In reply to @niklaskorz:matrix.org
hm I see, I just wonder where the build time increase could be coming from, the release profiles seem to be unchanged in the past months, so that’s at least not the origin

Idea smoke test: Problem: You probably don't understand what on earth the difference between cargoShortTarget, rustcTarget, and rustcTargetSpec is

nix-repl> stdenv.hostPlatform.rust
{
  cargoEnvVarTarget = "AARCH64_UNKNOWN_LINUX_GNU";
  cargoShortTarget = "aarch64-unknown-linux-gnu";
  isNoStdTarget = false;
  platform = { ... };
  rustcTarget = "aarch64-unknown-linux-gnu";
  rustcTargetSpec = "aarch64-unknown-linux-gnu";
}

These are only different if you specify rust.platform, in which case it behaves very counterintuitively:

nix-repl> (lib.systems.elaborate { config = "x86_64-unknown-linux-gnu"; rust.platform = { fake = "json"; }; }).rust
{
  cargoEnvVarTarget = "1P406CKXYZARK9J8L3LSZSJCD1VSDGNL_X86_64_UNKNOWN_LINUX_GNU";
  cargoShortTarget = "1p406ckxyzark9j8l3lszsjcd1vsdgnl-x86_64-unknown-linux-gnu";
  isNoStdTarget = false;
  platform = { ... };
  rustcTarget = "x86_64-unknown-linux-gnu";
  rustcTargetSpec = "/nix/store/1p406ckxyzark9j8l3lszsjcd1vsdgnl-x86_64-unknown-linux-gnu.json";
}

As it stands today ~nobody gets it right. Nixpkgs docs lies about the file name of rustcTargetSpec if it's a JSON. cargoShortTarget, rustcTarget, and rustcTargetSpec are used interchangably everywhere in Nixpkgs.

Idea: cargoShortTarget == rustcTarget. Snag: If there's a JSON spec, cargoShortTarget must be the base name sans extension of the JSON file, and AFAICT can't make a directory with only `lib

Proposal:

Phase 1:

• Create rustPlatform.targetSpec and rustcPlatform.targetSpecFor (latter being function taking a platform)
• rustcTarget = cargoShortTarget
• If rust.platform is set, make rustcTargetSpec error out, else rustcTargetSpec = cargoShortTarget
• Change docs to ask user to set rust.{cargoShortTarget,platform} (instead of rust.{rustcTarget,platform})
• Fix every user in Nixpkgs

Phase 2:

• Deprecate and remove rustcTarget and rustcTargetSpec

See: https://github.com/NixOS/nixpkgs/pull/420797#pullrequestreview-2969405491

In reply to @niklaskorz:matrix.org
right, thanks! I’ll start with the Rust 1.86 -> 1.87 commit then