Rust: 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)

[matthiaskrgr]

I think this also affects normal backtraces :(
Just got this rather unuseful backtrace while bootstrapping:
thread 'rustc' panicked at 'internal error: entered unreachable code', <::std::macros::panic macros>:2:4 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

full:

````
Compiling rustc-std-workspace-core v1.99.0 (/home/matthias/vcs/github/rust/src/tools/rustc-std-workspace-core)
thread 'rustc' panicked at 'internal error: entered unreachable code', <::std::macros::panic macros>:2:4
stack backtrace:
0: 0x7f92e585f0c8 - ::fmt::h98cd3dee97c4cf65
1: 0x7f92e58a60ec - core::fmt::write::h1f46f059eaf9219f
2: 0x7f92e585e795 - std::io::Write::write_fmt::h36c86a593cbd1590
3: 0x7f92e5836d85 - std::panicking::default_hook::{{closure}}::h9a3a41b67bb037fb
4: 0x7f92e5836a76 - std::panicking::default_hook::h01aac4b9fe280ce2
5: 0x7f92e6f11203 - rustc_driver::report_ice::hd2eba4ea9f27dbcc
6: 0x7f92e58375c5 - std::panicking::rust_panic_with_hook::h6deaeefc56672e26
7: 0x7f92e9a1ff9e - std::panicking::begin_panic::h880977661c4908d4
8: 0x7f92e9b52b7e - ::decode::hf67ea99b3e3a4dab
9: 0x7f92e9a6b75d - as core::iter::traits::iterator::Iterator>::fold::h22fb777da3d9a5a1
10: 0x7f92e9b74e3c - rustc_metadata::rmeta::decoder::::imported_source_files::h9946abe7c8f19abf
11: 0x7f92e9b3a73a - >::specialized_decode::h169c8f045b63ef76
12: 0x7f92e9b67d54 - rustc_metadata::rmeta::decoder::cstore_impl::::item_children_untracked::h0032eb94dc3ab968
13: 0x7f92e8f1e2cd - rustc_resolve::Resolver::resolutions::h79e8a0279a7176a7
14: 0x7f92e8e5b459 - rustc_resolve::imports::ImportResolver::resolve_imports::hf8687998229af9f1
15: 0x7f92e8f095be - rustc_resolve::macros::::resolve_imports::h71ad1801b43469a5
16: 0x7f92e9cd96dc - rustc_expand::expand::MacroExpander::fully_expand_fragment::haf86c5ef2edf203a
17: 0x7f92e9cd8ce4 - rustc_expand::expand::MacroExpander::expand_crate::h6eaf85d46e90c1c4
18: 0x7f92e7105d68 - rustc_session::utils::::time::h079848db3bcfd3fb
19: 0x7f92e71a1ac9 - rustc_interface::passes::configure_and_expand_inner::h7d9e62ef90bf2e65
20: 0x7f92e70c83ef - rustc_interface::passes::configure_and_expand::{{closure}}::h4fff5724d27e7258
21: 0x7f92e70ac6e1 - rustc_data_structures::box_region::PinnedGenerator::new::hb83bee823324c376
22: 0x7f92e71a0b53 - rustc_interface::passes::configure_and_expand::ha157eb8331b33b71
23: 0x7f92e70cb26e - rustc_interface::queries::Queries::expansion::h830b381995ac40d4
24: 0x7f92e6f667f9 - rustc_interface::interface::run_compiler_in_existing_thread_pool::h8aacb1822953387e
25: 0x7f92e6f1a3ca - std::sys_common::backtrace::__rust_begin_short_backtrace::hd0bcef876e2e5413
26: 0x7f92e5836ef4 - std::panicking::try::do_try::h68415d0eb0d1728a
27: 0x7f92e6f6a25f - core::ops::function::FnOnce::call_once{{vtable.shim}}::h4916feb8f3d5ccf7
28: 0x7f92e5835d1f - as core::ops::function::FnOnce>::call_once::hebff0696e6b8e665
29: 0x7f92e586422f - std::sys_common::thread::start_thread::h588d103079703071
30: 0x7f92e5844716 - std::sys::unix::thread::Thread::new::thread_start::hc2107903fae320c0
31: 0x7f92e54cb46f - start_thread
32: 0x7f92e56873d3 - clone
33: 0x0 -

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: rustc 1.44.0-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -Z macro-backtrace -Z save-analysis -Z binary-dep-depinfo -Z force-unstable-if-unmarked -C opt-level=3 -C debuginfo=0 -C target-cpu=native -C link-args=-Wl,-rpath,$ORIGIN/../lib -C prefer-dynamic -C debug-assertions=n --crate-type rlib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile rustc-std-workspace-core.

To learn more, run the command again with --verbose.
command did not execute successfully: "/home/matthias/vcs/github/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "4" "--release" "--features" "panic-unwind backtrace profiler compiler-builtins-c" "--manifest-path" "/home/matthias/vcs/github/rust/src/libtest/Cargo.toml" "--message-format" "json-render-diagnostics"
expected success, got: exit code: 101
failed to run: /home/matthias/vcs/github/rust/build/bootstrap/debug/bootstrap build
Build completed unsuccessfully in 0:00:01
````

[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.