In order to inform future perf improvements and prevent regressions,
lets add some benchmarks that stress `impl Debug for str`.

Also sort the features.

I tried simplifying `RegionCtxt`, which led me to finding that the
fields are printed in `sccs_info`.

The comment mentions that `ReBound` and `ReVar` aren't expected here.
Experimentation with the full test suite indicates this is true, and
that `ReErased` also doesn't occur. So the commit introduces `bug!` for
those cases. (If any of them show up later on, at least we'll have a
test case.)

The commit also remove the first sentence in the comment.
`RePlaceholder` is now handled in the match arm above this comment and
nothing is printed for it, so that sentence is just wrong. Furthermore,
issue rust-lang#13998 was closed some time ago.

xz crate consist of simple reexport of xz2 crate. Why? Idk.

`InferCtxt::next_{ty,const,int,float}_var_id` each have a single call
site, in `InferCtt::next_{ty,const,int,float}_var` respectively.

The only remaining method that creates a var_id is
`InferCtxt::next_ty_var_id_in_universe`, which has one use outside the
crate.

`InferCtxt::next_{ty,const}_var*` all take an origin, but the
`param_def_id` is almost always `None`. This commit changes them to just
take a `Span` and build the origin within the method, and adds new
methods for the rare cases where `param_def_id` might not be `None`.
This avoids a lot of tedious origin building.

Specifically:
- next_ty_var{,_id_in_universe,_in_universe}: now take `Span` instead of
  `TypeVariableOrigin`
- next_ty_var_with_origin: added

- next_const_var{,_in_universe}: takes Span instead of ConstVariableOrigin
- next_const_var_with_origin: added

- next_region_var, next_region_var_in_universe: these are unchanged,
  still take RegionVariableOrigin

The API inconsistency (ty/const vs region) seems worth it for the
large conciseness improvements.

The type name has changed.

It's unused.

The use of `Binder` was removed in the recent rust-lang#123900, but the comment
wasn't removed at the same time.

Thus eliminating a `FIXME` comment.

Add benchmarks for `impl Debug for str`

In order to inform future perf improvements and prevent regressions, lets add some benchmarks that stress `impl Debug for str`.

---

As I am currently working on improving the perf in rust-lang#121150, its nice to have these benchmarks.

Writing them, I also saw that escapes are written out one char at a time, even though other parts of the code are already optimizing that via `as_str`, which I intend to do as well as a followup improvement.

r? ``@cuviper``
☝🏻 as you were also assigned to rust-lang#121150, CC ``@the8472`` if you want to steal the review :-)

… r=bjorn3

`rustc_target` cleanups

Minor improvement I found while looking at this code.

r? ```@lqd```

Eliminate some `FIXME(lcnr)` comments

In some cases this involved changing code. In some cases the comment was able to removed or replaced.

r? ``@lcnr``

opt-dist: use xz2 instead of xz crate

xz crate consist of simple reexport of xz2 crate. Why? Idk.

Totally not a backdoor.

analyse visitor: build proof tree in probe

see inline comments

fixes rust-lang#124791
fixes rust-lang#124702

r? ```@compiler-errors```

…errors

always use `GenericArgsRef`

r? ```@compiler-errors```

Use fewer origins when creating type variables.

To reduce lots of repetitive boilerplate code. Details in the individual commit messages.

r? ``@lcnr``