rustdoc seems to ignore `edition2018` attribute

[Alexander-N]

Running

rustdoc -L target/debug/deps --test test.md

with the following content of test.md

```edition2018
// extern crate clap;
use clap::Arg;
```

gives me

running 1 test
test test.md -  (line 1) ... FAILED

failures:

---- test.md -  (line 1) stdout ----
error[E0432]: unresolved import `clap`
 --> test.md:3:5
  |
4 | use clap::Arg;
  |     ^^^^ use of undeclared type or module `clap`

thread 'test.md -  (line 1)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:351:13
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

The test passes when I uncomment extern crate clap.

This is a problem when code blocks in the documentation should get tested, see amethyst/amethyst#1364 (comment) for an example.

Meta

rustdoc --version --verbose:
rustdoc 1.34.0-nightly (aadbc45 2019-02-23)
binary: rustdoc
commit-hash: aadbc45
commit-date: 2019-02-23
host: x86_64-unknown-linux-gnu
release: 1.34.0-nightly
LLVM version: 8.0

[ehuss]

I think that is expected. Crates are only added to the extern prelude if the --extern flag is used. Since mdbook doesn't really know about extern crates (rust-lang/mdBook#706), you'll probably need to keep the extern crate lines (or figure out how to pass the correct --extern flags to rustdoc).

[Alexander-N]

Ok, thanks.