"rerun-if-changed" in build script causes causes fingerprint error and extra rebuild

[mbrubeck]

If a dependency contains the following build script:

fn main() { println!("cargo:rerun-if-changed=build.rs"); }

Then running cargo build twice in a row compile the dependency twice, instead of just once:

$ cargo build
   Compiling a v0.1.0 (file:///home/mbrubeck/src/test/cargotest)
   Compiling cargotest v0.1.0 (file:///home/mbrubeck/src/test/cargotest)
$ cargo build
   Compiling a v0.1.0 (file:///home/mbrubeck/src/test/cargotest)
   Compiling cargotest v0.1.0 (file:///home/mbrubeck/src/test/cargotest)
$ cargo build
$ 

Possibly related: the following message appears in the log output during the second build, but not the first or third build:

INFO:cargo::ops::cargo_rustc::fingerprint: fingerprint error for a v0.1.0 (file:///home/mbrubeck/src/test/cargotest): local fingerprint type has changed

As reported by @cdlm on IRC, this affects the current version of the glutin crate. And here is a minimal test case for this issue: https://github.com/mbrubeck/cargotest

[mbrubeck]

cc #2134

[alexcrichton]

Note that you don't need to depend on the build script's source to re-trigger compiling the build script, Cargo does that automatically. That being said this probably shouldn't happen either way, so there may still be a bug.

[mbrubeck]

This happens for any value of rerun-if-changed. For example, cargo:rerun-if-changed=src/lib.rs triggers the same issue.

[alexcrichton]

Fixed by #2279