4029: Fix various proc-macro bugs r=matklad a=edwin0cheng
This PRs does the following things:
1. Fixed#4001 by splitting `LIFETIME` lexer token to two mbe tokens. It is because rustc token stream expects `LIFETIME` as a combination of punct and ident, but RA `tt:TokenTree` treats it as a single `Ident` previously.
2. Fixed#4003, by skipping `proc-macro` for completion. It is because currently we don't have `AstNode` for `proc-macro`. We would need to redesign how to implement `HasSource` for `proc-macro`.
3. Fixed a bug how empty `TokenStream` merging in `proc-macro-srv` such that no L_DOLLAR and R_DOLLAR will be emitted accidentally.
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
Basically adds a From impl for tuple enum variants with one field. Added
to cover the fairly common case of implementing your own Error that can
be created from another one, although other use cases exist.
It improves compile time in `--release` mode quite a bit, it doesn't
really slow things down and, conceptually, it seems closer to what we
want the physical architecture to look like (we don't want to
monomorphise EVERYTHING in a single leaf crate).
The `ty` function in code_model returned the type with placeholders for type
parameters. That's nice for printing, but not good for completion, because
placeholders won't unify with anything else: So the type we got for `HashMap`
was `HashMap<K, V, T>`, which doesn't unify with `HashMap<?, ?, RandomState>`,
so the `new` method wasn't shown.
Now we instead return `HashMap<{unknown}, {unknown}, {unknown}>`, which does
unify with the impl type. Maybe we should just expose this properly as variables
though, i.e. we'd return something like `exists<type, type, type> HashMap<?0,
?1, ?2>` (in Chalk notation). It'll make the API more complicated, but harder to
misuse. (And it would handle cases like `type TypeAlias<T> = HashMap<T, T>` more
correctly.)
