This uses a dummy file to check if the filesystem being used supports the
executable bit in general.

btree/map.rs is approaching the 3000 line mark, splitting out the entry
code buys about 500 lines of headroom

It would incorrectly suggest adding it as a parameter to the closure instead of the
containing function.

resolve: Do not put nonexistent crate `meta` into prelude

Before the 2018 edition release there was some vague suggestion about adding a crate named `meta` to the standard distribution.
On this basis the name `meta` was "partially reserved" by putting `meta` into extern prelude (this means importing something named `meta` will result in an ambiguity error, for example).
This only caused confusion so far, and two years later there are no specific plans to add such crate.

If some standard crate (named `meta` or not) is added in the future, then cargo will hopefully already have ability to put it into extern prelude explicitly through `Cargo.toml`.
Otherwise, it could be added to extern prelude by the compiler at edition boundary.

Closes rust-lang#73948

Modify executable checking to be more universal

This uses a dummy file to check if the filesystem being used supports the executable bit in general.

Supersedes rust-lang#74753.

BTreeMap: refactor Entry out of map.rs into its own file

btree/map.rs is approaching the 3000 line mark, splitting out the entry
code buys about 500 lines of headroom.

I've created this PR because the changes I've made in rust-lang#77438 will push `map.rs` over the 3000 line limit and cause tidy to complain.

I picked `Entry` to factor out because it feels less tightly coupled to the rest of `BTreeMap` than the various iterator implementations.

Related: rust-lang#60302

Fix grammar in note for orphan-rule error [E0210]

Fixes the grammar in the error note for [E0210] from:

_"= note: implementing a foreign trait is only possible if at least one of the types for which **is it** implemented is local"_

to:

_"= note: implementing a foreign trait is only possible if at least one of the types for which **it is** implemented is local"_

The content of this commit is the result of running the following command at the repository root:

`find . \( -type d -name .git -prune \) -o -type f -print0 | xargs -0 sed -i 's/which is it implemented/which it is implemented/g'`

…t, r=lcnr

Suggest correct place to add `self` parameter when inside closure

It would incorrectly suggest adding it as a parameter to the closure instead of the containing function.

[For example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1936bcd1e5f981573386e0cee985c3c0):
```
help: add a `self` receiver parameter to make the associated `fn` a method
  |
5 |         let _ = || self&self;
  |                        ^^^^^
```

`DiagnosticMetadata.current_function` is only used for these messages so tweaking its behavior should be ok.

…=jyn514

Small CSS cleanup

r? @jyn514

Set `MDBOOK_OUTPUT__HTML__INPUT_404` on linkchecker

This is found in rust-lang/nomicon#240.
It seems the spurious failure shows up without this flag.