buildRustCrate: don't try to set CARGO_FEATURE_ variables for dep: features

These features are internal-only, have special characters that bash
doesn't support in variable names, and aren't normally given
environment variables by cargo as far as I can tell.

(cherry picked from commit ede639a8d6)
(cherry picked from commit a94a643de9)

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -257,9 +257,14 @@ stdenv.mkDerivation (rec {
 
     # Create a list of features that are enabled by the crate itself and
     # through the features argument of buildRustCrate. Exclude features
-    # with a forward slash, since they are passed through to dependencies.
+    # with a forward slash, since they are passed through to dependencies,
+    # and dep: features, since they're internal-only and do nothing except
+    # enable optional dependencies.
     crateFeatures = lib.optionals (crate ? features)
-      (builtins.filter (f: !lib.hasInfix "/" f) (crate.features ++ features));
+      (builtins.filter
+        (f: !(lib.hasInfix "/" f || lib.hasPrefix "dep:" f))
+        (crate.features ++ features)
+      );
 
     libName = if crate ? libName then crate.libName else crate.crateName;
     libPath = if crate ? libPath then crate.libPath else "";