buildRustCrate: make default value for codegenUnits configurable

[niklaskorz]

As of now, buildRustCrate defaults to using a single codegen unit per crate, which is far off from what cargo is doing:

The default value, if not specified, is 16 for non-incremental builds. For incremental builds the default is 256 which allows caching to be more granular.

Before debating whether the value itself should be increased, we should at least make it easy to override this for all crates at once, by introducing a parameter defaultCodegenUnits. In its current state, the codegenUnits parameter must be overriden per crate to increase codegen parallelization.

Note that this change does not affect anything inside nixpkgs, but mainly consumers of crate2nix (also see #373744).

Example usage for crate2nix:

{ pkgs, ... }:
let
  rustPkgs = pkgs.callPackage ./Cargo.nix {
    buildRustCrateForPkgs =
      pkgs:
      pkgs.buildRustCrate.override {
        defaultCodegenUnits = 16;
      };
  };
in
# ...

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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[flokli]

As of now, buildRustCrate defaults to using a single codegen unit per crate, which is far off from what cargo is doing:

The default value, if not specified, is 16 for non-incremental builds. For incremental builds the default is 256 which allows caching to be more granular.

Before debating whether the value itself should be increased, we should at least make it easy to override this for all crates at once, by introducing a parameter defaultCodegenUnits. In its current state, the codegenUnits parameter must be overriden per crate to increase codegen parallelization.

There's already been a bit of a debate on what this should be set to, see #130309 (and follow the breadcrumbs from there).

Also see https://discourse.nixos.org/t/crate2nix-setting-codegenunits-for-all-crates/53014/9 for some recent benchmarking.

[niklaskorz]

@flokli thank you for the feedback, but as I said, I'm not here to debate the default, but just want to make it easier to change the default for all crates at once

[drakon64]

Also see https://discourse.nixos.org/t/crate2nix-setting-codegenunits-for-all-crates/53014/9 for some recent benchmarking.

I'm not entirely sure this is a great benchmark since it's just one crate which was hastily tested, and included still-experimental compiler flags

[flokli]

@flokli thank you for the feedback, but as I said, I'm not here to debate the default, but just want to make it easier to change the default for all crates at once

Not objecting the PR, just providing additional context, especially on why it might be set to 1 currently.

[niklaskorz]

Also, fwiw, in our own monorepo, increasing codegen units to 16 decreased our cold build time from 9 to 7 minutes.