split NormalizesTo out of Projection

[lcnr]

implementing solution 3 of https://hackmd.io/noGGefTfQ0eCARFb3D0tJw to fix rust-lang/trait-system-refactor-initiative#1

r? @compiler-errors

[lcnr]

I don't think we should land this without first separately handling that FIXME.

With NormalizesTo as a goal getting proven by Projection in the environment it's otherwise far too easy to accidentally put a NormalizesTo into the environment which is useless. Even without bugs from this, it is still very confusing.

[compiler-errors]

should we skip all the nonsense and just emit an alias-eq here directly?

Well, actually, I guess this does behave differently on alias projection ?0 now 🤔

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---
   Compiling tidy v0.1.0 (/checkout/src/tools/tidy)
    Finished release [optimized] target(s) in 16.24s
##[endgroup]
fmt check
Diff in /checkout/compiler/rustc_trait_selection/src/solve/normalizes_to.rs at line 416:
             ty::Binder::dummy(ty::ProjectionPredicate {
                 projection_ty: ecx.tcx().mk_alias_ty(goal.predicate.def_id(), [self_ty]),
                 term,
-            }).to_predicate(tcx),
+            .to_predicate(tcx),
+            .to_predicate(tcx),
             // Technically, we need to check that the future type is Sized,
             // but that's already proven by the generator being WF.
             [],
Diff in /checkout/compiler/rustc_trait_selection/src/solve/normalizes_to.rs at line 457:
                     .tcx()
                     .mk_alias_ty(goal.predicate.def_id(), [self_ty, generator.resume_ty()]),
                 term,
-            }).to_predicate(tcx),
+            .to_predicate(tcx),
+            .to_predicate(tcx),
             // Technically, we need to check that the future type is Sized,
             // but that's already proven by the generator being WF.
             [],
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/rustfmt/bin/rustfmt" "--config-path" "/checkout" "--edition" "2021" "--unstable-features" "--skip-children" "--check" "/checkout/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs" "/checkout/compiler/rustc_trait_selection/src/traits/object_safety.rs" "/checkout/compiler/rustc_trait_selection/src/solve/alias_relate.rs" "/checkout/compiler/rustc_trait_selection/src/traits/util.rs" "/checkout/compiler/rustc_trait_selection/src/solve/normalizes_to.rs" "/checkout/compiler/rustc_trait_selection/src/traits/select/mod.rs" "/checkout/compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs" "/checkout/compiler/rustc_trait_selection/src/traits/specialize/mod.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.

[lcnr]

with this the following example currently fails:

fn foo(i: isize) -> isize { i + 1 }

fn apply<A, F>(f: F, v: A) where F: FnOnce(A) -> A { f(v); }

pub fn main() {
    apply(foo, 2)
}

the reason is that we try to prove Projection(<?0 as FnOnce(?1)>::Assoc, ?1) which equates <?0 as FnOnce(?1)>::Assoc with ?1 which incorrectly fails the occurs check.

I think that to fix this we can either change the generalizer to emit NormalizesTo goals in that case, going to do that in a separate PR.

[jackh726]

Huh, that's exactly the problem I ran into with Chalk :)

[bors]

☔ The latest upstream changes (presumably #108860) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

@rustbot author