rustdoc: make langstring parsing more robust

This changes the parsing of the language string
in code examples so that unrecognized examples
are not considered Rust code. This was, for example,
the case when a code example was marked `sh` for shell
code.

This relieves authors of having to mark those samples
as `notrust`.

Also adds recognition of the positive marker `rust`.

By default, unmarked examples are still considered rust.

If any rust-specific tags are seen, code is considered
rust unless marked as "notrust".

Adds test cases for the detection logic.
auto merge of #14569 : skade/rust/rustdoc-robust-langstring-parsing, r=alexcrichton

This changes the parsing of the language string
in code examples so that unrecognized examples
are not considered Rust code. This was, for example,
the case when a code example was marked `sh` for shell
code.

This relieves authors of having to mark those samples
as `notrust`.

Also adds recognition of the positive marker `rust`.

By default, unmarked examples are still considered rust.
Fix deriving Encodable trait for unit structs

Closes #14021
auto merge of #14596 : Sawyer47/rust/encodable-fix, r=alexcrichton

Closes #14021
doc: Remove use of `pub use` globs
syntax: Remove use of `pub use` globs

`quote_expr!` now injects two more (priv) `use` globs.
This may cause extra unused_imports warning.
test: Add tests for closed issue #10764

Closes #10764
auto merge of #14509 : klutzy/rust/de-pub-use-glob, r=alexcrichton

This patchset removes `pub use` usage except for `test/`.
cc #11870
auto merge of #14598 : alexcrichton/rust/triage, r=huonw

Closes #10764
Adding show impl for Rc