fixes rust-lang#45420

This is better writing

Closes rust-lang#44050.

Thanks-to: @kennytm

Fixes rust-lang#43780.

…steveklabnik

rustdoc: update pulldown + fix spurious rendering difference around footnotes

fixes rust-lang#45420

…r=sfackler

Improve docs for UdpSocket::set_nonblocking.

Closes rust-lang#44050.

…nnytm

Use expect for current_dir on librustc/session mod

Reference bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=871638

Like described on the reference bug report:

~~~
$ mkdir dir
$ cd dir
$ rm -rf ../dir
$ RUST_BACKTRACE=1 rustc -C target-cpu=help
error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "No such file or directory" } }', src/libcore/result.rs:837
stack backtrace:
   1:     0x7f7d23970dda - <unknown>
   2:     0x7f7d2398305f - <unknown>
   3:     0x7f7d2397f8a5 - <unknown>
   4:     0x7f7d2397ffc7 - std::panicking::rust_panic_with_hook::h109e116a3a861224
   5:     0x7f7d2397fe54 - <unknown>
   6:     0x7f7d2397fd79 - std::panicking::begin_panic_fmt::h26713cea9bce3ab0
   7:     0x7f7d2397fd07 - rust_begin_unwind
   8:     0x7f7d239cb41d - core::panicking::panic_fmt::hcfbb59eeb7f27f75
   9:     0x7f7d20be63d3 - <unknown>
  10:     0x7f7d20d6ebcc - rustc::session::build_session_::h7a3559f2373a5d05
  11:     0x7f7d20d6dd7e - rustc::session::build_session_with_codemap::h68bc7bcd2f34eee4
  12:     0x7f7d20d6d72c - rustc::session::build_session::h437fda3c327a8bde
  13:     0x7f7d23d26030 - <rustc_driver::RustcDefaultCalls as rustc_driver::CompilerCalls<'a>>::no_input::h8047df7741757d1c
  14:     0x7f7d23d21d27 - rustc_driver::run_compiler::hafe7bbfedf95a825
  15:     0x7f7d23c57378 - <unknown>
  16:     0x7f7d2398ae0a - __rust_maybe_catch_panic
  17:     0x7f7d23c76fa8 - <unknown>
  18:     0x7f7d2397eb74 - <unknown>
  19:     0x7f7d1ed4f493 - start_thread
  20:     0x7f7d23645afe - __clone
  21:                0x0 - <unknown>
~~~

With this patch this will give instead:

~~~
error: Current directory is invalid: No such file or directory (os error 2)
~~~

…kennytm

Return 0 as an exit status when no subcommand is given to bootstrap

Running `./x.py` emits usage and error messages when no subcommand is given:
```
Usage: x.py <subcommand> [options] [<paths>...]

Subcommands:
    build       Compile either the compiler or libraries
    test        Build and run some test suites
    bench       Build and run some benchmarks
    doc         Build documentation
    clean       Clean out build directories
    dist        Build distribution artifacts
    install     Install distribution artifacts

To learn more about a subcommand, run `./x.py <subcommand> -h`

failed to run: /home/topecongiro/rust/build/bootstrap/debug/bootstrap
```
IMHO the last line is unnecessary. This PR removes it by changing the return code of `bootstrap` to 0 when no sub command is given.

…sdreavus

Remove 'just' in diagnostics

This is better writing

…eveklabnik

Fixed rustc_on_unimplemented example in Unstable Book

I saw [this](https://doc.rust-lang.org/beta/unstable-book/language-features/on-unimplemented.html) example and it seemed wrong. This fixes the example to emit an error message that makes more sense.

…nytm

Remove 'future Rust version' code block in diagnostic text.

Fixes rust-lang#43780.