rust: fix building in debug mode

[arcnmx]

Motivation for this change

#56540 broke a few things regarding rust builds and did not have the intended effect:

• bdd3c3f intended to support debug builds, but passes an invalid --debug flag to cargo build
• 60761e6 intended to make sense of cargo target directories, but breaks cargo.
  1. it deletes $CARGO_TARGET_DIR/debug, and symlinking/moving $CARGO_TARGET_DIR/x86_64-unknown-linux-gnu/debug to it, assuming they're redundant?
  2. this is an incorrect change, as cargo seems to produce and use both directories for different purposes? Each directory has its own lock file, which cargo attempts to use. When both lock files are symlinked to be the same thing, cargo will deadlock itself.
  3. this prevents cargo test from working, the only reason it happens to work now is because this step mangles the release directory, while the default checkPhase builds tests in debug mode. Changing buildType to debug or using cargo test --release instead will expose the deadlock.
  4. buildRustPackage provides releaseDir for this, anything that hard-codes target/release are messy hm...
     • messageformat=json stuff could maybe be used to find binary paths for packaging purposes instead? This is a a rather annoying and long-standing limitation of cargo.

cc @illegalprime @Ericson2314

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
  • will try this soon, it might take a while..?
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[Mic92]

Could you also mention $releaseDir in doc/languages-frameworks/rust.section.md?
Maybe we should also mention this in the NixOS changelog for out-of-tree rust packages because people need to upgrade.

[arcnmx]

Could you also mention $releaseDir in doc/languages-frameworks/rust.section.md?

Seems appropriate yeah.

Maybe we should also mention this in the NixOS changelog for out-of-tree rust packages because people need to upgrade.

Hmm... If you want backward-compatibility we could set CARGO_TARGET_DIR so cargo doesn't build in $sourceRoot/target by default and then just ln -s $releaseDir target/$buildType for derivations that rely on that path?

[FRidh]

Status?

[arcnmx]

Oh, was looking for feedback on whether CARGO_TARGET_DIR was a better idea.

To itemize the concerns I'd like feedback on more specifically before proceeding:

• CARGO_TARGET_DIR is currently unset (and thus roughly defaults to $sourceRoot/target). Is setting it acceptable? This seems less likely to break existing derivations than moving $releaseDir.
• Re: Documenting/using $releaseDir as the way to find build outputs rather than hardcoding target/release - any sense in bikeshedding this into a less vague name while we're at it? There's nothing cargo/rusty about it and the build may not even be release mode!
• symlink $releaseDir to $sourceRoot/target/release to preserve compatibility with existing derivations seems like a good idea y/n?
• Should CARGO_TARGET_DIR and $releaseDir be relative to cwd (presumed to be $sourceRoot) or an absolute path? I'm not too familiar with stdenv conventions regarding this.

[Mic92]

CARGO_TARGET_DIR sounds better then relying on releaseDir which is nixpkgs specific.

[arcnmx]

CARGO_TARGET_DIR sounds better then relying on releaseDir which is nixpkgs specific

So the thing is we'll presumably need something nixpkgs specific anyway, since the point of $releaseDir is to point to where to get outputs; $CARGO_TARGET_DIR on the other hand only points to the root under which there's still $CARGO_TARGET_DIR/${hostPlatform.config}/$buildType (also hostPlatform.config is subtly incorrect on many platforms) so... some variations:

1. Set up CARGO_TARGET_DIR to be absolute and provide releaseDir as relative to it: $CARGO_TARGET_DIR/$releaseDir.
2. Set up CARGO_TARGET_DIR to be absolute but provide releaseDir as relative to pwd/$sourceRoot, so unless you've chdir'd you can just refer to $releaseDir
3. Set up both as absolute paths.

(I'm not going to suggest relative CARGO_TARGET_DIR because cargo is inconsistent and setting it to anything disables its usual heuristics regarding where to put the path; "target" is not the same as it being unset, and things just get messy if derivations chdir...)

I'm still pretty indecisive about the details but just pushed the following:

• CARGO_TARGET_DIR is set via .cargo/config because it seems like a more noncommittal way to go right now, though I don't really have any problem setting it to an absolute path and exposing it to derivations.
• releaseDir is relative to pwd (expected to roughly be sourceRoot)
• target/release is symlinked to $releaseDir, though all uses of it should be removed from nixpkgs so this is just for backwards compatibility
• set up the cargo and cc-rs env vars for the whole derivation, if we're going to cause rust rebuilds may as well fix the cross compiling environment up a bit.

[stale]

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[stale]

I marked this as stale due to inactivity. → More info

[arcnmx]

Enough has changed in the rust build helpers that this is mostly irrelevant now, closing.