add all-features devshell

Because the nix rocksdb build depends on the jemalloc feature, you need
to use a different devshell when passing --all-features to cargo than
the default.

flake.nix

@@ -23,6 +23,11 @@
 
         oci-image = self.callPackage ./nix/pkgs/oci-image {};
 
+        # Return a new scope with overrides applied to the 'default' package
+        overrideDefaultPackage = args: self.overrideScope (final: prev: {
+          default = prev.default.override args;
+        });
+
         shell = self.callPackage ./nix/shell.nix {};
 
         # The Rust toolchain to use
@@ -84,6 +89,9 @@
           );
 
         devShells.default = (mkScope pkgs).shell;
+        devShells.all-features = ((mkScope pkgs).overrideDefaultPackage {
+          all-features = true;
+        }).shell;
       }
     )
     //

nix/pkgs/default/default.nix

@@ -11,6 +11,7 @@
 
 # Options (keep sorted)
 , default-features ? true
+, all-features ? false
 , features ? []
 , profile ? "release"
 }:
@@ -18,10 +19,17 @@
 let
   # We perform default-feature unification in nix, because some of the dependencies
   # on the nix side depend on feature values.
-  allDefaultFeatures =
-    (lib.importTOML "${inputs.self}/Cargo.toml").features.default;
+  cargoManifest = lib.importTOML "${inputs.self}/Cargo.toml";
+  allDefaultFeatures = cargoManifest.features.default;
+  allFeatures = lib.unique (
+    lib.remove "default" (lib.attrNames cargoManifest.features) ++
+    lib.attrNames
+      (lib.filterAttrs (_: dependency: dependency.optional or false)
+        cargoManifest.dependencies));
   features' = lib.unique
-    (features ++ lib.optionals default-features allDefaultFeatures);
+    (features ++
+      lib.optionals default-features allDefaultFeatures ++
+      lib.optionals all-features allFeatures);
 
   featureEnabled = feature : builtins.elem feature features';