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");

[dtolnay]

I think the only way to solve this would be for Cargo to expose a way that crates can ask for a particular environment variable to be hashed into the crate's metadata. Similar to what was done in rust-lang/cargo#14830 for RUSTFLAGS. Either dynamically from a build script (cargo:rerun-if-env-changed-hashed=…) or statically from a manifest entry.

For rustc 1.85+ (current beta), one workaround is to use different RUSTFLAGS between your RUSTC_BOOTSTRAP builds and non-RUSTC_BOOTSTRAP builds.

Without RUSTFLAGS:

$ cargo +beta check
   Compiling proc-macro2 v1.0.93
   Compiling unicode-ident v1.0.16
   Compiling quote v1.0.38
   Compiling syn v2.0.98
   Compiling serde_derive v1.0.217
    Checking repro v0.0.0
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.01s
$ RUSTC_BOOTSTRAP=1 cargo +beta check
   Compiling proc-macro2 v1.0.93
   Compiling quote v1.0.38
   Compiling syn v2.0.98
   Compiling serde_derive v1.0.217
    Checking repro v0.0.0
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.54s
$ cargo +beta check
   Compiling proc-macro2 v1.0.93
   Compiling quote v1.0.38
   Compiling syn v2.0.98
   Compiling serde_derive v1.0.217
    Checking repro v0.0.0
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.57s

With RUSTFLAGS:

$ cargo +beta check
   Compiling proc-macro2 v1.0.93
   Compiling unicode-ident v1.0.16
   Compiling quote v1.0.38
   Compiling syn v2.0.98
   Compiling serde_derive v1.0.217
    Checking repro v0.0.0
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.92s
$ RUSTC_BOOTSTRAP=1 RUSTFLAGS=--cfg=__anything cargo +beta check
   Compiling proc-macro2 v1.0.93
   Compiling unicode-ident v1.0.16
   Compiling quote v1.0.38
   Compiling syn v2.0.98
   Compiling serde_derive v1.0.217
    Checking repro v0.0.0
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.92s
$ cargo +beta check
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
$ RUSTC_BOOTSTRAP=1 RUSTFLAGS=--cfg=__anything cargo +beta check
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s