Failure for some `--target` values

[vhbit]

To be precise - it fails if target is different from host, i.e.

cargo build --target=arm-apple-ios --verbose
task '<unnamed>' failed at 'called `Option::unwrap()` on a `None` value', /Users/vhbit/projects/auto-rust/src/libcore/option.rs:262

while cargo build --target=x86_64-apple-darwin works fine.

Running cargo build --target=x86_64-unknown-linux-gnu --verbose returns something like this:

Compiling objcrust v0.0.1 (file:///Users/vhbit/projects/ObjCrust/Rust)
/Users/vhbit/projects/ObjCrust/Rust/src/lib.rs:1:1: 1:1 error: can't find crate for `std`

The thing is that rustc was built with arm-apple-ios support and without x86_64-unknown-linux-gnu.

[alexcrichton]

This looks like it's because cargo is attempting to discover the dylib prefix/suffix, but it's being dropped for iOS. The relevant code is here and it would mean that cargo needs to understand that some targets don't have dylibs.