`cargo check` recompiles dependencies when checking lib and bin separately

[crumblingstatue]

For this project:

Cargo.toml:

[package]
name = "foo"
version = "0.1.0"
authors = ["Mika Attila <radiantstatue@gmail.com>"]

[dependencies]
rand = "*"

src/lib.rs:

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
    }
}

src/bin/bar.rs: fn main() {}

cargo check does this:

$ cargo check --lib
   Compiling libc v0.2.18
   Compiling rand v0.3.15
   Compiling foo v0.1.0 (file:///tmp/foo)
    Finished debug [unoptimized + debuginfo] target(s) in 4.87 secs
$ cargo check --bin bar
   Compiling libc v0.2.18
   Compiling rand v0.3.15
   Compiling foo v0.1.0 (file:///tmp/foo)
warning: function is never used: `main`, #[warn(dead_code)] on by default
 --> src/bin/bar.rs:1:1
  |
1 | fn main() {}
  | ^^^^^^^^^^^^

[alexcrichton]

I believe this is a dupe of #3418, so closing in favor of that, but thanks for the report!