Note that this adds the `span.allows_unstable` checking that this case
previously lacked.

The switch_stdout_to test was broken on Windows7, as the test
infrastructure would refuse to delete the temporary test folder because
the switch-stdout-output file we redirected the stdout to was still
opened.

To fix this issue, we make switch_stdout_to return the previous handle,
and add a new switch_stdout_to call at the end of the test to return the
stdio handles to their original state. The handle the second
switch_stdout_to returns will be automatically closed, which should
allow the temporary test folder to be deleted properly.

fixes: typo in `std::cmp::Ord` trait docs

Pluralizes the word "sequence"

Fix switch_stdout_to on Windows7

The `switch_stdout_to` test was broken on Windows7, as deleting the temporary test folder would fail since the `switch-stdout-output` file we redirected the stdout to is never closed, and it's impossible on Win7 to delete an opened file.

To fix this issue, we make `switch_stdout_to` return the previous handle. Using this, we add a new `switch_stdout_to` call at the end of the test to return the stdio handles to their original state, and recover the handle to the file we opened. This handle is automatically closed at the end of the function, which should allow the temporary test folder to be deleted properly.

Minor improvements to `rustc_ast_passes`

Some improvements I found while looking at this code.

r? `@compiler-errors`

…ound vars

Makes progress on #106655

Don't check for alias bounds in liveness when aliases have escaping bound vars

I actually have no idea how we *should* be treating aliases with escaping bound vars here... but the simplest behavior is just doing what we used to do before.

r? aliemjay

Fixes #117455

use global cache when computing proof trees

we're writing the solver while relying on the existence of the global cache to avoid exponential blowup. By disabling the global cache when building proof trees, it is easy to get hangs, e.g. when computing intercrate ambiguity causes.

Removes the unstable `-Zdump_solver_proof_tree_use_cache` option, as we now always return a full proof tree.

r? `@compiler-errors`

Clarify `Unsize` documentation

The documentation erroneously says that:

```rust
/// - Types implementing a trait `Trait` also implement `Unsize<dyn Trait>`.
/// - Structs `Foo<..., T, ...>` implement `Unsize<Foo<..., U, ...>>` if all of these conditions
///   are met:
///   - `T: Unsize<U>`.
///   - Only the last field of `Foo` has a type involving `T`.
///   - `Bar<T>: Unsize<Bar<U>>`, where `Bar<T>` stands for the actual type of that last field.
```

Specifically, `T: Unsize<U>` is not required to hold -- only the final field must implement `FinalField<T>: Unsize<FinalField<U>>`. This can be demonstrated by the test I added.

---

Second commit fleshes out the documentation a lot more.

Remove support for alias `-Z symbol-mangling-version`

(This is very similar to the removal of `-Z instrument-coverage` in #117111.)

`-C symbol-mangling-version` was stabilized back in rustc 1.59.0 (2022-02-24) via #90128, with the old unstable flag kept around (with a warning) as an alias to ease migration.

Expand mem::offset_of! docs

Makes progress on #106655

Rollup of 4 pull requests

Successful merges:

 - #117394 (use global cache when computing proof trees)
 - #117495 (Clarify `Unsize` documentation)
 - #117509 (Remove support for alias `-Z symbol-mangling-version`)
 - #117512 (Expand mem::offset_of! docs)

r? `@ghost`
`@rustbot` modify labels: rollup

…pe_bounds

…rors

dropck_outlives check whether generator witness needs_drop

see https://rust-lang.zulipchat.com/#narrow/stream/326866-t-types.2Fnominated/topic/.23116242.3A.20Code.20no.20longer.20compiles.20after.20-Zdrop-tracking-mir.20.E2.80.A6/near/398311627 for an explanation.

Fixes #116242 (or well, the repro by `@jamuraa` in rust-lang/rust#116242 (comment)). I did not add a regression test as it depends on other crates. We do have 1 test going from fail to pass, showing the intended behavior.

r? types

Add all RPITITs when augmenting param-env with GAT bounds in `check_type_bounds`

When checking that associated type definitions actually satisfy their associated type bounds in `check_type_bounds`, we construct a "`normalize_param_env`" which adds a projection predicate that allows us to assume that we can project the GAT to the definition we're checking. For example, in:

```rust
type Foo {
  type Bar: Display = i32;
}
```

We would add `<Self as Foo>::Bar = i32` as a projection predicate when checking that `i32: Display` holds.

That `normalize_param_env` was, for some reason, only being used to normalize the predicate before it was registered. This is sketchy, because a nested obligation may require the GAT bound to hold, and also the projection cache is broken and doesn't differentiate projection cache keys that differ by param-envs 😿.

This `normalize_param_env` is also not sufficient when we have nested RPITITs and default trait methods, since we need to be able to assume we can normalize both the RPITIT and all of its child RPITITs to sufficiently prove all of its bounds. This is the cause of #117104, which only starts to fail for RPITITs that are nested 3 and above due to the projection-cache bug above.[^1]

## First fix

Use the `normalize_param_env` everywhere in `check_type_bounds`. This is reflected in a test I've constructed that fixes a GAT-only failure.

## Second fix

For RPITITs, install projection predicates for each RPITIT in the same function in `check_type_bounds`. This fixes #117104.

not sure who to request, so...
r? `@lcnr` hehe feel free to reassign :3

[^1]: The projection cache bug specifically occurs because we try normalizing the `assumed_wf_types` with the non-normalization param-env. This causes us to insert a projection cache entry that keeps the outermost RPITIT rigid, and it trivially satisifes all its own bounds. Super sketchy![^2]

[^2]: I haven't actually gone and fixed the projection cache bug because it's only marginally related, but I could, and it should no longer be triggered here.

Run part of `rustc_codegen_gcc`'s tests in CI

Thanks to #112701 and `@bjorn3,` it made this much easier.

Also cc `@antoyo.`

r? `@bjorn3`

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