Impossible to use clang to build rust packages

[ninegua]

Can we just say that use setEnv to force set environment variables before the cargo command is a bad idea?

nixpkgs/pkgs/build-support/rust/lib/default.nix

Line 57 in ac6c93e

setEnv = ''

nixpkgs/pkgs/build-support/rust/hooks/cargo-build-hook.sh

Line 33 in ac6c93e

@setEnv@ cargo build -j $NIX_BUILD_CORES \

It forces HOST_CC to be set to gcc even when stdenv is switched to llvm, for example the code below would still run gcc when building native packages because of the HOST_CC setting, which would lead to linking failures:

makeRustPlatform {
        rustc = self.rust-stable;
        cargo = self.rust-stable;
        stdenv = self.llvmPackages.libcxxStdenv;
}

Note that this problem does not exist with Nixpkgs 23.05. It only becomes a problem after the setEnv approach is adopted in 23.11 and newer.

In general, we want environment variables to be configurable. But this setEnv thing is just very difficult to override. Would appreciate suggestions on how to override HOST_CC, or getting rid of setEnv completely. Thanks!

[RossComputerGuy]

Is this still a problem? Have you tried pkgsLLVM?

[ninegua]

It is still a problem. It is pretty clear where the problem is, namely, setEnv is too rigid and can't be overridden.

[RossComputerGuy]

Ok, how have you tried setting the CC? Have you tried overriding rustc's stdenv to clangStdenv?

[ninegua]

Ok, how have you tried setting the CC? Have you tried overriding rustc's stdenv to clangStdenv?

If you read the source code of setEnv, it gets CC from environment, but it gets HOST_CC from pkgsBuildHost.stdenv.cc, which is not something a user can easily have control over.

[RossComputerGuy]

I am reading the source code which is why I have been asking if you have tried changing the stdenv.

[ninegua]

Yes, I tried to pass a different stdenv as argument to makeRustPlatform, but it wasn't effective.

[RossComputerGuy]

makeRustPlatform isn't what needs to have stdenv overridden. pkgs/development/compilers/rust/default.nix imports pkgs/build-support/rust/lib/default.nix which the rust lib takes in stdenv. Which the rust default file is imported from pkgs/development/compilers/rust/1_80.nix.

[ninegua]

I don't know, but using pkgsLLVM is too much of a heavy lifting to solve a local environment variable problem. But thanks for the suggestion though!

[RossComputerGuy]

Yeah, these are two of the best (probably only) options. I suggest pkgsLLVM since it gives you an environment that is purely LLVM on target. Overriding a single package to use LLVM will result in an LLVM + GCC environment. If you have any issues with clangStdenv when doing this, I suggest giving pkgsLLVM a shot.

[emilazy]

The Rust build infrastructure should be fixed, though.

[RossComputerGuy]

@emilazy True, pkgs/development/compilers/rust/default.nix could be callPackaged and pkgs/development/compilers/rust/1_80.nix could just inherit it and pass through stdenv.

[reckenrode]

Rust and setEnv is what motivated #346043. Prior to that PR, it was impossible to override the SDK version of Rust packages that needed to build non-Rust code (particularly some -sys crates) because setEnv pulled in other clangs with the wrong SDK version (e.g., #239384).

Some of my early attempts to work around it involved turning makeRustPlatform into a fixed-point and allowing rustPlatform to be overridden, but it uses pkgsBuildHost, pkgsBuildTarget, and pkgsTargetTarget depending on the cross-compilation scenario. Just overriding the stdenv may not be enough.