feat: add type aliases

[Wodann]

We want to introduce type aliases, as follows:

type Typename = OtherTypeName;

In large, this change builds on existing architecture, but it will require work in several places of our stack:

• lexer: add the type keyword (src, example)
• parsing: add parsing of type TypeName = OtherTypeName (example#1, example#2)
• hir: add type aliases as type definition (src)
• tests: add test for all of the above stages

Good first issue process:

• read our contributing guidelines
• claim this issue (assign yourself or comment below)
• implement the feature and start a Pull Request with in the description: closes feat: add type aliases #110.

If this is your first PR, welcome 🎉 😄

[fominok]

Hey,
I think that this issue will make more sense if generics feature lands. Or are there any use cases in current language state?

[baszalmstra]

Yes there was previously a case where this was needed. But that was no longer the case so this has much lower priority.

[sinato]

I have a question about mun-lang syntax. Should type aliases be definable in a block?
For example, should Example 2 work the same as Example 1?
Example1:

type OtherTypename = i32;
type Typename = OtherTypename;
fn main() {
    let a: Typename = 10
}

Example2:

type OtherTypeName = i32;
fn main() {
    type Typename = OtherTypeName;  // <- Does mun-lang expect this?
    let a: Typename = 10;
}

[Wodann]

Yes, Rust supports this and my expectation for Mun was to mimick this.

[sinato]

@Wodann
Hi, may I create a PR to support only Example1?

rust-analyzer does not yet support Example2.

The current code in rust-analyzer just completely ignores local items and builds a Resolver instance for top-level scope.

rust-lang/rust-analyzer#1165
I think it would be better to wait a little longer for the implementation of Example 2 to reuse their consideration and implementation.

[baszalmstra]

Definitely! We prefer smaller increments.