`Either` is wasteful for a one-or-none iterator, especially since `Once`
is already an `option::IntoIter` internally. We don't really need any of
the iterator mechanisms in this case, just a single conditional insert.

This re-lookup of `&hir::Pat` by its ID appears to be an artifact of earlier
complexity that has since been removed from the compiler.

… r=petrochenkov

Emit an error for invalid use of the linkage attribute

fixes rust-lang#128486

Currently, the use of the linkage attribute for Mod, Impl,... is incorrectly permitted. This PR will correct this issue by generating errors, and I've also added some UI test cases for it.

Related: rust-lang#128552.

…eril

mir/pretty: use `Option` instead of `Either<Once, Empty>`

`Either` is wasteful for a one-or-none iterator, especially since `Once`
is already an `option::IntoIter` internally. We don't really need any of
the iterator mechanisms in this case, just a single conditional insert.

…r=compiler-errors

Return correct HirId when finding body owner in diagnostics

Fixes rust-lang#129145
Fixes rust-lang#128810

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

```rust
fn generic<const N: u32>() {}

trait Collate<const A: u32> {
    type Pass;
    fn collate(self) -> Self::Pass;
}

impl<const B: u32> Collate<B> for i32 {
    type Pass = ();
    fn collate(self) -> Self::Pass {
        generic::<{ true }>()
        //~^ ERROR: mismatched types
    }
}
```

When type checking the `{ true }` anon const we would error with a type mismatch. This then results in diagnostics code attempting to check whether its due to a type mismatch with the return type. That logic was implemented by walking up the hir until we reached the body owner, except instead of using the `enclosing_body_owner` function it special cased various hir nodes incorrectly resulting in us walking out of the anon const and stopping at `fn collate` instead.

This then resulted in diagnostics logic inside of the anon consts `ParamEnv` attempting to do trait solving involving the `<i32 as Collate<B>>::Pass` type which ICEs because it is in the wrong environment.

I have rewritten this function to just walk up until it hits the `enclosing_body_owner` and made some other changes since I found this pretty hard to read/understand. Hopefully it's easier to understand now, it also makes it more obvious that this is not implemented in a very principled way and is definitely missing cases :)

…r=GuillaumeGomez

rustdoc-json: Clean up serialization and printing.

Somewhat a followup to rust-lang#128963, but makes sense regardless.

- Renames `out_path` to `out_dir` because it's not the path to the JSON, but the directory
  - Also adds a comment explaining `None`
- Renames `write` to `serialize_and_write` because it does both.
  - Also renames the self-profile activity name to be clear this measures both IO cost and serialization CPU cost
  - Expands the timer to cover flushing
- Renames `output` to `output_crate`, to emphasize it's the contents, not the `--output` flag.

r? `@GuillaumeGomez`

…octest-attrs, r=notriddle

Remove useless attributes in merged doctest generated code

I took another look at the generated code for merged doctests and it seems like those attributes are only useful when running `rustc --test`, which isn't the case for merged doctests. Less code generated. \o/

r? `@notriddle`

…rors

Remove a useless ref/id/ref round-trip from `pattern_from_hir`

This re-lookup of `&hir::Pat` by its ID appears to be an artifact of earlier complexity that has since been removed from the compiler.

Merely deleting the let/match results in borrow errors, but sprinkling `'tcx` in the signature allows it to work again, so I suspect that this code's current function is simply to compensate for overly loose lifetimes in the signature. Perhaps it made more sense at a time when HIR lifetimes were not tied to `'tcx`.

I spotted this while working on some more experimental changes, which is why I've extracted it into its own PR.