save-analysis: inferred crates are not included

[nrc]

save-analysis includes crates declared with extern crate or in the implicit prelude, but does not include those inferred from imports due to the new extern crate elision feature. I assume because extern_crate on the tcx does not include info for those crates (see https://github.com/rust-lang/rust/blob/master/src/librustc_save_analysis/lib.rs#L113).

This is the root cause for part of rust-lang/rls#729

[nrc]

cc @petrochenkov who implemented the 'implicit extern crate' support in #47156

I can't see where the info about extern crates gets into the tcx, but if it is syntactic then perhaps this is just a bug of omission where #47156 should also have entered info about implicit extern crates into the tcx too?

[nrc]

ping @petrochenkov @rust-lang/compiler - anyone got advice on how to address this issue?

[sinkuu]

master...sinkuu:save_analysis_implicit_extern
I'd like to make a PR unless @petrochenkov has already been working on this.

[petrochenkov]

@sinkuu
Thanks for fixing this issue, I never got to it.