rustc: fix build for no_std targets

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

@@ -38,4 +38,9 @@ rec {
     if platform ? rustc.platform
     then builtins.toFile (toRustTarget platform + ".json") (builtins.toJSON platform.rustc.platform)
     else toRustTarget platform;
+
+  # Returns true if the target is no_std
+  # https://github.com/rust-lang/rust/blob/2e44c17c12cec45b6a682b1e53a04ac5b5fcc9d2/src/bootstrap/config.rs#L415-L421
+  IsNoStdTarget = platform: let rustTarget = toRustTarget platform; in
+    builtins.any (t: lib.hasInfix t rustTarget) ["-none" "nvptx" "switch" "-uefi"];
 }

pkgs/development/compilers/rust/default.nix

@@ -28,7 +28,7 @@ in
   lib = lib';
 
   # Backwards compat before `lib` was factored out.
-  inherit (lib') toTargetArch toTargetOs toRustTarget toRustTargetSpec;
+  inherit (lib') toTargetArch toTargetOs toRustTarget toRustTargetSpec IsNoStdTarget;
 
   # This just contains tools for now. But it would conceivably contain
   # libraries too, say if we picked some default/recommended versions from

pkgs/development/compilers/rust/rustc.nix

@@ -107,6 +107,8 @@ in stdenv.mkDerivation rec {
     "${setHost}.musl-root=${pkgsBuildHost.targetPackages.stdenv.cc.libc}"
   ] ++ optionals stdenv.targetPlatform.isMusl [
     "${setTarget}.musl-root=${pkgsBuildTarget.targetPackages.stdenv.cc.libc}"
+  ] ++ optionals (rust.IsNoStdTarget stdenv.targetPlatform) [
+    "--disable-docs"
   ];
 
   # The bootstrap.py will generated a Makefile that then executes the build.