rust: replaces fenix with rust-overlay now allows specifying version

src/modules/languages/rust.nix

@@ -3,16 +3,15 @@
 let
   cfg = config.languages.rust;
 
-  fenix = config.lib.getInput {
-    name = "fenix";
-    url = "github:nix-community/fenix";
-    attribute = "languages.rust.version";
+  rust-overlay = config.lib.getInput {
+    name = "rust-overlay";
+    url = "github:oxalica/rust-overlay";
+    attribute = "languages.rust.input";
     follows = [ "nixpkgs" ];
   };
 in
 {
   imports = [
-    (lib.mkRenamedOptionModule [ "languages" "rust" "version" ] [ "languages" "rust" "channel" ])
     (lib.mkRenamedOptionModule [ "languages" "rust" "packages" ] [ "languages" "rust" "toolchain" ])
   ];
 
@@ -34,7 +33,7 @@ in
       default = [ ];
       defaultText = lib.literalExpression ''[ ]'';
       description = ''
-        List of extra [targets](https://github.com/nix-community/fenix#supported-platforms-and-targets)
+        List of extra [targets](https://doc.rust-lang.org/nightly/rustc/platform-support.html)
         to install. Defaults to only the native target. 
       '';
     };
@@ -46,6 +45,16 @@ in
       description = "The rustup toolchain to install.";
     };
 
+    version = lib.mkOption {
+      type = lib.types.str;
+      default = "latest";
+      defaultText = lib.literalExpression ''"latest"'';
+      description = ''
+        Which version of rust to use, this value could be `latest`,`1.81.0`, `2021-01-01`.
+        Only works when languages.rust.channel is NOT nixpkgs.
+      '';
+    };
+
     rustflags = lib.mkOption {
       type = lib.types.str;
       default = "";
@@ -101,6 +110,17 @@ in
               The nixpkgs channel does not support cross-compiling with targets.
               Use the stable, beta, or nightly channels instead. For example:
 
+              languages.rust.channel = "stable";
+            '';
+          }
+          {
+            assertion = cfg.channel == "nixpkgs" -> (cfg.version == "latest");
+            message = ''
+              Cannot use `languages.rust.channel = "nixpkgs"` with `languages.rust.version`.
+
+              The nixpkgs channel does not contain all versions required, and is
+              therefore not supported to be used together.
+
               languages.rust.channel = "stable";
             '';
           }
@@ -153,23 +173,15 @@ in
 
     (lib.mkIf (cfg.channel != "nixpkgs") (
       let
-        rustPackages = fenix.packages.${pkgs.stdenv.system};
-        fenixChannel =
-          if cfg.channel == "nightly"
-          then "latest"
-          else cfg.channel;
-        toolchain = rustPackages.${fenixChannel};
+        toolchain = (rust-overlay.lib.mkRustBin {} pkgs)."${cfg.channel}"."${cfg.version}";
+        # filter toolchain, otherwise it contains a bunch of non-rust stuff (llvm-tools, etc.)
+        filteredToolchain = (lib.filterAttrs (n: _: builtins.elem n toolchain._manifest.profiles.complete) toolchain);
       in
       {
         languages.rust.toolchain =
-          (builtins.mapAttrs (_: pkgs.lib.mkDefault) toolchain);
+          (builtins.mapAttrs (_: pkgs.lib.mkDefault) filteredToolchain);
 
-        packages = [
-          (rustPackages.combine (
-            (map (c: toolchain.${c}) cfg.components) ++
-            (map (t: rustPackages.targets.${t}.${fenixChannel}.rust-std) cfg.targets)
-          ))
-        ];
+        packages = builtins.attrValues (lib.filterAttrs (n: _: builtins.elem n cfg.components) cfg.toolchain);
       }
     ))
   ]);