Cargo thinks project is fresh after moving files

[hannobraun]

If I move files around the src directory, Cargo will think the directory is still fresh and won't compile on the next cargo build.

I see this as a problem for two reasons:

• Ergonomics: I'd like to immediately build to see what files need to be changed after the move.
• Possibility for errors: If I'm not paying attention, I might think everything's okay, when in fact my program doesn't compile.

Here are the exact steps to reproduce. First, create a project:

$ cargo -V
cargo-nightly 0.1.0-pre (f720684 2014-07-26 04:21:30 +0000)
$ cargo new cargo-bug
$ cd cargo-bug/

Add a new module and build the project:

$ touch src/a.rs
$ echo "mod a;" > src/lib.rs
$ cargo build
   Compiling cargo-bug v0.0.1 (file:/home/hanno/Desktop/Stuff/tmp/cargo-bug)

Move the file and attempt to rebuild:

$ mv src/a.rs src/b.rs
$ cargo build
       Fresh cargo-bug v0.0.1 (file:/home/hanno/Desktop/Stuff/tmp/cargo-bug)

I get a green build, when in fact I expect to see this:

$ cargo build
   Compiling cargo-bug v0.0.1 (file:/home/hanno/Desktop/Stuff/tmp/cargo-bug)
/home/hanno/Desktop/Stuff/tmp/cargo-bug/src/lib.rs:1:5: 1:6 error: file not found for module `a`
/home/hanno/Desktop/Stuff/tmp/cargo-bug/src/lib.rs:1 mod a;
                                                         ^
Could not compile `cargo-bug`.

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