Support inferring&completing Self type in enum/struct/union definitions

[ice1000]

Signed-off-by: ice1000 ice1000kotlin@foxmail.com

An attempt to fix #1908.
This code works, but I believe the implementation is ugly. Please give me suggestions!

[ice1000]

I've updated the comment in #1908 with a screenshot.

[flodiebold]

Note that this should also work for structs; also, Self should refer to the whole type with its type parameters, not just the enum. E.g.:

struct List<T> {
    elem: T,
    next: Option<Box<Self>>,
}

(For a test, I'd shorten that to struct List<T> { next: Self } -- it doesn't matter that this wouldn't compile.)

We already resolve Self in impl blocks, and this is very similar. The Resolver works by having a stack of scopes of various kinds, one of them ImplBlock. When building a Resolver for an impl block, we push such a scope on the stack here. The ImplBlockScope is then handled in a few places in resolve.rs; when resolving Self as a type, we return TypeNs::SelfType. And that's then mainly handled here during type lowering to resolve the target type of the impl.

I think I'd suggest adding an AdtScope(Adt) in the Scope enum, and an AdtSelfType(Adt) in the TypeNs enum. (Note that this is different from the existing TypeNs::Adt, because that one takes type parameters, while Self always gets the type parameters from the definition.) The scope should be handled basically the same way as the impl block scope; we just need to take care to push it in the resolver methods of Struct, Enum and Union. And when handling TypeNs::AdtSelfType in lower.rs, you can just use the ty method to get the definition type.

(Handling the scope here will also mean we'll complete Self in struct/enum definitions, by the way.)

[ice1000]

Changed implementation into Scope::AdtSelfScope, please take a look @flodiebold

[flodiebold]

Looks good to me apart from what @matklad already wrote! You could add a test for completing Self in this situation (see here).

Feel free to r+ with @matklad's suggestions:

bors delegate+

[bors]

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[ice1000]

I'm gonna apply the review suggestions locally instead of using GitHub.

[ice1000]

I'm very nervous. It's my first time to summon bors for merging a PR.

[matklad]

@ice1000 we have a rather extensive test suite, so the probability that something breaks spectacularly is pretty low :) Just reply with bors r+ comment

[ice1000]

Between the darkness and the dawn, a PR bot rises!

bors r+

[bors]

Build succeeded

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