rustc doesn't complain if extern rlib is missing

[andir]

I just came across a weird case where rustc would not warn that a given extern dependency could not be found. I believe it should just warn the user or throw an error in that case instead of silently ignoring the error. It seems like the dependencies are only checked on a per-use basis whereas I believe being more strict here would probably not harm and help users find issues earlier.

Reproducer

$ echo 'fn main() {}' > foo.rs
$ rustc foo.rs --extern something=asdlflaksdjfljsaljfdlaskjdf

What happened

It did compile my trivial program without any issues.

Expected output

The compiler should have warned or thrown an error about the missing something dependency. Alternatively warned about an unused dependency that was provided.

[nagisa]

Likely an oversight in implementation of Rust 2018 implicit extern crate mechanism.

echo 'extern crate banana; fn main(){}' | rustc --edition=2015 --extern banana=fooooooo -

behaves just as you'd expect.

[petrochenkov]

This is by design.
Crates from --extern are looked up on the filesystem and loaded lazily, so if they are never used from source code, then they are never loaded and not put into dependency info.

Sometimes the number of crates on the command line is much larger than the number of actually used crates, so we don't want to load, or even search and touch them.

[petrochenkov]

Alternatively warned about an unused dependency that was provided.

This, on the other hand, would be a nice lint.
(And not requiring going to filesystem.)

[nagisa]

A lint for unused crates is being tracked #57274 AFAIK