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WIP: Normalize associated types in structs when performing unsizing coercion #87548

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WIP: Normalize associated types in structs when performing unsizing coercion #87548

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ebe2c08
Add basic coerce-normalization test
WaffleLapkin Jul 28, 2021
2d32445
Try to normalize when checking if unsize coercion is possible
WaffleLapkin Jul 28, 2021
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11 changes: 10 additions & 1 deletion compiler/rustc_typeck/src/check/coercion.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -184,7 +184,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
match unsize {
Ok(_) => {
debug!("coerce: unsize successful");
debug!("coerce: unsize successful: {:?}", unsize);
return unsize;
}
Err(TypeError::ObjectUnsafeCoercion(did)) => {
Expand Down Expand Up @@ -603,6 +603,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
continue;
}
};


let InferOk { value: trait_pred2, obligations: obls } =
self.normalize_associated_types_in_as_infer_ok(self.cause.span, trait_pred);

debug!("coerce_unsized: normalized {:?} to {:?}; obls: {:?}", trait_pred, trait_pred2, obls);
let trait_pred = trait_pred2;
// coercion.obligations.extend(obls);
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Why is this commented out? We definitely shouldn't drop these obligations.

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This is very WIP and actually even abandoned 😅

At the time, I was experimenting with this and was hitting problems I couldn't understand, so I was commenting out different parts in an attempt to understand what and how (doesn't) work.

We've discussed this with Niko in a zulip thread and he said that it would probably be better to remove the "special trait solver" altogether:

nikomatsakis: as it happens, I would love to remove this part of the coercion code that you are fixing
nikomatsakis: I'm not sure if we can get away with it, but it's going to be really painful to make a part of chalk
nikomatsakis: the reason I dislike this code is that it is basically reproducing part of the normal "evaluate whether a trait matches" logic
nikomatsakis: but doing it in a custom and different way
nikomatsakis: anyway, I imagine that this is part of why you are seeing different behavior
nikomatsakis: I'm debating whether I can convince you into trying to remove this logic, instead of fixing it

I wanted to work on this (the issue I'm trying to fix really bothers me), but this never ended up happening.

I can try to revive this PR or try to make the refactor Niko proposed. But I'd need some guidance since I haven't worked on the compiler much before, so I'm not sure if I'll help or just spend someones time 😅

I'm assuming since you've assigned yourself, you know this part of the compiler well?


match selcx.select(&obligation.with(trait_pred)) {
// Uncertain or unimplemented.
Ok(None) => {
Expand Down
29 changes: 29 additions & 0 deletions src/test/ui/coercion/coerce-normalization.rs
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@@ -0,0 +1,29 @@
// check-pass

trait Trait {}

impl<T0> Trait for T0 {}

impl<T1: ?Sized> Id for T1 {
type Assoc = T1;
}

trait Id {
type Assoc: ?Sized;
}

struct NewType<T2: ?Sized + Id>(T2::Assoc);

fn coerce_newtype_slice<'a, T3, const N: usize>(array: &'a NewType<[T3; N]>) -> &'a NewType<[T3]> {
array
}

fn coerce_newtype_trait<T4: Trait + 'static>(tr: &NewType<T4>) -> &NewType<dyn Trait> {
tr
}

fn main() {
let nt = NewType::<[i32; 1]>([0]);
coerce_newtype_slice(&nt);
coerce_newtype_trait(&nt);
}