buildRustCrate: make codegen-units configurable

This parameter is being set to `$NIX_BUILD_CORES` by default. This is a
standard practice but there's a suspicion that this can produce broken
builds. For some details see
https://github.com/cargo2nix/cargo2nix/issues/184 . As a
work-around/test, it'd be good if codegen-units can be set to something
constant, such as `1`. This PR allows it.

Note that the default of `$NIX_BUILD_CORES` is preserved so this MR
causes no change in default behaviour and no rebuilds.

pkgs/build-support/rust/build-rust-crate/build-crate.nix

@@ -4,14 +4,15 @@
   crateFeatures, crateRenames, libName, release, libPath,
   crateType, metadata, crateBin, hasCrateBin,
   extraRustcOpts, verbose, colors,
-  buildTests
+  buildTests,
+  codegenUnits
 }:
 
   let
     baseRustcOpts =
       [
         (if release then "-C opt-level=3" else "-C debuginfo=2")
-        "-C codegen-units=$NIX_BUILD_CORES"
+        "-C codegen-units=${codegenUnits}"
         "--remap-path-prefix=$NIX_BUILD_TOP=/"
         (mkRustcDepArgs dependencies crateRenames)
         (mkRustcFeatureArgs crateFeatures)

pkgs/build-support/rust/build-rust-crate/configure-crate.nix

@@ -2,6 +2,7 @@
 {
   build
 , buildDependencies
+, codegenUnits
 , colors
 , completeBuildDeps
 , completeDeps
@@ -24,7 +25,7 @@ let version_ = lib.splitString "-" crateVersion;
     version = lib.splitVersion (lib.head version_);
     rustcOpts = lib.foldl' (opts: opt: opts + " " + opt)
         (if release then "-C opt-level=3" else "-C debuginfo=2")
-        (["-C codegen-units=$NIX_BUILD_CORES"] ++ extraRustcOptsForBuildRs);
+        (["-C codegen-units=${codegenUnits}"] ++ extraRustcOptsForBuildRs);
     buildDeps = mkRustcDepArgs buildDependencies crateRenames;
     authors = lib.concatStringsSep ":" crateAuthors;
     optLevel = if release then 3 else 0;

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -228,6 +228,7 @@ crate_: lib.makeOverridable
         "colors"
         "edition"
         "buildTests"
+        "codegenUnits"
       ];
       extraDerivationAttrs = builtins.removeAttrs crate processedAttrs;
       nativeBuildInputs_ = nativeBuildInputs;
@@ -315,6 +316,7 @@ crate_: lib.makeOverridable
       colors = lib.attrByPath [ "colors" ] "always" crate;
       extraLinkFlags = lib.concatStringsSep " " (crate.extraLinkFlags or [ ]);
       edition = crate.edition or null;
+      codegenUnits = if crate ? codegenUnits then crate.codegenUnits else "$NIX_BUILD_CORES";
       extraRustcOpts =
         lib.optionals (crate ? extraRustcOpts) crate.extraRustcOpts
           ++ extraRustcOpts_
@@ -329,13 +331,13 @@ crate_: lib.makeOverridable
         inherit crateName buildDependencies completeDeps completeBuildDeps crateDescription
           crateFeatures crateRenames libName build workspace_member release libPath crateVersion
           extraLinkFlags extraRustcOptsForBuildRs
-          crateAuthors crateHomepage verbose colors;
+          crateAuthors crateHomepage verbose colors codegenUnits;
       };
       buildPhase = buildCrate {
         inherit crateName dependencies
           crateFeatures crateRenames libName release libPath crateType
           metadata hasCrateBin crateBin verbose colors
-          extraRustcOpts buildTests;
+          extraRustcOpts buildTests codegenUnits;
       };
       installPhase = installCrate crateName metadata buildTests;