doc tests fail without any information when missing back quotes.

[lcnr]

Doc test with missing closing back quotes fail without any explanation.

///```rust
/// let x = 7;
/// assert_eq!(x, a());
/// // <- HERE
///```rust
/// let x = 6;
/// assert_ne!(x, a());
///```
fn a() -> i32 {
    7
}

cargo test:

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Doc-tests temp

running 1 test
test src/lib.rs - a (line 1) ... FAILED

failures:

failures:
src/lib.rs - a (line 1)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--doc'

Expected outcome: error while compiling the tests.

tested versions:

rustc --version --verbose

rustc 1.35.0-nightly (cb2f34dc6 2019-03-22)
binary: rustc
commit-hash: cb2f34dc6d7e83d8bcfef79e0388d49f0c24aca7
commit-date: 2019-03-22
host: x86_64-unknown-linux-gnu
release: 1.35.0-nightly
LLVM version: 8.0

rustc 1.33.0 (2aa4c46cf 2019-02-28)
binary: rustc
commit-hash: 2aa4c46cfdd726e97360c2734835aa3515e8c858
commit-date: 2019-02-28
host: x86_64-unknown-linux-gnu
release: 1.33.0
LLVM version: 8.0

[lcnr]

https://play.rust-lang.org/?version=stable&mode=release&edition=2015&gist=6b2de799b02ae7255031b96133d2a697

[euclio]

I believe this is related to #56885. The second ```rust line is unable to be lexed, so the lexer bails with an unbuffered fatal error.

I was wrong. I have a fix incoming.