Rust build support doesn't use NIX_LDFLAGS in link flags

[kira-bruneau]

Describe the bug

I was trying to use NIX_LDFLAGS instead of LD_LIBRARY_PATH in ukmm to link extra runtime libraries, but ukmm would silently fail to start: #325139 (comment). I noticed that a bunch of other packages were defining RUSTFLAGS to handle this, so I just ended up using that instead, but it was surprising to see that NIX_LDFLAGS wouldn't work.

Steps To Reproduce

Steps to reproduce the behavior:

1. Create a rust package with NIX_LDFLAGS set
2. Notice that the libraries listed in NIX_LDFLAGS don't actually get linked in resulting binaries

Expected behavior

NIX_LDFLAGS should link extra libraries in rust like it does with packages built with c.

Notify maintainers

@zowoq @winterqt @figsoda

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.9.6, NixOS, 24.05 (Uakari), 24.05.20240623.e4509b3`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.18.2`
 - nixpkgs: `/nix/store/h5hhc58qp2rcv0f3d9di6569pxrpsi1j-source`

[nbdd0121]

I believe this is another case caused by rust-lang/rust#124129

[kira-bruneau]

I don't think this is a regression, it just looks like it was something that wasn't considered when the rust builder was created.

It seems like all other packages that use the same workaround I used for ukmm were merged before rust-lang/rust#124129:

• asusctl
• cosmic-applets
• cosmic-comp
• cosmic-edit
• cosmic-panel
• cosmic-store
• cosmic-term
• cosmic-workspaces-epoch
• xdg-desktop-portal-cosmic

[nbdd0121]

Cosmic ones use dlopen so they require the no-as-needed flag to have the libraries added even without any symbol reference.

What I have observed is that with Rust nightly even referenced symbols will not have rpath set properly. I guess what I see is a different bug with similar symptom.