Motivation for this change

Update rustc stable to be 1.40.0.

Things done

firefox and thunderbird still lock on rust 1.38.0, which are not affected.

Not sure whether the version of llvm matches.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
  • rustc: 796,307,712 -> 419,829,488 (rlibs of dependent crates in lib/rustlib/{triple}/lib/ now no longer exists)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @edolstra