rustcSrc: remove unrelated files

This package is used to provide semantic completion in tools like racer
and is equivalent to rust-src in rustup. This commit gets rid of a
number of files not required for completion.

pkgs/development/compilers/rust/rust-src.nix

@@ -0,0 +1,11 @@
+{ stdenv, rustc }:
+
+stdenv.mkDerivation {
+  name = "rust-src";
+  src = rustc.src;
+  phases = [ "unpackPhase" "installPhase" ];
+  installPhase = ''
+    mv src $out
+    rm -rf $out/{ci,doc,driver,etc,grammar,llvm,rt,rtstartup,rustllvm,test,tools,vendor}
+  '';
+}

pkgs/top-level/all-packages.nix

@@ -6239,13 +6239,9 @@ with pkgs;
         inherit rust;
       };
 
-      rustcSrc = stdenv.mkDerivation {
+      rustcSrc = callPackage ../development/compilers/rust/rust-src.nix {
-        name = "rust-src";
+        inherit (rust) rustc;
-        src = rust.rustc.src;
-        phases = ["unpackPhase" "installPhase"];
-        installPhase = "mv src $out";
       };
-
     });
 
   cargo-edit = callPackage ../tools/package-management/cargo-edit { };