Generate documentation for auto-trait impls
A new section is added to both both struct and trait doc pages.
On struct/enum pages, a new 'Auto Trait Implementations' section displays any synthetic implementations for auto traits. Currently, this is only done for Send and Sync.
On trait pages, a new 'Auto Implementors' section displays all types which automatically implement the trait. Effectively, this is a list of all public types in the standard library.
Synthesized impls for a particular auto trait ('synthetic impls') take generic bounds into account. For example, a type
```rust
struct Foo<T>(T)
```
will have 'impl<T> Send for Foo<T> where T: Send' generated for it.
Manual implementations of auto traits are also taken into account. If we have
the following types:
```rust
struct Foo<T>(T)
struct Wrapper<T>(Foo<T>)
unsafe impl<T> Send for Wrapper<T>' // pretend that Wrapper<T> makes this sound somehow
```
Then Wrapper will have the following impl generated:
```rust
impl<T> Send for Wrapper<T>
```
reflecting the fact that 'T: Send' need not hold for 'Wrapper<T>: Send' to hold
Lifetimes, HRTBS, and projections (e.g. '<T as Iterator>::Item') are taken into account by synthetic impls:
However, if a type can *never* implement a particular auto trait (e.g. `struct MyStruct<T>(*const T)`), then a negative impl will be generated (in this case, `impl<T> !Send for MyStruct<T>`)
All of this means that a user should be able to copy-paste a syntheticimpl into their code, without any observable changes in behavior (assuming the rest of the program remains unchanged).
Current document takes 2^4, which is equal to 4^2.
This example is not very helpful for those unfamiliar with math words in English and thus rely on example codes.
You can now choose between the following:
- `#[unwind(allowed)]`
- `#[unwind(aborts)]`
Per rust-lang/rust#48251, the default is `#[unwind(allowed)]`, though
I think we should change this eventually.
This re-introduces a `Config.libdir_relative` field, now derived from
`libdir` and made relative to `prefix` if necessary.
This fixes a regression from #46592 when `--libdir` is given an absolute
path. `Builder::sysroot_libdir` should always use a relative path so
its callers don't clobber system locations, and `librustc` also asserts
that `CFG_LIBDIR_RELATIVE` is really relative.
