rustdoc fails to account for `-Clinker` and `-Clink-arg` when building tests

[nagisa]

I’m working on a fairly peculiar system at the moment where the default linker (cc) is not able of linking properly, and thus I set the following environment variables

export RUSTFLAGS="-Clinker=clang -Clink-arg=--target=$HOST_TARGET"
export RUSTDOCFLAGS="-Clinker=clang -Clink-arg=--target=$HOST_TARGET"
export CC="clang"
export CFLAGS="--target=$HOST_TARGET"

which made cargo build pass. However cargo test still fails because doctests are still being compiled with the default linker. Running cargo test --verbose reveals that the flags are being passed through to rustdoc:

rustdoc ... -Clinker=clang -Clink-arg=--target=$HOST_TARGET

However, rustdoc still uses a default linker:

error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" ...

(Tested with rustdoc 1.38.0 (625451e37 2019-09-23))

[ollie27]

This has been fixed by #63827 + #63834 so should work on 1.39.0.