fix: Error on illegal `[const]`s inside blocks within legal positions #144741

ShoyuVanilla · 2025-07-31T15:32:24Z

Fixes #132067

I initially considered moving [const] validations to rustc_ast_lowering, but that approach would require adding constness information to AssocCtxt, which introduces significant changes - especially within rustc_expand - just to support a single use case here:

rust/compiler/rustc_ast_passes/src/ast_validation.rs

Lines 1596 to 1610 in 3fb1b53

    
           let parent_is_const = 
        
               self.outer_trait_or_trait_impl.as_ref().and_then(TraitOrTraitImpl::constness).is_some(); 
        
           match &item.kind { 
        
               AssocItemKind::Fn(func) 
        
                   if parent_is_const 
        
                       || ctxt == AssocCtxt::Trait 
        
                       || matches!(func.sig.header.constness, Const::Yes(_)) => 
        
               { 
        
                   self.visit_attrs_vis_ident(&item.attrs, &item.vis, &func.ident); 
        
                   let kind = FnKind::Fn(FnCtxt::Assoc(ctxt), &item.vis, &*func); 
        
                   self.visit_fn(kind, item.span, item.id); 
        
               } 
        
               AssocItemKind::Type(_) => { 
        
                   let disallowed = (!parent_is_const).then(|| match self.outer_trait_or_trait_impl {

Instead, I believe it's sufficient to simply "reset" [const] allowness whenever we enter a new block.

rustbot · 2025-07-31T15:32:29Z

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ShoyuVanilla · 2025-07-31T15:33:36Z

tests/ui/traits/const-traits/conditionally-const-in-struct-args.rs

@@ -1,7 +1,3 @@
-//@ compile-flags: -Znext-solver


I guess this flags were introduced to prevent triggering other ICEs but we don't need this anymore?

ShoyuVanilla · 2025-07-31T16:31:56Z

tests/ui/unpretty/exhaustive.rs

@@ -807,7 +807,6 @@ mod types {
        let _: impl Send + 'static;     //[hir]~ ERROR `impl Trait` is not allowed
        let _: impl 'static + Send;     //[hir]~ ERROR `impl Trait` is not allowed
        let _: impl ?Sized;             //[hir]~ ERROR `impl Trait` is not allowed
-        let _: impl [const] Clone;       //[hir]~ ERROR `impl Trait` is not allowed


This one used to pass ast passes - and failing tests with this PR - but I think it shouldn't

Hm, yeah this should definitely still be allowed. impl [const] Clone is valid in a const fn with the impl_trait_in_bindings, and it should for example work in this case:

const fn foo<T: [const] Foo>(x: &T) { let y: &impl [const] Foo = x; }

impl_trait_in_bindings is probably just broken with [const]-ness. If you could fix the AST validation, then that would be great. Otherwise, I'd at least like to understand why it is broken so it can have a proper FIXME in the codebase and so we can file an issue to fix it as a follow-up.

Oh, I wasn't aware of impl_trait_in_bindings 😅 I'll try fixing this AST validation with proper interaction with that feature. Thanks!

ShoyuVanilla · 2025-08-04T06:18:24Z

So, I’ve prohibited the following:

Any usage of [const] Trait bounds on ADTs, regardless of position—including inside const fn bodies.
Any local usages of [const] Trait bounds (I doubt there would be any case other than impl [const] Trait within impl_trait_in_binding) inside anonymous constants, such as in const generic arguments. They are still allowed in legal places like const fn bodies.

fee1-dead · 2025-08-04T06:49:55Z

@bors r+ rollup

bors · 2025-08-04T06:49:58Z

📌 Commit 7d78968 has been approved by fee1-dead

It is now in the queue for this repository.

… r=fee1-dead fix: Error on illegal `[const]`s inside blocks within legal positions Fixes rust-lang#132067 I initially considered moving `[const]` validations to `rustc_ast_lowering`, but that approach would require adding constness information to `AssocCtxt`, which introduces significant changes - especially within `rustc_expand` - just to support a single use case here: https://github.com/rust-lang/rust/blob/3fb1b53a9dbfcdf37a4b67d35cde373316829930/compiler/rustc_ast_passes/src/ast_validation.rs#L1596-L1610 Instead, I believe it's sufficient to simply "reset" `[const]` allowness whenever we enter a new block.

Rollup of 5 pull requests Successful merges: - #144741 (fix: Error on illegal `[const]`s inside blocks within legal positions) - #144779 (Implement debugging output of the bootstrap Step graph into a DOT file) - #144813 (Add a tidy check to prevent adding UI tests directly under `tests/ui/`) - #144866 (Remove `SHOULD_EMIT_LINTS` in favor of `should_emit`) - #144887 (`rust-analyzer` subtree update) r? `@ghost` `@rustbot` modify labels: rollup

… r=fee1-dead fix: Error on illegal `[const]`s inside blocks within legal positions Fixes rust-lang#132067 I initially considered moving `[const]` validations to `rustc_ast_lowering`, but that approach would require adding constness information to `AssocCtxt`, which introduces significant changes - especially within `rustc_expand` - just to support a single use case here: https://github.com/rust-lang/rust/blob/3fb1b53a9dbfcdf37a4b67d35cde373316829930/compiler/rustc_ast_passes/src/ast_validation.rs#L1596-L1610 Instead, I believe it's sufficient to simply "reset" `[const]` allowness whenever we enter a new block.

Rollup of 18 pull requests Successful merges: - #144467 (rustdoc template font links only emit `crossorigin` when needed) - #144548 (Rehome 21 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`) - #144596 (libtest: print the type of test being run) - #144741 (fix: Error on illegal `[const]`s inside blocks within legal positions) - #144776 (`Printer` cleanups) - #144779 (Implement debugging output of the bootstrap Step graph into a DOT file) - #144813 (Add a tidy check to prevent adding UI tests directly under `tests/ui/`) - #144817 (Properly reject tail calls to `&FnPtr` or `&FnDef`) - #144852 (Rename `rust_panic_without_hook` to `resume_unwind` ) - #144866 (Remove `SHOULD_EMIT_LINTS` in favor of `should_emit`) - #144867 (Use `as_array` in PartialEq for arrays) - #144872 (Document Poisoning in `LazyCell` and `LazyLock`) - #144877 (coverage: Various small cleanups) - #144887 (`rust-analyzer` subtree update) - #144890 (Add `InterpCx::project_fields`) - #144894 (Delete `tests/ui/threads-sendsync/tcp-stress.rs`) - #144905 (rustc-dev-guide subtree update) - #144920 (Dont print arg span in MIR dump for tail call) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 17 pull requests Successful merges: - #144467 (rustdoc template font links only emit `crossorigin` when needed) - #144548 (Rehome 21 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`) - #144741 (fix: Error on illegal `[const]`s inside blocks within legal positions) - #144776 (`Printer` cleanups) - #144779 (Implement debugging output of the bootstrap Step graph into a DOT file) - #144813 (Add a tidy check to prevent adding UI tests directly under `tests/ui/`) - #144817 (Properly reject tail calls to `&FnPtr` or `&FnDef`) - #144852 (Rename `rust_panic_without_hook` to `resume_unwind` ) - #144866 (Remove `SHOULD_EMIT_LINTS` in favor of `should_emit`) - #144867 (Use `as_array` in PartialEq for arrays) - #144872 (Document Poisoning in `LazyCell` and `LazyLock`) - #144877 (coverage: Various small cleanups) - #144887 (`rust-analyzer` subtree update) - #144890 (Add `InterpCx::project_fields`) - #144894 (Delete `tests/ui/threads-sendsync/tcp-stress.rs`) - #144905 (rustc-dev-guide subtree update) - #144920 (Dont print arg span in MIR dump for tail call) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144741 - ShoyuVanilla:tilde-const-in-block, r=fee1-dead fix: Error on illegal `[const]`s inside blocks within legal positions Fixes #132067 I initially considered moving `[const]` validations to `rustc_ast_lowering`, but that approach would require adding constness information to `AssocCtxt`, which introduces significant changes - especially within `rustc_expand` - just to support a single use case here: https://github.com/rust-lang/rust/blob/3fb1b53a9dbfcdf37a4b67d35cde373316829930/compiler/rustc_ast_passes/src/ast_validation.rs#L1596-L1610 Instead, I believe it's sufficient to simply "reset" `[const]` allowness whenever we enter a new block.

Rollup of 17 pull requests Successful merges: - rust-lang/rust#144467 (rustdoc template font links only emit `crossorigin` when needed) - rust-lang/rust#144548 (Rehome 21 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`) - rust-lang/rust#144741 (fix: Error on illegal `[const]`s inside blocks within legal positions) - rust-lang/rust#144776 (`Printer` cleanups) - rust-lang/rust#144779 (Implement debugging output of the bootstrap Step graph into a DOT file) - rust-lang/rust#144813 (Add a tidy check to prevent adding UI tests directly under `tests/ui/`) - rust-lang/rust#144817 (Properly reject tail calls to `&FnPtr` or `&FnDef`) - rust-lang/rust#144852 (Rename `rust_panic_without_hook` to `resume_unwind` ) - rust-lang/rust#144866 (Remove `SHOULD_EMIT_LINTS` in favor of `should_emit`) - rust-lang/rust#144867 (Use `as_array` in PartialEq for arrays) - rust-lang/rust#144872 (Document Poisoning in `LazyCell` and `LazyLock`) - rust-lang/rust#144877 (coverage: Various small cleanups) - rust-lang/rust#144887 (`rust-analyzer` subtree update) - rust-lang/rust#144890 (Add `InterpCx::project_fields`) - rust-lang/rust#144894 (Delete `tests/ui/threads-sendsync/tcp-stress.rs`) - rust-lang/rust#144905 (rustc-dev-guide subtree update) - rust-lang/rust#144920 (Dont print arg span in MIR dump for tail call) r? `@ghost` `@rustbot` modify labels: rollup

rustbot assigned fee1-dead Jul 31, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 31, 2025

ShoyuVanilla commented Jul 31, 2025

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ShoyuVanilla force-pushed the tilde-const-in-block branch from 5ec2c20 to c02624b Compare July 31, 2025 16:29

ShoyuVanilla commented Jul 31, 2025

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ShoyuVanilla marked this pull request as draft July 31, 2025 17:13

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 31, 2025

fmease added the PG-const-traits Project group: Const traits label Jul 31, 2025

fix: Error on illegal [const]s inside blocks within legal positions

7d78968

ShoyuVanilla force-pushed the tilde-const-in-block branch from c02624b to 7d78968 Compare August 3, 2025 16:49

ShoyuVanilla marked this pull request as ready for review August 3, 2025 16:49

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 Aug 3, 2025

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 4, 2025

Zalathar mentioned this pull request Aug 4, 2025

Rollup of 5 pull requests #144891

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samueltardieu mentioned this pull request Aug 4, 2025

Rollup of 18 pull requests #144929

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samueltardieu mentioned this pull request Aug 5, 2025

Rollup of 17 pull requests #144934

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bors merged commit 4cb0ebd into rust-lang:master Aug 5, 2025
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rustbot added this to the 1.91.0 milestone Aug 5, 2025

ShoyuVanilla deleted the tilde-const-in-block branch August 5, 2025 05:09

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fix: Error on illegal `[const]`s inside blocks within legal positions #144741

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	let parent_is_const =
	self.outer_trait_or_trait_impl.as_ref().and_then(TraitOrTraitImpl::constness).is_some();

	match &item.kind {
	AssocItemKind::Fn(func)
	if parent_is_const
	\|\| ctxt == AssocCtxt::Trait
	\|\| matches!(func.sig.header.constness, Const::Yes(_)) =>
	{
	self.visit_attrs_vis_ident(&item.attrs, &item.vis, &func.ident);
	let kind = FnKind::Fn(FnCtxt::Assoc(ctxt), &item.vis, &*func);
	self.visit_fn(kind, item.span, item.id);
	}
	AssocItemKind::Type(_) => {
	let disallowed = (!parent_is_const).then(\|\| match self.outer_trait_or_trait_impl {

fix: Error on illegal [const]s inside blocks within legal positions #144741

fix: Error on illegal [const]s inside blocks within legal positions #144741

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fix: Error on illegal `[const]`s inside blocks within legal positions #144741

fix: Error on illegal `[const]`s inside blocks within legal positions #144741

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