dev_dependency -> dependency cycle fails to compile crates in correct order/place

[huonw]

$ tree
.
|-- a
|   |-- Cargo.toml
|   `-- src
|       `-- lib.rs
|-- b
|   |-- Cargo.toml
|   `-- src
|       `-- lib.rs
`-- c
    |-- Cargo.toml
    `-- src
        `-- lib.rs

6 directories, 6 files

a

[package]

name = "a"
version = "0.0.1"
authors = []

[dev_dependencies.b]
path = "../b"
[dev_dependencies.c]
path = "../c"

#[cfg(test)] extern crate b;
#[cfg(test)] extern crate c;

b

[package]

name = "b"
version = "0.0.1"
authors = []

[dependencies.a]
path = "../a"

extern crate a;

c

[package]

name = "c"
version = "0.0.1"
authors = []

c/src/lib.rs empty.

---

.../a $ cargo build
   Compiling a v0.0.1 (.../a)

.../a $ cargo test
   Compiling c v0.0.1 (.../a)
   Compiling a v0.0.1 (.../a)
.../a/src/lib.rs:1:14: 1:29 error: can't find crate for `b`
.../a/src/lib.rs:1 #[cfg(test)] extern crate b;
                                ^~~~~~~~~~~~~~~
error: aborting due to previous error
Build failed, waiting for other jobs to finish...
Could not compile `a`.

To learn more, run the command again with --verbose.

Allowing progress by removing the offending line leads to this:

.../a $ cargo test
   Compiling c v0.0.1 (.../a)
   Compiling a v0.0.1 (.../a)
   Compiling b v0.0.1 (.../a)
Could not compile `b`.

--- stderr
error: extern location does not exist: .../a/target/deps/liba-609ef28031a9b714.rlib
.../b/src/lib.rs:1:1: 1:16 error: can't find crate for `a`
.../b/src/lib.rs:1 extern crate a;
                   ^~~~~~~~~~~~~~~
error: aborting due to 2 previous errors


To learn more, run the command again with --verbose.

If that extern crate is removed (leaving only a/src/lib.rs with anything significant, but b/Cargo.toml with the same dependency), the building becomes racy:

.../a $ rm -rf target && cargo test
   Compiling c v0.0.1 (.../a)
   Compiling a v0.0.1 (.../a)
.../a/src/lib.rs:2:14: 2:29 error: can't find crate for `c`
.../a/src/lib.rs:2 #[cfg(test)] extern crate c;
                                ^~~~~~~~~~~~~~~
error: aborting due to previous error
Build failed, waiting for other jobs to finish...
Could not compile `a`.

To learn more, run the command again with --verbose.

.../a $ rm -rf target && cargo test
   Compiling c v0.0.1 (.../a)
   Compiling a v0.0.1 (.../a)
   Compiling b v0.0.1 (.../a)
     Running target/a-e8d8fdc9319b934d

running 0 tests

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

   Doc-tests a

running 0 tests

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

---

In summary, the dependency cycle is causing the b library to be built after the a test runner (the first failure), and not even building the a library at all (the second failure), even though the dependency chain is actually:

        `a` library
              |
      [dependencies.a]
              |
        `b` library     `c` library
              |              |
[dev_dependencies.b]   [dev_dependencies.c]
               \           /
              `a` test runner

(which should be fine?)

[huonw]

Example, I want cargo test to test the macro code example there, but the macro crate depends on the main crate, and so the dev dep in the other direction hits this.