Spurious rebuilds (still…)

In https://github.com/servo/mozjs/pull/246 I tried to fix this and it seemed ok when building from this repository. But somehow building in Servo even after https://github.com/servo/servo/pull/26294 still has this bug:

$ ./mach build
[…]
$ CARGO_LOG=cargo::core::compiler::fingerprint=info ./mach build 
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] stale: changed "/home/simon/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/old-configure"
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]           (vs) "/home/simon/servo/target/release/build/mozjs_sys-b4b5f9376eb52130/output"
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]                FileTime { seconds: 1587715054, nanos: 830819920 } != FileTime { seconds: 1587715054, nanos: 862820022 }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for servo v0.0.1 (/home/simon/servo/ports/glutin)/Build/Target { name: "servo", tested: false, benched: false, doc: true, ..: with_path("/home/simon/servo/ports/glutin/main.rs", Edition2018) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for libservo v0.0.1 (/home/simon/servo/components/servo)/Build/Target { ..: lib_target("servo", ["rlib"], "/home/simon/servo/components/servo/lib.rs", Edition2018) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for layout_thread_2020 v0.0.1 (/home/simon/servo/components/layout_thread_2020)/Build/Target { ..: lib_target("layout_thread", ["lib"], "/home/simon/servo/components/layout_thread_2020/lib.rs", Edition2018) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for script v0.0.1 (/home/simon/servo/components/script)/Build/Target { ..: lib_target("script", ["lib"], "/home/simon/servo/components/script/lib.rs", Edition2018) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for mozjs v0.13.0 (https://github.com/servo/rust-mozjs#8615f863)/Build/Target { doctest: false, ..: lib_target("mozjs", ["lib"], "/home/simon/.cargo/git/checkouts/rust-mozjs-8611526964119dd6/8615f86/src/lib.rs", Edition2015) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for mozjs v0.13.0 (https://github.com/servo/rust-mozjs#8615f863)/RunCustomBuild/Target { ..: custom_build_target("build-script-build", "/home/simon/.cargo/git/checkouts/rust-mozjs-8611526964119dd6/8615f86/build.rs", Edition2015) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for mozjs_sys v0.68.1 (https://github.com/servo/mozjs?rev=9a6d8fc6b0cf13cd02efbd805ccdf62ddc5ce593#9a6d8fc6)/RunCustomBuild/Target { ..: custom_build_target("build-script-build", "/home/simon/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/build.rs", Edition2015) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint] fingerprint error for mozjs_sys v0.68.1 (https://github.com/servo/mozjs?rev=9a6d8fc6b0cf13cd02efbd805ccdf62ddc5ce593#9a6d8fc6)/Build/Target { doctest: false, ..: lib_target("mozjs_sys", ["lib"], "/home/simon/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/src/lib.rs", Edition2015) }
[2020-04-24T09:05:50Z INFO  cargo::core::compiler::fingerprint]     err: current filesystem status shows we're outdated
   Compiling mozjs_sys v0.68.1 (https://github.com/servo/mozjs?rev=9a6d8fc6b0cf13cd02efbd805ccdf62ddc5ce593#9a6d8fc6)
   Compiling mozjs v0.13.0 (https://github.com/servo/rust-mozjs#8615f863)
   Compiling script v0.0.1 (/home/simon/servo/components/script)
   Compiling layout_thread_2020 v0.0.1 (/home/simon/servo/components/layout_thread_2020)
   Compiling libservo v0.0.1 (/home/simon/servo/components/servo)
   Compiling servo v0.0.1 (/home/simon/servo/ports/glutin)

Relevant part:

stale: changed "/home/simon/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/9a6d8fc/mozjs/js/src/old-configure"

I don’t feel like investigating and making a fix right now, so here’s a bug report instead.

Reproduce the issue with ./mach build and the Cargo fingerprint logging shown in the report. Start by checking why mozjs/js/src/old-configure is reported newer than the build output, then trace the related build.rs entries for mozjs_sys and mozjs. Done means an unchanged Servo build no longer recompiles these targets spuriously.