build-rust-crate: parallelize build_bin via Makefile jobserver

Collect binary targets into a bash associative array (name→path), then
generate a Makefile on the fly and pipe it to `make -j`. Make handles
parallel execution and the jobserver protocol natively, so rustc
invocations share the same token pool via MAKEFLAGS.

Also fix build_bin error propagation: add `|| return 1` to the rustc
invocation so failures are not silently swallowed when the crate name
has no hyphens and the mv rename is skipped.

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

@@ -162,6 +162,8 @@ in
     fi
   ''}
 
+  declare -A BINS
+
   ${lib.optionalString (lib.length crateBin > 0) (
     lib.concatMapStringsSep "\n" (
       bin:
@@ -188,7 +190,7 @@ in
                 BIN_PATH='${bin.path}'
               ''
           }
-            ${build_bin} "$BIN_NAME" "$BIN_PATH"
+            BINS["$BIN_NAME"]="$BIN_PATH"
         ''
       else
         ''
@@ -222,13 +224,30 @@ in
   ${lib.optionalString (lib.length crateBin == 0 && !hasCrateBin) ''
     if [[ -e src/main.rs ]]; then
       mkdir -p target/bin
-      ${build_bin} ${crateName} src/main.rs
+      BINS["${crateName}"]="src/main.rs"
     fi
     for i in src/bin/*.rs; do #*/
       mkdir -p target/bin
-      ${build_bin} "$(basename $i .rs)" "$i"
+      BINS["$(basename $i .rs)"]="$i"
     done
   ''}
+
+  if [[ ''${#BINS[@]} -gt 0 ]]; then
+    export BIN_RUSTC_OPTS LINK EXTRA_LINK_ARGS EXTRA_LINK_ARGS_BINS EXTRA_LIB \
+           BUILD_OUT_DIR EXTRA_BUILD EXTRA_FEATURES EXTRA_RUSTC_FLAGS CAP_LINTS
+    export -f build_bin build_bin_test echo_build_heading noisily echo_colored echo_error
+    # Generate a Makefile and pipe it to make, which handles parallel execution
+    # and the jobserver protocol natively so rustc invocations share a token pool.
+    {
+      printf 'SHELL = %s\nall:' "$BASH"
+      for _n in "''${!BINS[@]}"; do printf ' %s' "$_n"; done
+      printf '\n'
+      for _n in "''${!BINS[@]}"; do
+        printf '%s:\n\t${build_bin} "%s" "%s"\n' "$_n" "$_n" "''${BINS[$_n]}"
+      done
+    } | make --no-print-directory -j"''${NIX_BUILD_CORES:-1}" -f -
+  fi
+
   # Remove object files to avoid "wrong ELF type"
   find target -type f -name "*.o" -print0 | xargs -0 rm -f
   runHook postBuild

pkgs/build-support/rust/build-rust-crate/lib.sh

@@ -58,7 +58,7 @@ build_bin() {
     $EXTRA_BUILD \
     $EXTRA_FEATURES \
     $EXTRA_RUSTC_FLAGS \
-    --color ${colors} \
+    --color ${colors} || return 1
 
   if [ "$crate_name_" != "$crate_name" ]; then
     if [ -f "$out_dir/$crate_name_.wasm" ]; then