Changes to RUSTC_BOOTSTRAP causes unwanted recompilations

[weiznich]

I have a rather large project and I need to sometimes change RUSTC_BOOTSTRAP for some reasons. This causes proc_macro2 to recompile, which in turn recompile a large portion of our dependency tree. I personally find it quite surprising that this happens as this behavior is not documented anywhere.

Cargo verbose log:

      Dirty proc-macro2 v1.0.86: the env variable RUSTC_BOOTSTRAP changed
     Compiling proc-macro2 v1.0.86

Offending source code:

proc-macro2/build.rs

Line 131 in aa9476b

println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");