This commit adds the headers for the top level documentation to
rustdoc's existing table of contents, along with associated items.

It only show two levels of headers. Going further would require the
sidebar to be wider, and that seems unnecessary (the crates that
have manually-built TOCs usually don't need deeply nested headers).

The main `arbitrary_self_types` feature gate will shortly be reused for
a new version of arbitrary self types which we are amending per [this
RFC](https://github.com/rust-lang/rfcs/blob/master/text/3519-arbitrary-self-types-v2.md).
The main amendments are:

* _do_ support `self` types which can't safely implement `Deref`
* do _not_ support generic `self` types
* do _not_ support raw pointers as `self` types.

This PR relates to the last of those bullet points: this strips pointer
support from the current `arbitrary_self_types` feature.
We expect this to cause some amount of breakage for crates using this
unstable feature to allow raw pointer self types. If that's the case, we
want to know about it, and we want crate authors to know of the upcoming
changes.

For now, this can be resolved by adding the new
`arbitrary_self_types_pointers` feature to such crates. If we determine
that use of raw pointers as self types is common, then we may maintain
that as an unstable feature even if we come to stabilize the rest of the
`arbitrary_self_types` support in future. If we don't hear that this PR
is causing breakage, then perhaps we don't need it at all, even behind
an unstable feature gate.

[Tracking issue](rust-lang#44874)

This is [step 4 of the plan outlined here](rust-lang#44874 (comment))

fixes rust-lang#129528

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Apparently tools like `rustfmt` require it in order to find the right
`librustc_driver.so` without extra tweaks.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

This change will also remove the current warnings in the build due to
`rustfmt` not being available (for `bindgen` output):

    error: 'rustfmt' is not installed for the custom toolchain 'local'.
    note: this is a custom toolchain, which cannot use `rustup component add`
    help: if you built this toolchain from source, and used `rustup toolchain link`, then you may be able to build the component with `x.py`
    Failed to run rustfmt: Internal rustfmt error (non-fatal, continuing)

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

This particular target does not expand into much code, so it is a good
first candidate to see if we could keep this in the CI.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

It will make it easier to add more in the future.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

…layout

Additionally teach compiletest to ignore tests that rely on deterministic layout.
Tests themselves aren't built with randomization but they can still observe
slight changes in std or rustc

…l struct sizes

Per the `waker_getters` FCP:
rust-lang#96992 (comment)

Per the `waker_getters` FCP:
rust-lang#96992 (comment)

Docs largely copied from `RawWaker::new`.