The compiler would previously fall back to using `-L` and normal lookup paths if
a `--extern` path was specified but it did not match (wrong architecture, for
example). This commit removes this behavior and forces the hand of the crate
loader to *always* use the `--extern` path if specified, no matter whether it is
correct or not.
This fixes a bug today where the compiler's own libraries are favored in cross
compilation by accident. For example when a crate using the crates.io version of
`log` was cross compiled, Cargo would compile `log` for the target architecture.
When loading the macros, however, the compiler currently favors using the *host*
architecture (for plugins), and because the `--extern log=...` pointed at an
rlib for the target architecture, that lookup failed. The crate loader then
fell back on `-L` paths to find the compiler-used `log` crate (the wrong one!)
and then a compile failure happened because the logging macros are slightly
different.
80627cd3cc