cargo doc --all does not document proc macro crates if target is explicitly specified

[leo60228]

If a target is explicitly specified on the command line, even if it is the same target as the host, then proc macro crates will not be documented by cargo doc --all. When a cargo configuration file is used to set a default target for cross-compliation, this can prevent documenting proc macro crates at all.

Minimal testcase: https://github.com/leo60228/minimal_testcase

[leo60228@leotop2:~/minimal_testcase]$ rustc --version --verbose
rustc 1.38.0-nightly (273f42b59 2019-07-21)
binary: rustc
commit-hash: 273f42b5964c29dda2c5a349dd4655529767b07f
commit-date: 2019-07-21
host: x86_64-unknown-linux-gnu
release: 1.38.0-nightly
LLVM version: 9.0

[jyn514]

Possible duplicate of rust-lang/cargo#7677

[ehuss]

Yea, I'm going to close as I think this is mostly a cargo issue.