bors[bot] 01836a0f35 Merge #3050 ... 3050: Refactor type parameters, implement argument position impl trait r=matklad a=flodiebold I wanted to implement APIT by lowering to type parameters because we need to do that anyway for correctness and don't need Chalk support for it; this grew into some more wide-ranging refactoring of how type parameters are handled 😅 - use Ty::Bound instead of Ty::Param to represent polymorphism, and explicitly count binders. This gets us closer to Chalk's way of doing things, and means that we now only use Param as a placeholder for an unknown type, e.g. within a generic function. I.e. we're never using Param in a situation where we want to substitute it, and the method to do that is gone; `subst` now always works on bound variables. (This changes how the types of generic functions print; previously, you'd get something like `fn identity<i32>(T) -> T`, but now we display the substituted signature `fn identity<i32>(i32) -> i32`, which I think makes more sense.) - once we do this, it's more natural to represent `Param` by a globally unique ID; the use of indices was mostly to make substituting easier. This also means we fix the bug where `Param` loses its name when going through Chalk. - I would actually like to rename `Param` to `Placeholder` to better reflect its use and get closer to Chalk, but I'll leave that to a follow-up. - introduce a context for type lowering, to allow lowering `impl Trait` to different things depending on where we are. And since we have that, we can also lower type parameters directly to variables instead of placeholders. Also, we'll be able to use this later to collect diagnostics. - implement argument position impl trait by lowering it to type parameters. I've realized that this is necessary to correctly implement it; e.g. consider `fn foo(impl Display) -> impl Something`. It's observable that the return type of e.g. `foo(1u32)` unifies with itself, but doesn't unify with e.g. `foo(1i32)`; so the return type needs to be parameterized by the argument type. This fixes a few bugs as well: - type parameters 'losing' their name when they go through Chalk, as mentioned above (i.e. getting `[missing name]` somewhere) - impl trait not being considered as implementing the super traits (very noticeable for the `db` in RA) - the fact that argument impl trait was only turned into variables when the function got called caused type mismatches when the function was used as a value (fixes a few type mismatches in RA) The one thing I'm not so happy with here is how we're lowering `impl Trait` types to variables; since `TypeRef`s don't have an identity currently, we just count how many of them we have seen while going through the function signature. That's quite fragile though, since we have to do it while desugaring generics and while lowering the type signature, and in the exact same order in both cases. We could consider either giving only `TypeRef::ImplTrait` a local id, or maybe just giving all `TypeRef`s an identity after all (we talked about this before)... Follow-up tasks: - handle return position impl trait; we basically need to create a variable and some trait obligations for that variable - rename `Param` to `Placeholder` Co-authored-by: Florian Diebold <florian.diebold@freiheit.com> Co-authored-by: Florian Diebold <flodiebold@gmail.com>		2020-02-09 11:35:08 +00:00
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src	Merge #3050	2020-02-09 11:35:08 +00:00
Cargo.toml	Optimize and profile	2019-12-21 18:46:21 +01:00