Repro steps:

1. Create build.rs and make appropriate entry in Cargo.toml
2. Have build.rs conditionally generate a file src/foo.rs (eg generate file if it doesn't exist, or generate file if mtime is older than its configuration)
3. cargo build --verbose (should run build script, generate file, and build foo.rs)
4. cargo build --verbose again. This time, I would expect it to be a noop, but it rebuilds foo.rs
5. cargo build --verbose again. This time it's a noop as expected.

I am running into this issue because I am compiling a .proto file into a protobuf rs file during the build.rs phase.