Don't build `ParamEnv` and do trait solving in `ItemCtxt`s when lowering IATs #140247

BoxyUwU · 2025-04-24T10:15:25Z

Fixes #125879
This was due to updating inhabited predicate stuff which I had to do to make constructing ADTs with IATs in fields not ICE

Fixes #136678 (but no test added, I don't rly care about weird IAT edge cases under GCE)

Fixes #138240
Fixes #138266
These ICEs got masked but we have 3 other ICEs that look like dupes so I'm not too worried about this. Caused by mgca not properly lowering paths in type signatures to forbid infers or something, and now we happen to emit an error when we previously didnt.

Avoids doing "fully correct" candidate selection for IATs during hir ty lowering when in item signatures as it almost always leads to a query cycle from trying to build a ParamEnv. I replaced it with a use DeepRejectCtxt which should be able to handle this kind of conservative "could these types unify" while in a context where we don't want to do type equality.

A potential alternative would be to replace all the generic parameters in the self type with inference variables as then it would be "truly" environment agnostic and we could just use an empty environment. I think this is somewhat subpar as we would be weakening how much we used the self type to guide candidate selection and that's the only way for the user to attempt to specify which IAT to resolve to.

In general that's also why I think doing something hacky here instead of just throwing our hands up and saying "ItemCtxts shouldnt make inference variables" and not filtering any candidates out, is a good idea (there is no way for users to disambiguate such cases).

I'm not really sure how this interacts with #126651, though I'm also not really sure its super important to support projecting IATs from IAT self types given we don't even support T::Assoc::Other for trait-associated types so didn't give much thought to how this might fit in with that.

r? @compiler-errors
cc @fmease

rustbot · 2025-04-24T10:15:31Z

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

HIR ty lowering was modified

cc @fmease

compiler/rustc_hir_analysis/src/collect.rs

bors · 2025-04-28T21:32:11Z

☔ The latest upstream changes (presumably #140388) made this pull request unmergeable. Please resolve the merge conflicts.

compiler-errors · 2025-05-22T09:49:10Z

@bors r+

bors · 2025-05-22T09:49:13Z

📌 Commit 34ce403 has been approved by compiler-errors

It is now in the queue for this repository.

…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#135562 (Add ignore value suggestion in closure body) - rust-lang#139635 (Finalize repeat expr inference behaviour with inferred repeat counts) - rust-lang#139668 (Handle regions equivalent to 'static in non_local_bounds) - rust-lang#140218 (HIR ty lowering: Clean up & refactor the lowering of type-relative paths) - rust-lang#140435 (use uX::from instead of _ as uX in non - const contexts) - rust-lang#141130 (rustc_on_unimplemented cleanups) - rust-lang#141286 (Querify `coroutine_hidden_types`) Failed merges: - rust-lang#140247 (Don't build `ParamEnv` and do trait solving in `ItemCtxt`s when lowering IATs) r? `@ghost` `@rustbot` modify labels: rollup

bors · 2025-05-23T01:02:00Z

☔ The latest upstream changes (presumably #141396) made this pull request unmergeable. Please resolve the merge conflicts.

BoxyUwU · 2025-05-23T14:28:28Z

I should probably update this to use DeepRejectCtxt and handle LTAs correctly like discussed at the all hands 🤔 I was just gonna do it in a followup PR but if I'm having to rebase and figure out new crashes tests then I may aswell just do it all now

compiler/rustc_hir_analysis/src/collect.rs

BoxyUwU · 2025-05-23T16:23:00Z

compiler/rustc_hir_analysis/src/collect.rs

+        // We make an infcx and replace any escaping vars with placeholders so that IAT res
+        // with type/const bound vars in arguments is slightly smarter. `for<T> <Foo<T>>::IAT`
+        // would otherwise unify with `impl Foo<u8>` when ideally we would not.
+        let infcx = self.tcx().infer_ctxt().build(TypingMode::non_body_analysis());


Creating an infcx just to make placeholders feels pretty bad lol

do u need to? id expect expand free aliases to handle escaping bound vars. so does deep reject

deep reject "handles" escaping vars by allowing them to unify with anything, which is weaker IAT resolution than replacing with placeholders. See tests/ui/associated-inherent-types/bound_vars_in_args.rs which fails if we don't replace with placeholders

BoxyUwU · 2025-05-23T19:06:37Z

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

-        if candidates.is_empty() {
-            return Ok(None);
-        }


Removing this masked a bunch of mgca ICEs I think. We now emit errors in all cases where IAT resolution fails to find any candidates which makes some delayed bugs no longer trigger. I've replaced a few of the crashes tests with examples that don't rely on IATs and I removed the two others that I couldn't find a reproducer for that didn't seem like a dupe of the other three.

…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#135562 (Add ignore value suggestion in closure body) - rust-lang#139635 (Finalize repeat expr inference behaviour with inferred repeat counts) - rust-lang#139668 (Handle regions equivalent to 'static in non_local_bounds) - rust-lang#140218 (HIR ty lowering: Clean up & refactor the lowering of type-relative paths) - rust-lang#140435 (use uX::from instead of _ as uX in non - const contexts) - rust-lang#141130 (rustc_on_unimplemented cleanups) - rust-lang#141286 (Querify `coroutine_hidden_types`) Failed merges: - rust-lang#140247 (Don't build `ParamEnv` and do trait solving in `ItemCtxt`s when lowering IATs) r? `@ghost` `@rustbot` modify labels: rollup

BoxyUwU · 2025-05-27T17:28:50Z

tests/ui/associated-inherent-types/impl_params_are_infers.rs

+    // It would be illegal to resolve to `Foo<T, T, u8>::IAT`  as  `T` and `U` are
+    // different types. However, currently we treat all impl-side params sort of like
+    // they're infers and assume they can unify with anything, so we consider it a
+    // valid candidate.
+    field: Foo<T, U, <u16 as Identity>::This>::IAT,
+    //~^ ERROR: multiple applicable items in scope


Honestly not sure how I feel about using DeepRejectCtxt knowing it doesn't let this code work 🤔 This PR is already a big step forwards though so I think I'm in favour of just continuing with this impl and revisiting before stabilization.

rustbot assigned compiler-errors Apr 24, 2025

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fmease mentioned this pull request Apr 29, 2025

Tracking Issue for RFC 3591: useing a function from a trait #134691

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Rollup of 7 pull requests #141396

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Don't build ParamEnv and do trait solving in ItemCtxts

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Test whether we use DeepRejectCtxt

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Don't build `ParamEnv` and do trait solving in `ItemCtxt`s when lowering IATs #140247

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