feat(nix): allow override of RUSTFLAGS for certain features

- enabling the `http3` features requires unstable features, namely `reqwest_unstable`
- the main suggestion of cargo is to enable this through RUSTFLAGS
- we had no way to customize RUSTFLAGS, now we do
- changed the max-perf package to showcase this feature
- also turn on http3 by default in both max-perf and the default build
  (jade approved this)

nix/packages/continuwuity.nix

@@ -8,6 +8,7 @@
   callPackage,
   rustPlatform,
   cargoExtraArgs ? "",
+  rustflags ? "",
   rocksdb ? callPackage ./rocksdb.nix { },
   profile ? "release",
 }:
@@ -37,6 +38,7 @@ let
       ROCKSDB_INCLUDE_DIR = "${rocksdb}/include";
       ROCKSDB_LIB_DIR = "${rocksdb}/lib";
       CARGO_PROFILE = profile;
+      RUSTFLAGS = rustflags;
     };
   };
 in

nix/packages/default.nix

@@ -13,7 +13,14 @@
     {
       packages = {
         rocksdb = pkgs.callPackage ./rocksdb.nix { };
-        default = pkgs.callPackage ./continuwuity.nix { inherit self craneLib; };
+        default = pkgs.callPackage ./continuwuity.nix {
+          inherit self craneLib;
+          # extra features via `cargoExtraArgs`
+          cargoExtraArgs = "-F http3";
+          # extra RUSTFLAGS via `rustflags`
+          # the stuff below is required for http3
+          rustflags = "--cfg reqwest_unstable";
+        };
         # users may also override this with other cargo profiles to build for other feature sets
         #
         # other examples include: