fix(nix): build kcov in the dev shell on macOS

`zig build build-ci` builds the custom kcov coverage tool from source on every
platform, but the dev shell only provided kcov's link dependencies on Linux, so
on macOS the build failed with "unable to find dynamic system library 'z'/'curl'".

Provide curl/zlib/pkg-config on all platforms, and on macOS add libdwarf (kcov
links it as "dwarf"). kcov resolves system libraries through pkg-config but asks
for the module names "z"/"dwarf", which nixpkgs ships as "zlib"/"libdwarf"; add
alias .pc files so those resolve (the nix pkg-config wrapper only reads .pc dirs
from buildInputs, not an exported PKG_CONFIG_PATH).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jaredramirez

src/flake.nix

@@ -37,12 +37,32 @@
         pkgs = import nixpkgs { inherit system; };
         isLinux = pkgs.stdenv.isLinux;
 
-        # kcov build dependencies for Linux (coverage uses custom kcov fork built from source)
+        # kcov (built from source for coverage on every platform) links system
+        # libs via pkg-config, but asks for "z"/"curl"/"dwarf" while nixpkgs
+        # ships "zlib"/"libcurl"/"libdwarf". Provide alias .pc files so those
+        # resolve; they must be packages since the nix pkg-config wrapper only
+        # reads .pc dirs from buildInputs. On macOS the bare "-l" fallback only
+        # searches the SDK (which has neither libz nor libcurl), so every lib
+        # must resolve through pkg-config here.
+        pcConfigAlias = name: pkg: pcName: pkgs.runCommand "pkgconfig-alias-${name}" { } ''
+          mkdir -p "$out/lib/pkgconfig"
+          cp "$(find ${pkg} -name '${pcName}.pc' | head -n1)" "$out/lib/pkgconfig/${name}.pc"
+        '';
         kcovBuildDeps = with pkgs; [
-          elfutils
           pkg-config
           curl
           zlib
+          (pcConfigAlias "z" zlib.dev "zlib")
+          (pcConfigAlias "curl" curl.dev "libcurl")
+        ];
+        # elfutils (libdw/libelf for DWARF parsing) is used by kcov on Linux.
+        kcovLinuxOnlyDeps = with pkgs; [
+          elfutils
+        ];
+        # macOS kcov uses libdwarf (linked as "dwarf") instead of elfutils.
+        kcovDarwinOnlyDeps = with pkgs; [
+          libdwarf
+          (pcConfigAlias "dwarf" libdwarf.dev "libdwarf")
         ];
         zig = pkgs.zig_0_16;
         dependencies = [
@@ -53,7 +73,9 @@
           pkgs.jq # see ci/benchmarks_zig.sh
           (pkgs.writeShellScriptBin "zon2nix" "nix run github:Cloudef/zig2nix -- zon2nix")
         ]
-        ++ pkgs.lib.optionals isLinux kcovBuildDeps;
+        ++ kcovBuildDeps
+        ++ pkgs.lib.optionals isLinux kcovLinuxOnlyDeps
+        ++ pkgs.lib.optionals (!isLinux) kcovDarwinOnlyDeps;
 
         shellFunctions = ''
           buildcmd() {