Use an explicit `Option` instead of requiring a `Default` bound

There is room for improvement on some of these, but something is better
than nothing.

We can just get the error level in the `match` and then use
`DiagnosticBuilder::new`. This then means a number of `DiagCtxt`
functions are no longer needed, because this was the one place that used
them.

Note: the commit changes the treatment of spans for `Expect`, which was
different to all the other cases, but this has no apparent effect.

There are a bunch of them about 400 lines down, which is weird and
annoying. This commit moves them up and puts them in a more sensible
order.

`IntoDiagnostic` defaults to `G = ErrorGuaranteed`. Take advantage of
this in one place that currently doesn't.

`IntoDiagnostic` defaults to `ErrorGuaranteed`, because errors are the
most common diagnostic level. It makes sense to do likewise for the
closely-related (and much more widely used) `DiagnosticBuilder` type,
letting us write `DiagnosticBuilder<'a, ErrorGuaranteed>` as just
`DiagnosticBuilder<'a>`. This cuts over 200 lines of code due to many
multi-line things becoming single line things.

- Take a `Vec` instead of an iterator, because that's all that is
  needed.
- Do an early return for the "no bugs" case.
- Use `enumerate` and an `i == 0` test to identify the first bug.

Those changes mean the `no_bug` variable can be removed, which I found
hard to read.

To fix a FIXME.

`DiagCtxt::span_bug` is different to the other `DiagCtxt::span_*`
methods. This commit makes it the same, which requires changing
`DiagCtxt::span_delayed_bug` to not do everything within the
`inner.borrow_mut()`.

- `early_error_no_abort` -> `early_err`
- `early_error` -> `early_fatal`
- `early_struct_error` -> `early_struct_fatal`

Its single use can be replaced with `Diagnostic::new_with_messages`.

Lots of vectors of messages called `message` or `msg`. This commit
pluralizes them.

Note that `emit_message_default` and `emit_messages_default` both
already existed, and both process a vector, so I renamed the former
`emit_messages_default_inner` because it's called by the latter.

They're always `None`.

It's only ever set to `None`.

It has three variants, but only `LabelKind::Label` is ever used. This
means `SingleLabelManySpans::kind` can also be removed.

Some of the `Level::` qualifiers in this file are omitted. This commit
removes the remainder.

There are quite a few hand-written `IntoDiagnostic` impls.

There are only three. It's simpler to make the type
`DiagnosticBuilder<'_, ()>` from the start, no matter the level, than to
change the guarantee later.

It's unused. And this means `DiagnosticBuilderInner` no longer needs to
be separate from `DiagnosticBuilder`.

…-fn, r=b-naber

Extract `layout_of_{struct,enum}` fn

While writing rust-lang#118974 I noticed it was annoying to navigate a huge, several hundred line function, which handles many subcases, and make confident declarations about what part of the flow of execution the compiler would be in. To help with that, this breaks out `layout_of_struct_or_enum`'s fundamental logic into a pair of functions, one for each case. It changes essentially none of that logic, merely moves it around.

Because "the layout of an ADT" feels like a somewhat nebulous subject, I chose to deliberately avoid any expansions to LayoutCalculator's public API, though such does feel like a possible logical next step. There are, indeed, many logical next steps. I'm not taking any of them here, yet, because this comparatively tiny refactor is a prerequisite for all of them.

Separate MIR lints from validation

Add a MIR lint pass, enabled with -Zlint-mir, which identifies undefined or
likely erroneous behaviour.

The initial implementation mostly migrates existing checks of this nature from
MIR validator, where they did not belong (those checks have false positives and
there is nothing inherently invalid about MIR with undefined behaviour).

Fixes rust-lang#104736
Fixes rust-lang#104843
Fixes rust-lang#116079
Fixes rust-lang#116736
Fixes rust-lang#118990

…mpiler-errors

Cleanup error handlers: round 4

More `rustc_errors` cleanups. A sequel to rust-lang#118933.

r? `@compiler-errors`

Split coroutine desugaring kind from source

What a coroutine is desugared from (gen/async gen/async) should be separate from where it comes (fn/block/closure).

…ler-errors,dtolnay

Add `IntoAsyncIterator`

This introduces the `IntoAsyncIterator` trait and uses it in the desugaring of the unstable `for await` loop syntax. This is mostly added for symmetry with `Iterator` and `IntoIterator`.

r? `@compiler-errors`

cc `@rust-lang/libs-api,` `@rust-lang/wg-async`

…r=compiler-errors

Exhaustiveness: clean up after librarification

This cleans up some things that weren't done nicely by rust-lang#118842.

r? `@compiler-errors`