flake: impl builder interface lib.mkRustBin

This allows construction of `rust-bin` attrset on an existing `pkgs`,
instead of staging another overlayed nixpkgs. This can reduce redundant
evaluation of nixpkgs for flake-heavy users who also don't like or
cannot (due to flake dependency hell) manually instantiate nixpkgs.

default.nix

@@ -24,7 +24,7 @@ let
 in {
  rust-bin = (prev.rust-bin or { }) // {
  # The overridable dist url for fetching.
- distRoot = "https://static.rust-lang.org/dist";
+ distRoot = import ./lib/dist-root.nix;
  } // import ./lib/rust-bin.nix {
  inherit lib manifests;
  inherit (final.rust) toRustTarget;

flake.nix

@@ -10,11 +10,10 @@
  };
 
  outputs = { self, nixpkgs, flake-utils }: let
- inherit (nixpkgs.lib)
+ inherit (nixpkgs) lib;
+ inherit (lib)
  elem filterAttrs head mapAttrs' optionalAttrs replaceStrings;
 
- overlay = import ./.;
-
  allSystems = [
  "aarch64-darwin"
  "aarch64-linux"
@@ -30,13 +29,43 @@
  "x86_64-linux"
  ];
 
+ overlay = import ./.;
+
+ defaultDistRoot = import ./lib/dist-root.nix;
+ mkManifests = distRoot: import ./lib/manifests.nix { inherit lib distRoot; };
+
+ # Builder to construct `rust-bin` interface on an existing `pkgs`.
+ # This would be immutable, non-intrusive and (hopefully) can benefit from
+ # flake eval-cache.
+ #
+ # Note that this does not contain compatible attrs for mozilla-overlay.
+ mkRustBin =
+ { distRoot ? defaultDistRoot }:
+ pkgs:
+ lib.fix (rust-bin: import ./lib/rust-bin.nix {
+ inherit lib pkgs;
+ inherit (pkgs.rust) toRustTarget;
+ inherit (rust-bin) nightly;
+ manifests = mkManifests distRoot;
+ });
+
  in {
+ lib = {
+ # Internal use only!
+ _internal = {
+ defaultManifests = mkManifests defaultDistRoot;
+ };
+
+ inherit mkRustBin;
+ };
+
  overlays = {
  default = overlay;
  rust-overlay = overlay;
  };
  } // flake-utils.lib.eachSystem allSystems (system: let
- pkgs = import nixpkgs { inherit system; overlays = [ overlay ]; };
+ pkgs = nixpkgs.legacyPackages.${system};
+ rust-bin = mkRustBin {} pkgs;
  in {
  # TODO: Flake outputs except `overlay[s]` are not stabilized yet.
 
@@ -54,25 +83,27 @@
  then "rust"
  else "rust_${replaceStrings ["."] ["_"] version}";
  value = select version comps;
- }) pkgs.rust-bin.stable //
+ }) rust-bin.stable //
  mapAttrs' (version: comps: {
  name = if version == "latest"
  then "rust-nightly"
  else "rust-nightly_${version}";
  value = select version comps;
- }) pkgs.rust-bin.nightly //
+ }) rust-bin.nightly //
  mapAttrs' (version: comps: {
  name = if version == "latest"
  then "rust-beta"
  else "rust-beta_${version}";
  value = select version comps;
- }) pkgs.rust-bin.beta;
+ }) rust-bin.beta;
  result' = filterAttrs (name: drv: drv != null) result;
  in result' // { default = result'.rust; };
 
  checks = let
- inherit (pkgs) rust-bin rustChannelOf;
- inherit (pkgs.rust-bin) fromRustupToolchain fromRustupToolchainFile stable beta nightly;
+ inherit (rust-bin) fromRustupToolchain fromRustupToolchainFile stable beta nightly;
+
+ pkgs-compat = import nixpkgs { inherit system; overlays = [ overlay ]; };
+ inherit (pkgs-compat) latest rustChannelOf;
 
  rustHostPlatform = pkgs.rust.toRustTarget pkgs.hostPlatform;
 
@@ -100,9 +131,9 @@
  rename-unavailable = assertEq (stable."1.30.0" ? rustfmt) false;
  rename-available = assertEq stable."1.48.0".rustfmt stable."1.48.0".rustfmt-preview;
 
- latest-stable-legacy = assertEq pkgs.latest.rustChannels.stable.rustc stable.latest.rustc;
- latest-beta-legacy = assertEq pkgs.latest.rustChannels.beta.rustc beta.latest.rustc;
- latest-nightly-legacy = assertEq pkgs.latest.rustChannels.nightly.rustc nightly.latest.rustc;
+ latest-stable-legacy = assertEq latest.rustChannels.stable.rustc stable.latest.rustc;
+ latest-beta-legacy = assertEq latest.rustChannels.beta.rustc beta.latest.rustc;
+ latest-nightly-legacy = assertEq latest.rustChannels.nightly.rustc nightly.latest.rustc;
 
  rust-channel-of-stable = assertEq (rustChannelOf { channel = "stable"; }).rustc stable.latest.rustc;
  rust-channel-of-beta = assertEq (rustChannelOf { channel = "beta"; }).rustc beta.latest.rustc;

lib/dist-root.nix

@@ -0,0 +1 @@
+"https://static.rust-lang.org/dist"