Ungreat change to doctest test names

[ehuss]

Doctest test names have changed due to #66364. Doc comments that pass through a macro defined in another crate now seem to report a different span.

lazy_static::lazy_static! {
    /// Comments
    /// ```
    /// assert!(true);
    /// ```
    static ref FOO: bool = true;
}

running with cargo test --doc previously looked like this:

test <::lazy_static::__lazy_static_internal macros> - FOO (line 26) ... ok

Now the test names look like this:

test /Users/eric/.cargo/registry/src/github.com-1ecc6299db9ec823/lazy_static-1.4.0/src/lib.rs - FOO (line 159) ... ok

Frankly, neither are particularly great. It would be nice if the test name used the span from where the macro was invoked, not where it was defined.

rustc 1.43.0-nightly (c20d7eecb 2020-03-11)

[Centril]

cc @eddyb @petrochenkov

[eddyb]

@matthiaskrgr What you're describing is entirely in the opposite direction, <::std::macros::panic macros> is the old (and arguably bad) kind of Span that's impossible to get anymore.
I've hidden your comment to focus on the reported issue.

[eddyb]

@ehuss That makes sense, we have a way to get the outermost invocation, that is used by file!()/line! and #[track_caller]:

span.ctxt().outer_expn().expansion_cause().unwrap_or(span)

We'd just need to use this in rustdoc.

[Centril]

cc @GuillaumeGomez

[Centril]

cc @rust-lang/rustdoc

[spastorino]

Removed T-compiler label, left nomination for T-rustdoc to decide about this issue.

[GuillaumeGomez]

Well, using what @eddyb suggested seems like the best thing to do.