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
into account generic bounds. For example, a type '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:

'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:
'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 synthetic
impl into their code, without any observable changes in behavior
(assuming the rest of the program remains unchanged).

Fixes rust-lang#47708.

This span covers the whole visibility expression: e.g. `pub (in path)`.

… r=petrochenkov

Fix span of visibility

This PR

1. adds a closing parenthesis to the span of `Visibility::Crate` (e.g. `pub(crate)`). The current span only covers `pub(crate`.
2. adds a `span` field to `Visibility::Restricted`. This span covers the entire visibility expression (e.g. `pub (in self)`). Currently all we can have is a span for `Path`.

This PR is motivated by the bug found in rustfmt (rust-lang/rustfmt#2398).

The first change is a strict improvement IMHO. The second change may not be desirable, as it adds a field which is currently not used by the compiler.

…aumeGomez,QuietMisdreavus

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.

![Auto trait implementations for Cloned](https://i.imgur.com/XtTV6IJ.png)

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.

![Auto trait implementors for Send](https://i.imgur.com/3GRBpTy.png)

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:

![A ridiculous demonstration type](https://i.imgur.com/TkZMWuN.png)

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).

…Mark-Simulacrum

Doc test command

r? @Mark-Simulacrum

Add a warning to File about mutability.

Fixes rust-lang#47708.

…arkor

fix more typos found by codespell.

…th, r=kennytm

Fix spelling in core::iter::repeat_with: s/not/note

Fixes spelling error in rust-lang#48156 (comment).
Tracking issue: rust-lang#48169