rustPlatform.cargoSetupHook: remove references to build directory in binaries

[winterqt]

By default, Rust binaries include debug information (e.g. for the panic handler) that specifies source file paths. On sandboxed Linux, this isn't much of an issue since the build will always be in /build, but on any other platform (including sandboxed macOS), this will result in potential impurities (e.g. when using --keep-failed or building multiple derivations with the same name concurrently).

Luckily, rustc provides an option to remap path prefixes when writing this debug info, in the form of --remap-path-prefix. This allows the replacement of any path prefix that would be written to the binary. This change sets this to $NIX_BUILD_TOP, falling back to a noop if there is no $NIX_BUILD_TOP during build (as cases where it's unset, like devshells, don't really benefit from purity).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[lilyball]

I haven't looked at these hooks before but this certainly looks like it'll work. I suppose we can assume that $NIX_BUILD_TOP will never contain a path that has a " embedded in it?

[winterqt]

I realized I also need to add this to rustc itself -- I have a diff, but want to test, and don't have the time to run it now.

[winterqt]

Ideally, we'd wait for rust-lang/rust#140778 to be fixed before landing this, as without it, rustc source paths get embedded in the resulting binaries.

[winterqt]

Something interesting between the setup hook commit and the rustc commit:

[nix-shell:~/src/nixpkgs]$ strings result-2/bin/rg | rg "alloc\/"
/private/tmp/nix-build-rustc-1.89.0.drv-1/rustc-1.89.0-src/library/alloc/src/string.rs
/private/tmp/nix-build-rustc-1.89.0.drv-1/rustc-1.89.0-src/library/alloc/src/slice.rs
/private/tmp/nix-build-rustc-1.89.0.drv-1/rustc-1.89.0-src/library/alloc/src/collections/btree/node.rs
/private/tmp/nix-build-rustc-1.89.0.drv-1/rustc-1.89.0-src/library/alloc/src/collections/btree/map/entry.rs

[nix-shell:~/src/nixpkgs]$ strings result/bin/.cargo-wrapped | rg "alloc\/"
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/collections/btree/map/entry.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/collections/btree/node.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/string.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/raw_vec/mod.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/borrow.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/string.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/slice.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/vec/spec_from_iter_nested.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/vec/mod.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/vec/in_place_collect.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/slice.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/slice.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/sync.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/slice.rs
/rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/alloc/src/string.rs

Where's that /rustc/.../... path coming from?!

All good now, thank you!

[alyssais]

Could we put this in build.rustflags and avoid having to repeat it?

[winterqt]

Seems a better solution will be in-flight eventually: https://matrix.to/#/!UNVBThoJtlIiVwiDjU:nixos.org/$pghG2JN87PUdBDTSG9Wda8Zs15w72PQZxuZk_0TxXXs?via=nixos.org&via=matrix.org&via=tchncs.de