rustc doesn't check nested WFness if it's in a higher-ranked predicate #162200

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1. compiler-errors commented on Apr 8, 2024

   

   compiler-errors

   on Apr 8, 2024

   Last edited by lcnr

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   Minimal I think:

   trait Bound {}
   
   struct W<'a, T: Bound>(&'a T);
   
   fn wf<T>() where for<'a> W<'a, T>: Sized {}

   We should recurse on W<'a, T> and see T: Bound must hold, but we don't even recurse on the self arg of the predicate because the trait has escaping bound vars.
2. jackh726 commented on Apr 11, 2024

   

   jackh726

   on Apr 11, 2024

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   That regression list sucks. I think this is something we should consider slowly moving towards error (or at least deny-by-default) by starting with a warn-by-default lint.

   I'm also curious if there's a good MCVE for things that are so "obvious" as the example above. This modification, for example, is pretty clear what the problem is when trying to call wf:

   use std::fmt::Debug;
   
   trait Bound {}
   
   struct W<'a, T: Bound>(&'a T);
   
   fn wf<T>() where for<'a> W<'a, T>: Debug {}
   
   fn main() {
       wf::<u32>();
   }

   But, I could imagine that we can have move subtle things we don't check in e.g. super trait bounds. These could be good candidates to also move towards erroring quicker.
3. lcnr commented on Jul 25, 2025

   

   lcnr

   on Jul 25, 2025

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   For where-clauses this check happens during item wf checking, could we FCW on these missing bounds? I think implying T: Bound here is undesirable and if we start the FCW we'll likely be in a fairly good spot once we actually want to finish the hr-implied bounds work
4. lcnr

   transferred this issue fromrust-lang/trait-system-refactor-initiativeon Sep 2, 2026
5. rustbot

   added

   needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triagingThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging

   on Sep 2, 2026
6. lcnr

   added

   T-typesRelevant to the types team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.

   A-higher-rankedArea: Higher-ranked things (e.g., lifetimes, types, trait bounds aka HRTBs)Area: Higher-ranked things (e.g., lifetimes, types, trait bounds aka HRTBs)

   and removed

   needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triagingThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging

   on Sep 3, 2026