rls: fix darwin build

rls links to internal rustc crates, which introduces an atypical
dependency on rustc. A superficial installCheck ensures that the
binary is linked properly and able to run on its own.

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

@@ -19,6 +19,9 @@ rustPlatform.buildRustPackage rec {
   # a nightly compiler is required unless we use this cheat code.
   RUSTC_BOOTSTRAP=1;
 
+  # rls-rustc links to rustc_private crates
+  CARGO_BUILD_RUSTFLAGS = if stdenv.isDarwin then "-C rpath" else null;
+
   nativeBuildInputs = [ pkgconfig cmake ];
   buildInputs = [ openssh openssl curl zlib libiconv ]
     ++ (stdenv.lib.optionals stdenv.isDarwin [ CoreFoundation Security ]);
@@ -29,6 +32,11 @@ rustPlatform.buildRustPackage rec {
     rm tests/client.rs
   '';
 
+  doInstallCheck = true;
+  installCheckPhase = ''
+    $out/bin/rls --version
+  '';
+
   meta = with stdenv.lib; {
     description = "Rust Language Server - provides information about Rust programs to IDEs and other tools";
     homepage = https://github.com/rust-lang/rls/;