--build-plan fails when dev-dependencies are present

[KodrAus]

Hi! It looks like there's a problem attempting to grab a build plan from libraries that use dev-dependencies.

Toolchain that fails:

rustc 1.30.0-nightly (f49f6e73a 2018-09-23)
cargo 1.31.0-nightly (de314a8b2 2018-09-21)

Toolchain that succeeds:

rustc 1.30.0-nightly (63c75d375 2018-09-21)
cargo 1.30.0-nightly (a5d829494 2018-09-13)

If I have a Cargo.toml that looks like:

[package]
name = "build-plan-test"
version = "0.0.0"

[dev-dependencies]
libc = "*"

When running cargo build -Z unstable-options --build-plan --verbose, I get the following output:

     Running `rustc
        --crate-name build_plan_test src/lib.rs
        --color always
        --crate-type lib
        --emit=dep-info,link
        -C debuginfo=2
        -C metadata=2746e0b9ac45f8b8
        -C extra-filename=-2746e0b9ac45f8b8
        --out-dir /Users/user/dev/build-plan-test/target/debug/deps
        -C incremental=/Users/user/dev/build-plan-test/target/debug/incremental
        -L dependency=/Users/user/dev/build-plan-test/target/debug/deps`
error: failed to find libc v0.2.43 

[KodrAus]

Sorry, I forgot to note that this Cargo.toml does output a build plan successfully:

[package]
name = "build-plan-test"
version = "0.0.0"

[dependencies]
libc = "*"

The difference there is that libc is declared as a regular dependency rather than a dev-dependency.

[ehuss]

@alexcrichton This seems to be caused by #6005.

[alexcrichton]

Indeed! Should be fixed by #6086