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

fmease · 2026-01-08T22:07:15Z

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.

rustbot · 2026-01-08T22:07:18Z

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.

mGCA: Make trait object types with type associated consts dyn compatible if the latter are specified via bindings

rustbot · 2026-01-08T22:36:09Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

@BoxyUwU

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

@BoxyUwU

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

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 · 2026-01-15T15:37:38Z

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_trait.rs

+        // Collect all associated items that weren't specified and compute the list of projection
+        // bounds which we'll later turn into existential ones.
+        // For why we keep projections whose associated item has a `Self: Sized` bound,
+        // see <https://github.com/rust-lang/rust/pull/140684>.


can we say sth like "we keep them around so that we the resulting ty after HIR ty lowering still contains the user written types on the rhs of the assoc item binding, see https://github.com/...". clicking the link is annoying 😅

I now say:

We intentionally keep around projections whose associated item has a Self: Sized
bound in order to be able to wfcheck the RHS, allow the RHS to constrain generic
parameters and to imply bounds.
See also #140684.

https://github.com/rust-lang/rust/pull/150843/files#diff-c15dc5fc785375aaa6be6b5168da389dbcf933c328a8bace5fdb9950bf2d40a1R310-R313

BoxyUwU · 2026-01-15T16:00:41Z

tests/ui/const-generics/associated-const-bindings/dyn-compat-non-type-assoc-const.rs

+}
+
+fn main() {
+    let _: dyn Trait; //~ ERROR the trait `Trait` is not dyn compatible


can you add another case with dyn Trait<K = 0> and check that errors

BoxyUwU · 2026-01-15T16:01:57Z

tests/ui/const-generics/associated-const-bindings/dyn-compat-symbol-mangling.rs

@@ -0,0 +1,27 @@
+// Ensure that we can successfully mangle & demangle trait object types w/ assoc const bindings.
+
+// FIXME(fmease): Legacy mangling still crashes.


FIXME(mgca)

i wanna know this when looking at mgca issues not jsut when looking at your FIXMEs :3

BoxyUwU · 2026-01-15T16:15:13Z

tests/ui/const-generics/associated-const-bindings/dyn-compat-self-const-projections.rs

+
+// This is a run-pass test to ensure that codegen can actually deal with such method instances
+// (e.g., const projections normalize flawlessly to something concrete, symbols get mangled
+// properly properly) and to ensure that the generated code "received" the correct values from


properly properly

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

BoxyUwU

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

...but require all assoc consts to be specified via bindings.

…soc consts

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.

…would otherwise ref `Self` The added test used to ICE prior to this change.

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.

… `Self` (barring `Self` projections)

fmease · 2026-01-19T23:29:40Z

I've addressed all review comments (in one way or another).

I've now added a 10th commit in which I render traits dyn incompatible if the type of the type assoc const mentions the Self type param unless it's part of a "Self projection" as discussed. I've added a known-bug test exercising the "problematic nature" of "Self projections" in the type of type assoc consts (the fact that they leak dummy self & thus don't typeck correctly). That commit also contains the extra tests you wished for.

Ready for rereview.

fmease assigned BoxyUwU Jan 8, 2026

fmease added this to Associated Const Equality (ACE) Jan 8, 2026

fmease added the F-min_generic_const_args `#![feature(min_generic_const_args)]` label Jan 8, 2026

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

Auto merge of #150843 - fmease:dyn-ace, r=<try>

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mGCA: Make trait object types with type associated consts dyn compatible if the latter are specified via bindings

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 8, 2026

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fmease force-pushed the dyn-ace branch from d8c1ca1 to c11d680 Compare January 8, 2026 22:34

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rust-bors bot added a commit that referenced this pull request Jan 8, 2026

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

fmease force-pushed the dyn-ace branch from c11d680 to 5242b36 Compare January 8, 2026 22:36

rustbot added the T-clippy Relevant to the Clippy team. label Jan 8, 2026

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

fmease force-pushed the dyn-ace branch from 19c584d to fb5b179 Compare January 11, 2026 18:44

fmease force-pushed the dyn-ace branch from fb5b179 to 964d80f Compare January 13, 2026 09:11

fmease force-pushed the dyn-ace branch from 964d80f to 7e656f3 Compare January 15, 2026 15:59

BoxyUwU reviewed Jan 15, 2026

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compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs Outdated Show resolved Hide resolved

BoxyUwU reviewed Jan 15, 2026

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compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs Outdated Show resolved Hide resolved

BoxyUwU approved these changes Jan 15, 2026

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fmease removed the S-blocked Status: Blocked on something else such as an RFC or other implementation work. label Jan 19, 2026

fmease force-pushed the dyn-ace branch from 7e656f3 to 711cf17 Compare January 19, 2026 19:46

fmease added 10 commits January 20, 2026 00:12

mGCA: Make traits with type assoc consts dyn compatible...

49aaa37

...but require all assoc consts to be specified via bindings.

Fix v0 symbol mangling for assoc const bindings

b88d3ab

mGCA: Permit certain const projections in dyn-compatible traits

058307e

Generalize diag for missing assoc types to account for assoc consts

feed826

Generalize diag for conflicting assoc type bindings to account for as…

8b822cc

…soc consts

Introduce AssocTag::descr & refactor in the vicinity

835f905

Fix handling of const params defaults that ref Self & generalize diag

37bfc23

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.

mGCA: Force users to specify type assoc consts from supertraits that …

498cf33

…would otherwise ref `Self` The added test used to ICE prior to this change.

mGCA: Render traits dyn incompatible if the ty of an assoc const refs…

87fe6b1

… `Self` (barring `Self` projections)

fmease force-pushed the dyn-ace branch from 711cf17 to 87fe6b1 Compare January 19, 2026 23:17

fmease marked this pull request as ready for review January 19, 2026 23:24

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 Jan 19, 2026

		@@ -0,0 +1,27 @@
		// Ensure that we can successfully mangle & demangle trait object types w/ assoc const bindings.

		// FIXME(fmease): Legacy mangling still crashes.

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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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