…les.

This reduces the number of `associated_item` queries done here.

Quickly skip impls that do not define any items with the same name

`InferCtxt` contains six structures within `RefCell`s. Every time we
create and dispose of (commit or rollback) a snapshot we have to
`borrow_mut` each one of them.

This commit moves the six structures under a single `RefCell`, which
gives significant speed-ups by reducing the number of `borrow_mut`
calls. To avoid runtime errors I had to reduce the lifetimes of dynamic
borrows in a couple of places.

…rCtxt, r=varkor

Reduce the number of `RefCell`s in `InferCtxt`.

`InferCtxt` contains six structures within `RefCell`s. Every time we
create and dispose of (commit or rollback) a snapshot we have to
`borrow_mut` each one of them.

This commit moves the six structures under a single `RefCell`, which
gives significant speed-ups by reducing the number of `borrow_mut`
calls. To avoid runtime errors I had to reduce the lifetimes of dynamic
borrows in a couple of places.

r? @varkor

…agisa

rustc_codegen_llvm: always set AlwaysPreserve on all debuginfo variables

Making this depend on the optimization level appears to have been a copy-paste mistake (other LLVM functions called in this module also take a `bool` argument, but there it means something unrelated).
Also see rust-lang#8855 (comment).

I don't believe we have any reason to let LLVM omit user variables from DWARF, and we were already setting this to `true` when LLVM *could* optimize them away, so this PR should have no effect anyway.

r? @michaelwoerister or @nagisa cc @rkruppe @nikomatsakis

…=petrochenkov

Speed up the inherent impl overlap check

This gives a ~7% improvement in compile times for the stm32f0(x2) crate.

Also addresses @eddyb's comment in rust-lang#68837 (comment).

Pretty-print generic params and where clauses on associated types

closes rust-lang#67509