`cargo +nightly doc` panics for crates depending on gfx-backend-empty

[grovesNL]

If I create a new crate with

cargo new --lib rustdocissue

Then add a dependency on gfx-backend-empty version 0.5:

[dependencies]
gfx-backend-empty = "0.5"

Then run cargo doc on stable:

cargo +stable doc --lib --no-deps

cargo doc succeeds with warnings. I'm not sure if the warnings are relevant, but the warnings in cargo +stable doc are for some empty blocks in examples that haven't been populated, such as:

/// ```no_run
///
/// ```

Now if I run the same command on nightly (nightly-2020-04-06):

cargo +nightly doc --lib --no-deps

cargo doc panics with the following error:

thread 'rustc' panicked at 'could not find markdown in source', src/librustdoc/passes/mod.rs:411:31

The panic originates in

rust/src/librustdoc/passes/mod.rs

Line 411 in 1ccb0b4

let source_line = src_lines.next().expect("could not find markdown in source");

Besides stable and nightly, I tried the nightly-2020-03-19 toolchain which succeeds with warnings (like stable).

I also created a repo which reproduces the issue at https://github.com/grovesNL/rustdocissue if it's convenient to clone that instead of following the steps above.

[jonas-schievink]

cc @rust-lang/rustdoc

[GuillaumeGomez]

Oh that's bad. We should make a warning about empty code blocks though. I'll send a PR shortly.

[eddyb]

cc @arlosi - we're discussing this on Discord and @GuillaumeGomez has bisected it to #69718.

[arlosi]

It seems that before #69718, the hash would not have matched, so the external source would be "not present". After #69718, the external source would be present, but not-normalized. Which I suspect would cause any offsets into it to be incorrect.

I'm not exactly sure how it worked before (given that it would have absent external source).