The small-buffer path in Stdin::read allocates a one-element UTF-16
scratch buffer. When a pending surrogate exists,
read_u16s_fixup_surrogates requires temporary storage for both the
buffered surrogate and one newly read UTF-16 code unit.

Increase the scratch buffer to two UTF-16 code units so the surrogate
path can complete without panicking.

When bootstrap runs a step by default, without explicit paths, it will normally
act as though the user had explicitly requested all of the paths and aliases
that the step registered through `ShouldRun`.

For the coverage test suite, that gives the wrong outcome. Running a command
like `./x test --skip=tests` or `./x test --skip=coverage` should skip the
coverage tests, but instead the coverage tests would run anyway, due to the
`coverage-map` and `coverage-run` aliases being treated as implied command-line
arguments.

This commit fixes that problem by adding a special flag to `ShouldRun`. When
creating pathsets for a step that is being run by default, if the step has set
the `default_to_suites_only` flag, all non-suite pathsets are discarded. That
gives the desired behavior for skipping coverage tests, without affecting other
steps, since other steps don't set the flag.

… `error_helper.rs` into the `diagnostics` folder in `rustc_resolve`

… r=JonathanBrouwer

Apply MCP 1003 and move diagnostics.rs into its own module

Part of rust-lang#158699.

r? @JonathanBrouwer

…ulacrum

Simplify the unwind crate

Use cfg_select! and unify type definitions.

bootstrap: Allow path-based skipping of the coverage test suite

- Alternative to rust-lang#159079
---

When bootstrap runs a step by default, without explicit paths, it will normally act as though the user had explicitly requested all of the paths and aliases that the step registered through `ShouldRun`.

For the coverage test suite, that gives the wrong outcome. Running a command like `./x test --skip=tests` or `./x test --skip=coverage` should skip the coverage tests, but instead the coverage tests would run anyway, due to the `coverage-map` and `coverage-run` aliases being treated as implied command-line arguments.

This PR fixes that problem by adding a special flag to `ShouldRun`. When creating pathsets for a step that is being run by default, if the step has set the `default_to_suites_only` flag, all non-suite pathsets are discarded. That gives the desired behaviour for skipping coverage tests, without affecting other steps, since other steps don't set the flag.

The end result is that `./x test --skip=tests` should now skip the coverage tests, as intended. This lets us remove some `--skip` arguments from CI scripts, which were only required by the previous incorrect behaviour.

---

The `default_to_suites_only` flag is a bit of a hack, but to me it seems like the cleanest way to resolve this problem without having to completely overhaul how CLI paths work, which is a much bigger task. And I think being able to remove the weird extra `--skip` arguments from CI scripts makes this a net positive.

r? jieyouxu

…, r=Darksonn

std: fix panic in Windows Stdin::read_vectored with pending surrogate

Fixes rust-lang#158096.

The small-buffer path in `Stdin::read_vectored` allocates a one-element UTF-16 scratch buffer before calling `read_u16s_fixup_surrogates`.

When a pending surrogate exists, `read_u16s_fixup_surrogates` needs temporary storage for both the buffered surrogate and one newly read UTF-16 code unit. The one-element buffer causes an out-of-bounds slice before `ReadConsoleW` is called.

Increase the scratch buffer to two UTF-16 code units, matching the  function's requirements while leaving the normal read path unchanged.

…f, r=jieyouxu

compiletest: use VecDeque::pop_front_if

`VecDeque::pop_front_if` has been stable since Rust 1.93 and is now available to bootstrap.

Replace compiletest's local compatibility helper with the standard library method and remove the obsolete FIXME.