layout_of unexpected rigid alias delayed bug by lcnr · Pull Request #152575 · rust-lang/rust

lcnr · 2026-02-13T12:55:40Z

fixes #152545. The trait solver can keep aliases as rigid even if they are not well-formed

rust/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

Lines 315 to 345 in d7daac0

    
           if !cx.has_item_definition(target_item_def_id) { 
        
               // If the impl is missing an item, it's either because the user forgot to 
        
               // provide it, or the user is not *obligated* to provide it (because it 
        
               // has a trivially false `Sized` predicate). If it's the latter, we cannot 
        
               // delay a bug because we can have trivially false where clauses, so we 
        
               // treat it as rigid. 
        
               if cx.impl_self_is_guaranteed_unsized(impl_def_id) { 
        
                   match ecx.typing_mode() { 
        
                       // Trying to normalize such associated items is always ambiguous 
        
                       // during coherence to avoid cyclic reasoning. See the example in 
        
                       // tests/ui/traits/trivial-unsized-projection-in-coherence.rs. 
        
                       // 
        
                       // As this ambiguity is unavoidable we emit a nested ambiguous 
        
                       // goal instead of using `Certainty::AMBIGUOUS`. This allows us to 
        
                       // return the nested goals to the parent `AliasRelate` goal. This 
        
                       // would be relevant if any of the nested goals refer to the `term`. 
        
                       // This is not the case here and we only prefer adding an ambiguous 
        
                       // nested goal for consistency. 
        
                       ty::TypingMode::Coherence => { 
        
                           ecx.add_goal(GoalSource::Misc, goal.with(cx, PredicateKind::Ambiguous)); 
        
                           return then(ecx, Certainty::Yes); 
        
                       } 
        
                       ty::TypingMode::Analysis { .. } 
        
                       | ty::TypingMode::Borrowck { .. } 
        
                       | ty::TypingMode::PostBorrowckAnalysis { .. } 
        
                       | ty::TypingMode::PostAnalysis => { 
        
                           ecx.structurally_instantiate_normalizes_to_term( 
        
                               goal, 
        
                               goal.predicate.alias, 
        
                           ); 
        
                           return then(ecx, Certainty::Yes);

r? types

compiler/rustc_ty_utils/src/layout.rs

jackh726 · 2026-02-13T18:22:46Z

@bors r+ rollup

rust-bors · 2026-02-13T18:22:49Z

📌 Commit b3d9fbc has been approved by jackh726

It is now in the queue for this repository.

…r=jackh726 layout_of unexpected rigid alias delayed bug fixes rust-lang#152545. The trait solver can keep aliases as rigid even if they are not well-formed https://github.com/rust-lang/rust/blob/d7daac06d87e1252d10eaa44960164faac46beff/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs#L315-L345 r? types

rustbot assigned jackh726 Feb 13, 2026

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. beta-nominated Nominated for backporting to the compiler in the beta channel. labels Feb 13, 2026

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cyrgani reviewed Feb 13, 2026

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compiler/rustc_ty_utils/src/layout.rs Outdated Show resolved Hide resolved

lcnr added 2 commits February 13, 2026 13:50

add regression test

b4f38c1

ICE to delayed bug

b3d9fbc

lcnr force-pushed the layout-error-to-delayed-bug branch from 53fe5ec to b3d9fbc Compare February 13, 2026 13:51

rust-bors bot 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 Feb 13, 2026

jhpratt mentioned this pull request Feb 14, 2026

Rollup of 11 pull requests #152602

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layout_of unexpected rigid alias delayed bug#152575

layout_of unexpected rigid alias delayed bug#152575
lcnr wants to merge 2 commits intorust-lang:mainfrom
lcnr:layout-error-to-delayed-bug

lcnr commented Feb 13, 2026 •

edited by rustbot

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	if !cx.has_item_definition(target_item_def_id) {
	// If the impl is missing an item, it's either because the user forgot to
	// provide it, or the user is not obligated to provide it (because it
	// has a trivially false `Sized` predicate). If it's the latter, we cannot
	// delay a bug because we can have trivially false where clauses, so we
	// treat it as rigid.
	if cx.impl_self_is_guaranteed_unsized(impl_def_id) {
	match ecx.typing_mode() {
	// Trying to normalize such associated items is always ambiguous
	// during coherence to avoid cyclic reasoning. See the example in
	// tests/ui/traits/trivial-unsized-projection-in-coherence.rs.
	//
	// As this ambiguity is unavoidable we emit a nested ambiguous
	// goal instead of using `Certainty::AMBIGUOUS`. This allows us to
	// return the nested goals to the parent `AliasRelate` goal. This
	// would be relevant if any of the nested goals refer to the `term`.
	// This is not the case here and we only prefer adding an ambiguous
	// nested goal for consistency.
	ty::TypingMode::Coherence => {
	ecx.add_goal(GoalSource::Misc, goal.with(cx, PredicateKind::Ambiguous));
	return then(ecx, Certainty::Yes);
	}
	ty::TypingMode::Analysis { .. }
	\| ty::TypingMode::Borrowck { .. }
	\| ty::TypingMode::PostBorrowckAnalysis { .. }
	\| ty::TypingMode::PostAnalysis => {
	ecx.structurally_instantiate_normalizes_to_term(
	goal,
	goal.predicate.alias,
	);
	return then(ecx, Certainty::Yes);

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lcnr commented Feb 13, 2026 •

edited by rustbot

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