…lang#109592.

Since structs are always `VariantIdx(0)`, there's a bunch of files where the only reason they had `VariantIdx` or `vec::Idx` imported at all was to get the first variant.

So this uses a constant for that, and adds some doc-comments to `VariantIdx` while I'm there, since it doesn't have any today.

…rs-rustdoc, r=GuillaumeGomez

rustdoc + rustdoc-json support for `feature(non_lifetime_binders)`

Makes `for<T> T: Trait` and `for<const N: usize> ..` in where clause operate correctly.

Fixes rust-lang#108158

… r=GuillaumeGomez

Rustdoc Book refer to rustdoc::missing_doc_code_examples. Fixes rust-lang#109592.

Fixes rust-lang#109592. But, please see a related question at rust-lang#109592 (item `2.`).

…to-hide-trait, r=notriddle

Improve "Auto-hide trait implementation documentation" GUI test

Part of rust-lang#66181.

I'll start working on the `include` command for `browser-ui-test` so we can greatly reduce the duplicated code between setting tests.

r? `@notriddle`

…urther-specializable, r=BoxyUwU

Still-further-specializable projections are ambiguous in new solver

Fixes https://github.com/rust-lang/rust/pull/108896/files#r1148450781

r? `@BoxyUwU` (though feel free to re-roll)

---

This can be used to create an unsound transmute function with the new solver:

```rust
#![feature(specialization)]

trait Default {
   type Id;

   fn intu(&self) -> &Self::Id;
}

impl<T> Default for T {
   default type Id = T;

   fn intu(&self) -> &Self::Id {
        self
   }
}

fn transmute<T: Default<Id = U>, U: Copy>(t: T) -> U {
    *t.intu()
}

use std::num::NonZeroU8;
fn main() {
    let s = transmute::<u8, Option<NonZeroU8>>(0);
    assert_eq!(s, None);
}
```

Correct typo (`back_box` -> `black_box`)

…iler-errors

Refactor: `VariantIdx::from_u32(0)` -> `FIRST_VARIANT`

Since structs are always `VariantIdx(0)`, there's a bunch of files where the only reason they had `VariantIdx` or `vec::Idx` imported at all was to get the first variant.

So this uses a constant for that, and adds some doc-comments to `VariantIdx` while I'm there, since [it doesn't have any today](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/abi/struct.VariantIdx.html).