fix missing match arm false positive for enum with no variants

[JoshMcguigan]

fixes #3974

[JoshMcguigan]

I think the solution in this PR should work, but cx.infer.variant_resolution_for_expr(cx.match_expr) returns None in the following case:

enum Never {}

fn test_fn(never: Never) {
	match never {}
}

I think this is an inference issue related to the match expression coming from a function argument. Any thoughts on this @flodiebold? Places I should start looking?

Note that in the IDE I do see Never as the type when I hover over the match expression, so perhaps this is some issue which only affects variant_resolution_for_expr?

[JoshMcguigan]

Sorry for the noise here. Converting back to draft because I have not yet covered the reference case (i.e. &Never).

This seems a bit tricky, since TypeCtor::Ref seems to not contain any information about the type of the underlying thing being referenced.

[flodiebold]

This seems a bit tricky, since TypeCtor::Ref seems to not contain any information about the type of the underlying thing being referenced.

The referenced type is a type parameter (&'x T is basically no different from Ref<'x, T>). There's actually ty.as_reference() for this purpose already.

[JoshMcguigan]

I've added a deref method on Ty, which recursively derences types until it gets a non-reference. This makes the test pass which was matching on a reference to the Never enum.

Let me know if there is a more appropriate name for this method. Or if it should go somewhere else. I thought about putting it in an impl block on ApplicationTy, but in that case I wasn't sure what the return type should be, since parameters.as_single() would still return a Ty and I wasn't sure if it was safe to assume that Ty would be an ApplicationTy in all cases.

[flodiebold]

bors r+

[bors]

Build succeeded:

• Rust (ubuntu-latest)
• TypeScript
• Rust (macos-latest)
• Rust (windows-latest)