Migrate `UnsizedConstParamTy` to unstable impl of `ConstParamTy_` #145095

tiif · 2025-08-08T13:10:21Z

Now that we have #[unstable_feature_bound], we can remove UnsizedConstParamTy that was meant to be an unstable impl of stable type and ConstParamTy_ trait.

r? @BoxyUwU

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? `@BoxyUwU`

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ``@BoxyUwU``

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ```@BoxyUwU```

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ````@BoxyUwU````

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? `````@BoxyUwU`````

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ``````@BoxyUwU``````

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ```````@BoxyUwU```````

Rollup merge of #145251 - tiif:support_trait, r=BoxyUwU Support using #[unstable_feature_bound] on trait This is needed to unblock #145095 r? ```````@BoxyUwU```````

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang/rust#145095 r? ```````@BoxyUwU```````

BoxyUwU · 2025-08-19T12:22:20Z

compiler/rustc_hir_analysis/src/check/wfcheck.rs

-                    Ok(())
-                })
-            } else if tcx.features().adt_const_params() {
+            if tcx.features().adt_const_params() || tcx.features().unsized_const_params() {


we should make unsized_const_params no longer be a lang feature. you can do this by removing its entry from unstable.rs

tiif · 2025-08-22T20:51:04Z

Currently all tests in tests/ui passed locally, but some test change feels suspicious, will take a closer look soon 🤔

bors · 2025-08-23T09:22:49Z

☔ The latest upstream changes (presumably #145773) made this pull request unmergeable. Please resolve the merge conflicts.

library/core/src/tuple.rs

tiif · 2025-08-31T14:29:04Z

compiler/rustc_hir_analysis/src/check/wfcheck.rs

+            if tcx.features().adt_const_params()
+                || tcx.features().enabled(sym::unsized_const_params)
+            {


It turns out that we need to have this change in order for the test here not to request for adt_const_params to be enabled.

I think that test should just enable adt_const_params

tiif · 2025-09-02T09:59:19Z

tests/ui/const-generics/adt_const_params/unsized_field-1.rs

weird, why do we no longer emit error for this?

Foo implements ConstParamTy with no unstable_feature_bound so is useable by all downstream crates. This is fine I think

tiif · 2025-09-02T09:59:39Z

tests/ui/feature-gates/feature-gate-unsized-const-params.rs

There is a diagnostic regression here, it'd be nice to keep the help message.

tiif · 2025-09-02T10:00:11Z

tests/ui/layout/thaw-transmute-invalid-enum.stderr

Diagnostic is a bit chatty here, we already have the same diagnostic above, it'd be nice to only say it once.

I suspect this happen because we have both #[unstable] and #[unstable_feature_bound] on the trait, and they emit two errors separately. I think I will just leave this as it is and write a test later.

The test is in https://github.com/rust-lang/rust/pull/146401/files, I think we can leave this as it is, and improve the diagnostic in another PR.

BoxyUwU · 2025-09-02T10:26:07Z

compiler/rustc_trait_selection/src/traits/misc.rs

@@ -101,7 +102,8 @@ pub fn type_allowed_to_implement_const_param_ty<'tcx>(
    lang_item: LangItem,
    parent_cause: ObligationCause<'tcx>,
 ) -> Result<(), ConstParamTyImplementationError<'tcx>> {
-    assert_matches!(lang_item, LangItem::ConstParamTy | LangItem::UnsizedConstParamTy);
+    assert_matches!(lang_item, LangItem::ConstParamTy);


no need to accept a lang_item argument if it can only ever be one thing. just replace all uses of lang_item with LangItem::ConstParamTy

BoxyUwU · 2025-09-02T10:27:53Z

library/core/src/tuple.rs

@@ -45,17 +45,11 @@ macro_rules! tuple_impls {
        maybe_tuple_doc! {
            $($T)+ @
            #[unstable(feature = "adt_const_params", issue = "95174")]
+            #[unstable_feature_bound(adt_const_params)]


Suggested change

#[unstable_feature_bound(adt_const_params)]

#[unstable_feature_bound(unsized_const_params)]

dont want this impl insta-stable when we stabilize adt const params. its not really "unsized" but right now its the only other feature gate we can put impls under 😅

I expect we'll wind up with a bunch of different feature gates for "support X

BoxyUwU · 2025-09-02T10:31:26Z

compiler/rustc_hir_analysis/src/check/wfcheck.rs

+            if tcx.features().adt_const_params()
+                || tcx.features().enabled(sym::unsized_const_params)
+            {


I think that test should just enable adt_const_params

BoxyUwU · 2025-09-02T10:32:55Z

tests/ui/const-generics/adt_const_params/unsized_field-1.rs

Foo implements ConstParamTy with no unstable_feature_bound so is useable by all downstream crates. This is fine I think

BoxyUwU · 2025-09-02T10:33:32Z

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -142,7 +139,7 @@ fn visit_implementation_of_const_param_ty(
    checker: &Checker<'_>,
    kind: LangItem,
 ) -> Result<(), ErrorGuaranteed> {
-    assert_matches!(kind, LangItem::ConstParamTy | LangItem::UnsizedConstParamTy);
+    assert_matches!(kind, LangItem::ConstParamTy);


u can remove the kind parameter, no need to accept an argument that can only be one value

BoxyUwU · 2025-09-02T10:46:19Z

tests/ui/feature-gates/feature-gate-adt_const_params.stderr

@@ -5,14 +5,6 @@ LL | struct Foo<const NAME: &'static str>;
   |                        ^^^^^^^^^^^^
   |
   = note: the only supported types are integers, `bool`, and `char`
-help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types


driveby: can you change this test to use a const generic parameter that would actually be supported under adt_const_params. e.g. struct Foo<const N: Bar>; struct Bar(u8);

BoxyUwU · 2025-09-02T10:50:08Z

compiler/rustc_trait_selection/src/traits/misc.rs

+                parent_cause.clone(),
+                param_env,
+                ty::ClauseKind::UnstableFeature(sym::unsized_const_params),
+            ))


I believe the diagnostic regression where we lost the help: enable this feature is caused by the fact that type_allowed_to_implement_const_param_ty no longer returns a special error for when its not allowed because of a unsized_const_params only type.

calling ocx.select_where_possible here and returning ConstParamTyImplementationError::UnsizedConstParamsFeatureRequired if there were any errors should fix that

ah. needs to be ocx.select_all_or_error actually. i forgot that unstable feature bounds never return NoSolution. You only register the bound here, you dont actually try and prove it and then return a custom failure reason which is what will bring back all the "help" messages

amazing, this works!

BoxyUwU · 2025-09-10T14:32:30Z

tests/ui/const-generics/generic_const_parameter_types/check-type-in-mir.stderr

@@ -1,27 +1,19 @@
-warning: the feature `unsized_const_params` is incomplete and may not be safe to use and/or cause compiler crashes


ah hmm :(

I kinda forgot that switching to a library feature would mean we no longer consider feature(unsized_const_params) to be incomplete 🤔 Do you think you could re-add unsized_const_params as a langauge feature just so that we continue getting these warnings? It shouldn't actually be used anywhere in the compiler, I just want people to get pushed away from actually using this for now lol

Yeah I brought it back now. Initially I was thinking if there is any way to have similar error message for libs feature, but I didn't find anything. 🤔

tiif · 2025-09-12T06:48:46Z

tests/ui/feature-gates/feature-gate-unsized-const-params.stderr

+error[E0741]: `Bar` must implement `ConstParamTy` to be used as the type of a const generic parameter
+  --> $DIR/feature-gate-unsized-const-params.rs:5:21
   |
-LL | struct Foo<const N: [u8]>;
-   |                     ^^^^
+LL | struct Foo<const N: Bar>;
+   |                     ^^^
   |
-   = note: the only supported types are integers, `bool`, and `char`
-help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+help: add `#[derive(ConstParamTy, PartialEq, Eq)]` to the struct
   |
-LL + #![feature(adt_const_params)]
-   |
-help: add `#![feature(unsized_const_params)]` to the crate attributes to enable references to implement the `ConstParamTy` trait
-   |
-LL + #![feature(unsized_const_params)]
+LL - struct Bar(u8);
+LL + #[derive(ConstParamTy, PartialEq, Eq)]


The help message here is a bit weird, but feels like a pre-existing issue, can take a look at this after tihs PR.

This reverts commit 675ad69.

…e warning

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tiif mentioned this pull request Aug 11, 2025

Support using #[unstable_feature_bound] on trait #145251

Merged

fmease added a commit to fmease/rust that referenced this pull request Aug 11, 2025

Rollup merge of rust-lang#145251 - tiif:support_trait, r=BoxyUwU

6434c4b

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? `@BoxyUwU`

fmease added a commit to fmease/rust that referenced this pull request Aug 12, 2025

Rollup merge of rust-lang#145251 - tiif:support_trait, r=BoxyUwU

0bce3c2

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ``@BoxyUwU``

fmease added a commit to fmease/rust that referenced this pull request Aug 12, 2025

Rollup merge of rust-lang#145251 - tiif:support_trait, r=BoxyUwU

5238288

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ```@BoxyUwU```

fmease added a commit to fmease/rust that referenced this pull request Aug 12, 2025

Rollup merge of rust-lang#145251 - tiif:support_trait, r=BoxyUwU

b1777e3

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ````@BoxyUwU````

fmease added a commit to fmease/rust that referenced this pull request Aug 12, 2025

Rollup merge of rust-lang#145251 - tiif:support_trait, r=BoxyUwU

556c571

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? `````@BoxyUwU`````

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Rollup merge of rust-lang#145251 - tiif:support_trait, r=BoxyUwU

cfae921

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ``````@BoxyUwU``````

Zalathar added a commit to Zalathar/rust that referenced this pull request Aug 12, 2025

Rollup merge of rust-lang#145251 - tiif:support_trait, r=BoxyUwU

42af95b

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang#145095 r? ```````@BoxyUwU```````

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Unrolled build for #145251

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Rollup merge of #145251 - tiif:support_trait, r=BoxyUwU Support using #[unstable_feature_bound] on trait This is needed to unblock #145095 r? ```````@BoxyUwU```````

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Rollup merge of #145251 - tiif:support_trait, r=BoxyUwU

8131632

Support using #[unstable_feature_bound] on trait This is needed to unblock rust-lang/rust#145095 r? ```````@BoxyUwU```````

rust-cloud-vms bot force-pushed the unstable_const_param branch from a4bd0a8 to 262cf11 Compare August 18, 2025 15:00

BoxyUwU reviewed Aug 19, 2025

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tiif added 5 commits September 13, 2025 14:21

wip0: compile pass

8003b61

wip1: use unstable_feature_bound on trait

0d76177

wip2: remove the error of needing to implement unsizedconstparamty

1bc49ad

wip3: Remove UnsizedConstParamTy related stuff

5cefaeb

wip with boxy

9a52da8

tiif added 14 commits September 13, 2025 14:21

Make build pass

a23ab7e

Fix tests

617b0ae

Clean up and add comment

2280b9c

Fix regression: the feature is not needed

784406b

Revert "Fix regression: the feature is not needed"

13a886d

This reverts commit 675ad69.

Directly use LangItem::ConstParamTy when necessary

32365f2

Add ocx.select_where_possible

93c35eb

Make the test case to be actually supported under adt_const_params

f2e1406

Use select_or_all_error

dfeeb5e

Bring back the help message

652dcf4

Use unsized_const_params for the impl

5746b32

Re-register unsized_const_params as language feature to bring back th…

5265f12

…e warning

Fix spurious test change

f889958

Fix rebase error

1f70531

rust-cloud-vms bot force-pushed the unstable_const_param branch from 8d88452 to 1f70531 Compare September 13, 2025 14:31

	#[unstable_feature_bound(adt_const_params)]
	#[unstable_feature_bound(unsized_const_params)]

		@@ -1,27 +1,19 @@
		warning: the feature `unsized_const_params` is incomplete and may not be safe to use and/or cause compiler crashes

Migrate UnsizedConstParamTy to unstable impl of ConstParamTy_ #145095

Are you sure you want to change the base?

Migrate UnsizedConstParamTy to unstable impl of ConstParamTy_ #145095

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Migrate `UnsizedConstParamTy` to unstable impl of `ConstParamTy_` #145095

Migrate `UnsizedConstParamTy` to unstable impl of `ConstParamTy_` #145095

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