Add support for trait aliases

[weiznich]

Trait aliases are currently a nightly feature.

Example code:

pub trait ValidGrouping<G> {
    type IsAggregate;
}

pub trait MixedAggregates {
    type Output;
}

mod is_aggregate {
    pub struct No;
}

pub trait NonAggregate = ValidGrouping<()>
where
    <Self as ValidGrouping<()>>::IsAggregate:
        MixedAggregates<is_aggregate::No, Output = is_aggregate::No>;

rust_analyzer seems not to be able to parse this correctly and returns a bunch of syntax errors:

383: rust-analyzer: Syntax Error: expected `{`
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected EXCL
383: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
383: rust-analyzer: Syntax Error: expected SEMI
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected an item
384: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected EXCL
385: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
385: rust-analyzer: Syntax Error: expected SEMI
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected EXCL
385: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
385: rust-analyzer: Syntax Error: expected SEMI
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected EXCL
385: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
385: rust-analyzer: Syntax Error: expected SEMI
385: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected an item

[kiljacken]

Parsing gets implemented in #2779.

It currently just gets mashed into the existing AST format so:

trait Z<U> = T<U> where Self: Debug + U: Clone;

is stored as:

trait Z<U>: T<U> where Self: Debug + U: Clone {}

I haven't read the RFC in detail apart from the parsing section, so I'm pretty sure more work need to be done to handle some of the special semantics trait aliases seems to introduce.

If the semantics differ from the above, we'd probably need a way to distinguish the two in the AST.

[weiznich]

Thanks for the fast fix. For my current use case it's sufficient to just not generate any parser errors and otherwise ignore the alias. (It's just an unused unstable feature deep in a crate…)