Before, the `Range` method was called with `end = slice.len()`.
Unfortunately, because `Range::index` first checks the order of the
indices (start has to be smaller than end), an out of bounds index
leads to `core::slice::slice_index_order_fail` being called. This
prints the message 'slice index starts at 27 but ends at 10', which is
worse than 'index 27 out of range for slice of length 10'. This is not
only useful to normal users reading panic messages, but also for people
inspecting assembly and being confused by `slice_index_order_fail`
calls.

Co-authored-by: Joshua Nelson <[email protected]>

We were setting these in both Builder::cargo and here, which ended up only
setting the first of the two.

With this, we can now use a target JSON file to build a
cross-compiler:

```
x.py install --host ../aarch64-apple-darwin.json --target aarch64-apple-darwin
```

Documentation for the false keyword

Partial fix of rust-lang#34601.

Short documentation for the false keyword mainly pointing to the `true` keyword.

…les, r=Mark-Simulacrum

Teach bootstrap install and dist commands about TargetSelection

With this, we can now use a target JSON file to build a
cross-compiler:

```
x.py install --host ../aarch64-apple-darwin.json --target aarch64-apple-darwin
```

r? @Mark-Simulacrum

…x-panic, r=hanna-kruppe

Fix panic message when `RangeFrom` index is out of bounds

Before, the `Range` method was called with `end = slice.len()`. Unfortunately, because `Range::index` first checks the order of the indices (start has to be smaller than end), an out of bounds index leads to `core::slice::slice_index_order_fail` being called. This prints the message 'slice index starts at 27 but ends at 10', which is worse than 'index 27 out of range for slice of length 10'. This is not only useful to normal users reading panic messages, but also for people inspecting assembly and being confused by `slice_index_order_fail` calls.

You can see the produced assembly [here](https://rust.godbolt.org/z/GzMGWf) and try on Playground [here](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=aada5996b2f3848075a6d02cf4055743). (By the way. this is only about which panic function is called; I'm pretty sure it does not improve anything about performance).

… r=ollie27

Do not clobber RUSTDOCFLAGS

We were setting these in both Builder::cargo and here, which ended up only setting the first of the two.

Fixes rust-lang#74511

Improve documentation for `core::fmt` internals

The public interface of `core::fmt` is well-documented, but the internals have very minimal documentation.

…aintainers, r=Mark-Simulacrum

Add myself to toolstate change notifications for rustfmt

I'd like to get the toolstate change notifications for rustfmt as well so that I'll know when things break.

Have spoken with @topecongiro about this offline already.