cargo test prefers dylib over rlib in doc-tests

[crumblingstatue]

If both dylib and rlib are specified as crate type, cargo test prefers the dylib over the rlib, making the doc tests fail due to (probably) linking failures.

Cargo.toml

[package]
name = "foo"
version = "0.0.1"

[lib]
name = "foo"
crate-type = ["dylib", "rlib"]

src/lib.rs

//! ```rust
//! extern crate foo;
//! ```

Output of cargo test --verbose:

   Compiling foo v0.0.1 (file:///home/snake/projects/temp/foo)
     Running `rustc src/lib.rs --crate-name foo --crate-type dylib --crate-type rlib -g -C metadata=03341f3fb3154edb -C extra-filename=-03341f3fb3154edb --out-dir /home/snake/projects/temp/foo/target/debug --emit=dep-info,link -L dependency=/home/snake/projects/temp/foo/target/debug -L dependency=/home/snake/projects/temp/foo/target/debug/deps`
     Running `rustc src/lib.rs --crate-name foo --crate-type dylib --crate-type rlib -g --test -C metadata=defca50fba1c8277 -C extra-filename=-defca50fba1c8277 --out-dir /home/snake/projects/temp/foo/target/debug --emit=dep-info,link -L dependency=/home/snake/projects/temp/foo/target/debug -L dependency=/home/snake/projects/temp/foo/target/debug/deps`
     Running `/home/snake/projects/temp/foo/target/debug/foo-defca50fba1c8277`

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests foo
     Running `rustdoc --test /home/snake/projects/temp/foo/src/lib.rs --crate-name foo -L dependency=/home/snake/projects/temp/foo/target/debug/deps --extern foo=/home/snake/projects/temp/foo/target/debug/libfoo-03341f3fb3154edb.so --extern foo=/home/snake/projects/temp/foo/target/debug/libfoo-03341f3fb3154edb.rlib`

running 1 test
test _0 ... FAILED

failures:

---- _0 stdout ----
    error: cannot satisfy dependencies so `std` only shows up once
note: having upstream crates all available in one format will likely make this go away
error: cannot satisfy dependencies so `core` only shows up once
note: having upstream crates all available in one format will likely make this go away
error: cannot satisfy dependencies so `collections` only shows up once
note: having upstream crates all available in one format will likely make this go away
error: cannot satisfy dependencies so `unicode` only shows up once
note: having upstream crates all available in one format will likely make this go away
error: cannot satisfy dependencies so `alloc` only shows up once
note: having upstream crates all available in one format will likely make this go away
error: cannot satisfy dependencies so `libc` only shows up once
note: having upstream crates all available in one format will likely make this go away
error: cannot satisfy dependencies so `rand` only shows up once
note: having upstream crates all available in one format will likely make this go away
error: aborting due to 7 previous errors
thread '_0' panicked at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:152



failures:
    _0

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured