It has been possible to clone closures for a while now

`next_back` is a method of DoubleEndedIterator, not Iterator.

…mber of exported symbols for each crate

Restored underlying num_def_ids_method

Update compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

Changed name to fit with naming convention

Co-authored-by: bjorn3 <bjorn3@users.noreply.github.com>

Update compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

Replace regular doc with Rustdoc comment

Co-authored-by: Joshua Nelson <jyn514@gmail.com>

Clarifies third-party use of num_def_ids_untracked

Document `From` impls for cow.rs

Remove unused feature gates

The first commit removes a usage of a feature gate, but I don't expect it to be controversial as the feature gate was only used to workaround a limitation of rust in the past. (closures never being `Clone`)

The second commit uses `#[allow_internal_unstable]` to avoid leaking the `trusted_step` feature gate usage from inside the index newtype macro. It didn't work for the `min_specialization` feature gate though.

The third commit removes (almost) all feature gates from the compiler that weren't used anyway.

…access_docs, r=Mark-Simulacrum

Fix typo in internal documentation for `TrustedRandomAccess`

`next_back` is a method of DoubleEndedIterator, not Iterator.

Restoring the `num_def_ids` function in the CStore API

## The context

I am the maintainer of https://github.com/hacspec/hacspec, an embedded Rust DSL aimed at cryptographic specifications. As it is normal for an embedded DSL, Hacspec's compiler relies on being plugged to the internal API of the Rust compiler, which is unstable and subject to changes.

## The problem

The Hacspec compiler features its own typechecker, that performs an additional, more restrictive typechecking pass over the Rust code of a crate. To complete this typechecking, the Hacspec compiler needs to retrieve the signature of functions defined in non-local imported crates. Rather than retrieving these signatures on-demand, the Hacspec compiler pre-populates its typechecking context with all the Hacspec-compatible symbols defined in non-local crates first. This requires having a way to iterate over all the definitions in a non-local crate.

I used to do this with `CrateMetadata::all_def_path_hashes_and_def_ids`, but this function was deleted in 908bf5a. Then, I fellback on `CStore::num_def_ids`, exploiting the fact that all the `DefIds` for a crate have the same `krate_num` and range from `0` to `num_def_ids(krate_num)`. But `num_def_ids` was deleted in b6120bf.

I looked to the `Cstore::item_children_untracked` function to replicate the feature of traversing through all the `DefId` for a crate, using `CRATE_DEF_INDEX` as the root, but this does not work as recursive `Cstore::item_children_untracked` calls do not reach all the symbols I was able to reach using the two previous methods.

## Description of this PR

This PR simply restores in the public API of `CStore` the `num_def_ids` function, giving the size of the definition table for a given crate.

jsondocck small cleanup

updated `shlex` (there was some fix comex/rust-shlex@6db4704)
replaced `lazy_static` with `once_cell`
removed `serde` direct dependency (`serde_json` will pull it)

…tebank

Fix bad suggestions for code from proc_macro

Fixes rust-lang#85932

This disables these suggestions for spans from external macros, while keeping them for macros defined locally:

Before:
```
3 | #[hello]
  | ^^^^^^^^
  | |
  | expected `&mut i32`, found integer
  | help: consider mutably borrowing here: `&mut #[hello]`
```

After:
```
3 | #[hello]
  | ^^^^^^^^ expected `&mut i32`, found integer
```

Unchanged:
```
26 | macro_rules! bla { () => { x(123); } }
   |                              ^^^
   |                              |
   |                              expected `&mut i32`, found integer
   |                              help: consider mutably borrowing here: `&mut 123`
...
29 |     bla!();
   |     ------- in this macro invocation
```

Show `::{{constructor}}` in std::any::type_name().

Fix rust-lang#84666

Before:
```
[src/main.rs:6] type_name::<T>() = "playground::Velocity"
[src/main.rs:6] type_name::<T>() = "playground::Velocity"
```

After:
```
[src/main.rs:6] type_name::<T>() = "scratchpad::Velocity::{{constructor}}"
[src/main.rs:6] type_name::<T>() = "scratchpad::Velocity"
```

cc ``@scottmcm``

…ulacrum

Fix linkcheck script from getting out of sync.

When there are changes to the linkcheck script, the CI jobs used in the books would download the latest version on master, but run it against nightly. During that 24 hour window, the CI can fail if the script has changes that are incompatible with the last nightly. This fixes it so that it downloads the linkchecker that matches the version of nightly.

This also includes a fix to build with release to make it run much faster (I forgot to add this in rust-lang#85652).