7e11379f3b
Support const args in type dependent paths (Take 2) once more, except it is sound this time 🥰 previously #71154 ----- ```rust #![feature(const_generics)] struct A; impl A { fn foo<const N: usize>(&self) -> usize { N } } struct B; impl B { fn foo<const N: usize>(&self) -> usize { 42 } } fn main() { let a = A; a.foo::<7>(); } ``` When calling `type_of` for generic const arguments, we now use the `TypeckTables` of the surrounding body to get the expected type. This alone causes cycle errors though, as we now have `typeck_tables_of(main)` -> `...` -> `type_of(main_ANON0 := 7)` -> `typeck_tables_of(main)` ⚡ (see https://github.com/rust-lang/rust/issues/68400#issuecomment-611760290) To prevent this we must not call `type_of(const_arg)` during `typeck_tables_of`. This is achieved by calling `type_of(param_def_id)` instead. We have to somehow remember the `DefId` of the param through all of typeck, which is done using the struct `ty::WithOptConstParam<DefId>`, which replaces `DefId` where needed and contains an `Option<DefId>` to be able to store the const parameter in case it exists. Queries which are currently cached on disk are split into two variants: `query_name`(cached) and `query_name_(of|for)_const_arg`(not cached), with `query_name_of_const_arg` taking a pair `(did, param_did): (LocalDefId, DefId)`. For some queries a method `query_name_of_opt_const_arg` is added to `TyCtxt` which takes a `ty::WithOptConstParam` and either calls `query_name` or `query_name_of_const_arg` depending on the value of `const_param_did`. r? @eddyb @varkor
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.