buildRustPackage: Add check phase and enable it by default

Also disable check phase in cargo as there are lots of failures (some
probably due to trying to access the network).

pkgs/development/tools/build-managers/cargo/default.nix

@@ -25,6 +25,10 @@ buildRustPackage rec {
 
   buildPhase = "make";
 
+  # Disable check phase as there are lots of failures (some probably due to
+  # trying to access the network).
+  doCheck = false;
+
   installPhase = ''
     make install
     ${postInstall}

pkgs/development/tools/rust/racer/default.nix

@@ -15,6 +15,10 @@ buildRustPackage rec {
 
   buildInputs = [ makeWrapper ];
 
+  preCheck = ''
+    export RUST_SRC_PATH="${rustc.src}/src"
+  '';
+
   installPhase = ''
     mkdir -p $out/bin
     cp -p target/release/racer $out/bin/