Should been part of rust-lang#74163

suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.

Also improve the docs for it a bit.

Co-authored-by: Esteban Küber <esteban@kuber.com.ar>

Some errors, like if rustc is broken, or dylib search path is wrong,
will only display non-JSON errors. Show those, too, to make it easier to
debug a problem.

Co-authored-by: Amanieu <amanieu@gmail.com>

Co-authored-by: Amanieu <amanieu@gmail.com>

Prefer asm! over llvm_asm! in core

Replace llvm_asm! with asm! in core.

x86 asm compare (in somecases I replaced generic type with String).
* https://rust.godbolt.org/z/59eEMv
* https://rust.godbolt.org/z/v78s6q
* https://rust.godbolt.org/z/7qYY41

Small improvements to asm documentation

Save people from searching and reading tons of comments in tracking issues.

remove orphaned files

Should been part of rust-lang#74163

…r=davidtwco

Introduce a PartitioningCx struct

This contains all the data used by the partitioning algorithm and allows that data to be used at each stage of the partitioning. This is useful for other approaches to partitioning which may want different pieces of the data available at each step.

cc @rust-lang/wg-incr-comp

…ion, r=estebank

fix syntax error in suggesting generic constraint in trait parameter

suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.

the test case i added here is derived from [this reproduction](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=0bf3ace9f2a183d0bdbd748c6b8e3971):
```
struct B<T: Copy> {
    t: T
}

trait A<T = ()> {
    fn returns_constrained_type(&self, t: T) -> B<T> {
        B { t }
    }
}
```
where the suggested fix,
```
trait A<T = ()>, T: Copy { ... }
```
is in fact invalid syntax!

i also found an error in the existing suggestion for `trait Base<T = String>: Super<T>` where rustc would suggest `trait Base<T = String>: Super<T>, T: Copy`, but `T: Copy` is the first of the trait's `where` clauses and should be `where T: Copy` as well. the test for that suggestion expects invalid syntax, and has been revised to a compiler-pleasing `trait Base<T = String>: Super<T> where T: Copy`.

judging by rust-lang#70009 i'll.. cc @estebank ?

improve const infer error

cc rust-lang#72328

reduces it from
```
error[E0282]: type annotations needed
  --> src/main.rs:17:5
   |
17 |     Foo.bar().bar().bar().bar().baz();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: unable to infer the value of a const parameter
```
to
```
error[E0282]: type annotations needed
  --> $DIR/method-chain.rs:21:33
   |
LL |     Foo.bar().bar().bar().bar().baz();
   |                                 ^^^
   |
   = note: cannot infer the value of the const parameter `N`
```

r? @varkor

Simplify iter flatten struct doc

… r=jonas-schievink

⬆️ rust-analyzer

r? @ghost

Small docs improvements

@rustbot modify labels: T-doc T-compiler

…-Simulacrum

Fix generating rustc docs with non-default lib directory.

If `libdir` is set in `config.toml`, then the tool to generate the rustc docs was unable to run `rustc` because it could not find the shared libraries. The solution is to set the dylib search path to include the libdir.

I changed the API of `add_rustc_lib_path` to take `Command` instead of `Cargo` to try to share the code in several places. This is how it worked before rust-lang#64316, and I think this still retains the spirit of that change.

Fixes rust-lang#76702