Consider emitting cargo:rerun-if-changed line for argument of cxx::Build::bridge

[dtolnay]

https://docs.rs/cxx/0.1.2/cxx/struct.Build.html#method.bridge could automatically do:

println!("cargo:rerun-if-changed={}", rust_source_file);

According to https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script:

rerun-if-changed=PATH is a path to a file or directory which indicates that the build script should be re-run if it changes (detected by a more-recent last-modified timestamp on the file). Normally build scripts are re-run if any file inside the crate root changes, but this can be used to scope changes to just a small set of files.

[dtolnay]

I am leaning against doing this because the pit of success is bigger if we don't. If we don't emit rerun-if-changed, the downsides are:

• build.rs might be rerun by Cargo more often than necessary, whenever anything under the crate's root directory changes;
• the user must add their own rerun-if-changed line in their build.rs to avoid those reruns.

If we do emit rerun-if-changed:

• and the user's build script does something other than cxx build, then those other parts no longer rerun automatically when anything under the crate's root directory changes, and it's hard to discover why.

[dtolnay]

Since rust-lang/cc-rs#738 the cc crate has started printing some rerun lines by default, which means build scripts that use cc now must print their own rerun-if-changed lines to avoid incorrect behavior (missing runs).