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

@@ -8,7 +8,9 @@
   callPackage,
   rustPlatform,
   cargoExtraArgs ? "",
+  rustflags ? "",
   rocksdb ? callPackage ./rocksdb.nix { },
+  profile ? "release",
 }:
 let
   # see https://crane.dev/API.html#cranelibfiltercargosources
@@ -35,6 +37,8 @@ let
     env = {
       ROCKSDB_INCLUDE_DIR = "${rocksdb}/include";
       ROCKSDB_LIB_DIR = "${rocksdb}/lib";
+      CARGO_PROFILE = profile;
+      RUSTFLAGS = rustflags;
     };
   };
 in

nix/packages/default.nix

@@ -5,6 +5,7 @@
 {
   perSystem =
     {
+      self',
       pkgs,
       craneLib,
       ...
@@ -12,7 +13,22 @@
     {
       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:
+        #
+        # - release-high-perf
+        max-perf = self'.packages.default.override {
+          profile = "release-max-perf";
+        };
       };
     };
 }