only link to one jemalloc build

Without setting JEMALLOC_OVERRIDE, we end up linking to two different
jemalloc builds. Once dynamically, as a transitive dependency through
rocksdb, and a second time to the static jemalloc that tikv-jemalloc-sys
builds.

This fixes dynamically-linked jemalloc builds, for the reasons described
in <https://github.com/girlbossceo/conduwuit/pull/400#issue-2316700200>.

nix/pkgs/default/default.nix

@@ -6,6 +6,7 @@
 , pkgsBuildHost
 , rocksdb
 , rust
+, rust-jemalloc-sys
 , stdenv
 
 # Options (keep sorted)
@@ -15,10 +16,23 @@
 }:
 
 let
+  featureEnabled = feature : builtins.elem feature features;
+
+  # This derivation will set the JEMALLOC_OVERRIDE variable, causing the
+  # tikv-jemalloc-sys crate to use the nixpkgs jemalloc instead of building it's
+  # own. In order for this to work, we need to set flags on the build that match
+  # whatever flags tikv-jemalloc-sys was going to use. These are dependent on
+  # which features we enable in tikv-jemalloc-sys.
+  rust-jemalloc-sys' = (rust-jemalloc-sys.override {
+    # tikv-jemalloc-sys/unprefixed_malloc_on_supported_platforms feature
+    unprefixed = true;
+  });
+
   buildDepsOnlyEnv =
     let
       rocksdb' = rocksdb.override {
-        enableJemalloc = builtins.elem "jemalloc" features;
+        jemalloc = rust-jemalloc-sys';
+        enableJemalloc = featureEnabled "jemalloc";
       };
     in
     {
@@ -60,6 +74,8 @@ let
       ];
     };
 
+    buildInputs = lib.optional (featureEnabled "jemalloc") rust-jemalloc-sys';
+
     nativeBuildInputs = [
       # bindgen needs the build platform's libclang. Apparently due to "splicing
       # weirdness", pkgs.rustPlatform.bindgenHook on its own doesn't quite do the

nix/shell.nix

@@ -30,5 +30,9 @@ mkShell {
     toolchain
   ]
   ++
-  default.nativeBuildInputs;
+  default.nativeBuildInputs
+  ++
+  default.propagatedBuildInputs
+  ++
+  default.buildInputs;
 }