build(nix): allow overriding TARGET_CPU

nix/packages/continuwuity.nix

@@ -9,6 +9,7 @@
   rustPlatform,
   cargoExtraArgs ? "",
   rustflags ? "",
+  target_cpu ? null,
   rocksdb ? callPackage ./rocksdb.nix { },
   profile ? "release",
 }:
@@ -39,7 +40,10 @@ let
       ROCKSDB_LIB_DIR = "${rocksdb}/lib";
       CARGO_PROFILE = profile;
       RUSTFLAGS = rustflags;
-    };
+    }
+    // (lib.optionalAttrs (target_cpu != null) {
+      TARGET_CPU = target_cpu;
+    });
   };
 in
 craneLib.buildPackage (

nix/packages/default.nix

@@ -22,11 +22,13 @@
           rustflags = "--cfg reqwest_unstable";
         };
         # users may also override this with other cargo profiles to build for other feature sets
-        #
-        # other examples include:
-        #
-        # - release-high-perf
-        max-perf = self'.packages.default.override {
+        # for features configuration see `default` package which enables http3 by default
+
+        # example: different compilation profile and different target_cpu
+        max-perf-haswell = self'.packages.default.override {
+          # compiles explicitly for haswell arch cpus
+          target_cpu = "haswell";
+          # compiles slower but with more thorough optimizations
           profile = "release-max-perf";
         };
       };