat obvious underlying principle: If the test checks basic or core parts of assoc const bindings and nothing else, move it, otherwise leave it even if it contains ACEs.

Motivation: It makes a lot easier for me to continue working on ACE efficiently.

r? @BoxyUwU
github-actions bot pushed a commit to rust-lang/rust-analyzer that referenced this pull request Jan 15, 2026
@bors
Auto merge of #150844 - fmease:mv-ace-tests, r=BoxyUwU
ea61c1c 
mGCA: Move tests for assoc const bindings (formerly ACE) into dedicated directory & replace more mentions of ACE

Split out of PR rust-lang/rust#150843.
As discussed.

Somewhat obvious underlying principle: If the test checks basic or core parts of assoc const bindings and nothing else, move it, otherwise leave it even if it contains ACEs.

Motivation: It makes a lot easier for me to continue working on ACE efficiently.

r? @BoxyUwU
github-actions bot pushed a commit to rust-lang/miri that referenced this pull request Jan 15, 2026
@bors
Auto merge of #150844 - fmease:mv-ace-tests, r=BoxyUwU
55aaa7b 
mGCA: Move tests for assoc const bindings (formerly ACE) into dedicated directory & replace more mentions of ACE

Split out of PR rust-lang/rust#150843.
As discussed.

Somewhat obvious underlying principle: If the test checks basic or core parts of assoc const bindings and nothing else, move it, otherwise leave it even if it contains ACEs.

Motivation: It makes a lot easier for me to continue working on ACE efficiently.

r? @BoxyUwU
BoxyUwU
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BoxyUwU reviewed Jan 15, 2026
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I've looked over all the commits, got some nits and you've still got the other thing to fix with type consts' types having Self in them until approval though

View changes since this review

@fmease fmease removed the S-blocked Status: Blocked on something else such as an RFC or other implementation work. label Jan 19, 2026
fmease added 10 commits January 19, 2026 20:17
@fmease
mGCA: Make traits w/ type assoc consts dyn compatible
97624b1 
but require all assoc consts to be specified via bindings.
@fmease
Fix v0 symbol mangling for assoc const bindings
c2ffd4a
@fmease
mGCA: Permit certain const projections in dyn-compatible traits
f09f14a
@fmease
Generalize diag for missing assoc types to account for assoc consts
c3a5d8f
@fmease
Generalize diag for conflicting assoc type bindings to account for as…
071e81d 
…soc consts
@fmease
Introduce AssocTag::descr & refactor in the vicinity
c3b6b68
@fmease
Fix handling of const params defaults that ref Self & generalize diag
2727871 
We used to lower such bad defaulted const args in trait object types to
`{type error}`; now correctly lower them to `{const error}`.

The added tests used to ICE prior to this change.
@fmease
mGCA: Force users to specify type assoc consts from supertraits that …
001c3bd 
…would otherwise ref `Self`

The added test used to ICE prior to this change.
@fmease
Reject const projections behind trait aliases that mention Self
095ede9 
This fully rewords the diagnostic that was previously only emitted for assoc ty bindings.
That's because it incorrectly called trait aliases *type aliases* and didn't really
make it clear what the root cause is.

The added test used to ICE prior to this change.

I've double-checked that the preexisting test I've modified still ICEs in
nightly-2025-03-29.
@fmease
mGCA: Render traits dyn incompatible if the ty of an assoc const refs…
711cf17 
… `Self` (barring `Self` projections)
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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-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. 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)
Status: In Progress

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