impl blocks.

For example, the following is now correctly illegal:

```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}
```

"Overlapping" here is determined the same way it is for traits (and in
fact shares the same code path): roughly, there must be some way of
substituting any generic types to unify the impls, such that none of the
`where` clauses are provably unsatisfiable under such a unification.

Closes #22889

This commit introduces the idea of an "impl header", which consists of
everything outside the impl body: the Self type, the trait
reference (when applicable), and predicates from `where` clauses. This
type is usable with the type folding machinery, making it possible to
work with impl headers at a higher and more generic level.

rustc: Add an i586-pc-windows-msvc target

Similarly to #31629 where an i586-unknown-linux-gnu target was added, there is
sometimes a desire to compile for x86 Windows as well where SSE2 is disabled.
This commit mirrors the i586-unknown-linux-gnu target and simply adds a variant
for Windows as well.

This is motivated by a recent [Gecko bug][ff] where crashes were seen on 32-bit
Windows due to users having CPUs that don't support SSE2 instructions. It was
requested that we could have non-SSE2 builds of the standard library available
so they could continue to use vanilla releases and nightlies.

[ff]: https://bugzilla.mozilla.org/show_bug.cgi?id=1253202

Add help for E0514

I fixed #30622.

r? @arielb1

std: Add impl of FnOnce to AssertRecoverSafe

This was originally intended, but forgot to land by accident!

cc #27719

rustdoc: correct src-link url

It would have been nice for htmldocck to catch this, especially since there are rustdoc tests specifically for checking src-links.

fixes #32113

Never return an error after a partial write

If LineWriter fails to flush, return the number of bytes written instead
of an error.

Fixes #32085

Add info in `rustdoc --passes list`

CC @mitaa, https://github.com/rust-lang/rust/pull/32055/files#r55205006

Rollup of 8 pull requests

- Successful merges: #31830, #32091, #32125, #32136, #32147, #32148, #32149, #32150
- Failed merges:

Remove inaccurate claim about inline assembly

It's not like GCC's.

Fixes #20213

Add other primitive types to the reference

Fixes #31628

Small grammar fix in Guessing Game

When it was Option.expect(), there was an .ok().expect(), but now that it uses Result.expect(), there's only one method, not two.

Fixes #31912

Clarify that try_unwrap needs exactly one

Fixes #31950

Add missing documentation examples for BTreeSet.

As part of the ongoing effort to document all methods with examples,
this commit adds the missing examples for the `BTreeSet` collection
type.

This is part of issue #29348.

r? @steveklabnik

Remove final note from testing chapter.

The information that documentation tests cannot be run in binary crates is already given at the beginning of the section.

Clarify documentation of hash::SipHasher

The docs were making assertions/recommendations they shouldn't have. This clarifies them and adds some helpful links.

Fixes #32043.

r? @sfackler

Prefer 'associated function' over 'static method' in msg.

TRPL seems to refer to 'static functions' as 'associated functions'.
This terminology should be used consistently.

"can be built on Ref::map"… how?

Now that `std::cell::Ref::filter_map` and `RefMut::filter_map` are deprecated, using them gives a warning like:
```
script/dom/element.rs:754:9: 754:24 warning: use of deprecated item: can be built on Ref::map, #[warn(deprecated)] on by default
```

But it’s not at all obvious *how* the functionality can be built on `Ref::map`. This PR adds to the warning message a crates.io URL for a crate that does.

Fix a regression in import resolution

This fixes #32089 (caused by #31726) by deducing that name resolution has failed (as opposed to being determinate) in more cases.

r? @nikomatsakis

TRPL seems to refer to 'static functions' as 'associated functions'.
This terminology should be used consistently.

Distinguish fn item types to allow reification from nothing to fn pointers.

The first commit is a rebase of #26284, except for files that have moved since.

This is a [breaking-change], due to:
* each FFI function has a distinct type, like all other functions currently do
* all generic parameters on functions are recorded in their item types, e.g.:
`size_of::<u8>` & `size_of::<i8>`'s types differ despite their identical signature.
* function items are zero-sized, which will stop transmutes from working on them

The first two cases are handled in most cases with the new coerce-unify logic,
which will combine incompatible function item types into function pointers,
at the outer-most level of if-else chains, match arms and array literals.

The last case is specially handled during type-checking such that transmutes
from a function item type to a pointer or integer type will continue to work for
another release cycle, but are being linted against. To get rid of warnings and
ensure your code will continue to compile, cast to a pointer before transmuting.

Optimize some functions in std::process

* Be sure that `read_to_end` gets directed towards `read_to_end_uninitialized` for all handles
* When spawning a child that guaranteed doesn't need a stdin, don't actually create a stdin pipe for that process, instead just redirect it to /dev/null
* When calling `wait_with_output`, don't spawn threads to read out the pipes of the child. Instead drain all pipes on the calling thread and *then* wait on the process.

Functionally, it is intended that nothing changes as part of this PR

---

Note that this was the same as #31613, and even after that it turned out that fixing Windows was easier than I thought! To copy a comment from over there:

> As some rationale for this as well, it's always bothered me that we've spawned threads in the standard library for this (seems a bit overkill), and I've also been curious lately as to our why our build times for Windows are so much higher than Unix (on the buildbots we have). I have done basically 0 investigation into why, but I figured it can't help to try to optimize Command::output which I believe is called quite a few times during the test suite.

collections: Fix broken doc example

PR #32135 was accidentally merged without tests passing, and unfortunately one
of the tests added was broken, so this fixes that.

PR #32135 was accidentally merged without tests passing, and unfortunately one
of the tests added was broken, so this fixes that.

Add missing documentation examples for BTreeMap.

Semantically there's actually no reason for us to spawn threads as part of the
call to `wait_with_output`, and that's generally an incredibly heavyweight
operation for just reading a few bytes (especially when stderr probably rarely
has bytes!). An equivalent operation in terms of what's implemented today would
be to just drain both pipes of all contents and then call `wait` on the child
process itself.

On Unix we can implement this through some convenient use of the `select`
function, whereas on Windows we can make use of overlapped I/O. Note that on
Windows this requires us to use named pipes instead of anonymous pipes, but
they're semantically the same under the hood.

Fix incorrect trait privacy error

This PR fixes #21670 by using the crate metadata instead of `ExternalExports` to determine if an external item is public.

r? @nikomatsakis