flake: use nightly rust-analyzer and add rust-src component (#735)

this also improves the application of overlays to be more uniform; what
was previously done was just Wrong

flake.nix

@@ -22,93 +22,9 @@
       rust-overlay,
     }:
     let
-      inherit (nixpkgs) lib;
+      niri-package =
-      # Support all Linux systems that the nixpkgs flake exposes
-      systems = lib.intersectLists lib.systems.flakeExposed lib.platforms.linux;
-
-      forAllSystems = lib.genAttrs systems;
-      nixpkgsFor = forAllSystems (system: nixpkgs.legacyPackages.${system});
-    in
-    {
-      checks = forAllSystems (system: {
-        # We use the debug build here to save a bit of time
-        inherit (self.packages.${system}) niri-debug;
-      });
-
-      devShells = forAllSystems (
-        system:
-        let
-          pkgs = nixpkgsFor.${system};
-          inherit (self.packages.${system}) niri;
-        in
-        {
-          default = pkgs.mkShell {
-            packages = [
-              # NOTE: Nixpkgs' Rust toolchain isn't used here as we prefer
-              # a nightly toolchain for development, and *require* a nightly
-              # `rustfmt`
-              rust-overlay.packages.${system}.rust-nightly
-
-              pkgs.rust-analyzer
-            ];
-
-            nativeBuildInputs = [
-              pkgs.clang
-              pkgs.pkg-config
-              pkgs.wrapGAppsHook4 # For `niri-visual-tests`
-            ];
-
-            buildInputs = niri.buildInputs ++ [
-              pkgs.libadwaita # For `niri-visual-tests`
-            ];
-
-            env = {
-              inherit (niri) LIBCLANG_PATH;
-
-              # WARN: Do not overwrite this variable in your shell!
-              # It is required for `dlopen()` to work on some libraries; see the comment
-              # in the package expression
-              #
-              # This should only be set with `CARGO_BUILD_RUSTFLAGS="$CARGO_BUILD_RUSTFLAGS -C your-flags"`
-              inherit (niri) CARGO_BUILD_RUSTFLAGS;
-            };
-          };
-        }
-      );
-
-      formatter = forAllSystems (system: nixpkgsFor.${system}.nixfmt-rfc-style);
-
-      packages = forAllSystems (
-        system:
-        let
-          inherit (self.overlays.default nixpkgsFor.${system} null) niri;
-        in
-        {
-          inherit niri;
-
-          # NOTE: This is for development purposes only
-          #
-          # It is primarily to help with quickly iterating on
-          # changes made to the above expression - though it is
-          # also not stripped in order to better debug niri itself
-          niri-debug = niri.overrideAttrs (
-            newAttrs: oldAttrs: {
-              pname = oldAttrs.pname + "-debug";
-
-              cargoBuildType = "debug";
-              cargoCheckType = newAttrs.cargoBuildType;
-
-              dontStrip = true;
-            }
-          );
-
-          default = niri;
-        }
-      );
-
-      overlays.default = final: _: {
-        niri = final.callPackage (
         {
+          lib,
           cairo,
           clang,
           dbus,
@@ -228,8 +144,108 @@
             mainProgram = "niri";
             platforms = lib.platforms.linux;
           };
+        };
+
+      inherit (nixpkgs) lib;
+      # Support all Linux systems that the nixpkgs flake exposes
+      systems = lib.intersectLists lib.systems.flakeExposed lib.platforms.linux;
+
+      forAllSystems = lib.genAttrs systems;
+      nixpkgsFor = forAllSystems (system: nixpkgs.legacyPackages.${system});
+    in
+    {
+      checks = forAllSystems (system: {
+        # We use the debug build here to save a bit of time
+        inherit (self.packages.${system}) niri-debug;
+      });
+
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgsFor.${system};
+          rust-bin = rust-overlay.lib.mkRustBin { } pkgs;
+          inherit (self.packages.${system}) niri;
+        in
+        {
+          default = pkgs.mkShell {
+            packages = [
+              # We don't use the toolchain from nixpkgs
+              # because we prefer a nightly toolchain
+              # and we *require* a nightly rustfmt
+              (rust-bin.selectLatestNightlyWith (
+                toolchain:
+                toolchain.default.override {
+                  extensions = [
+                    # includes already:
+                    # rustc
+                    # cargo
+                    # rust-std
+                    # rust-docs
+                    # rustfmt-preview
+                    # clippy-preview
+                    "rust-analyzer"
+                    "rust-src"
+                  ];
                 }
-        ) { };
+              ))
+            ];
+
+            nativeBuildInputs = [
+              pkgs.clang
+              pkgs.pkg-config
+              pkgs.wrapGAppsHook4 # For `niri-visual-tests`
+            ];
+
+            buildInputs = niri.buildInputs ++ [
+              pkgs.libadwaita # For `niri-visual-tests`
+            ];
+
+            env = {
+              inherit (niri) LIBCLANG_PATH;
+
+              # WARN: Do not overwrite this variable in your shell!
+              # It is required for `dlopen()` to work on some libraries; see the comment
+              # in the package expression
+              #
+              # This should only be set with `CARGO_BUILD_RUSTFLAGS="$CARGO_BUILD_RUSTFLAGS -C your-flags"`
+              inherit (niri) CARGO_BUILD_RUSTFLAGS;
+            };
+          };
+        }
+      );
+
+      formatter = forAllSystems (system: nixpkgsFor.${system}.nixfmt-rfc-style);
+
+      packages = forAllSystems (
+        system:
+        let
+          niri = nixpkgsFor.${system}.callPackage niri-package { };
+        in
+        {
+          inherit niri;
+
+          # NOTE: This is for development purposes only
+          #
+          # It is primarily to help with quickly iterating on
+          # changes made to the above expression - though it is
+          # also not stripped in order to better debug niri itself
+          niri-debug = niri.overrideAttrs (
+            newAttrs: oldAttrs: {
+              pname = oldAttrs.pname + "-debug";
+
+              cargoBuildType = "debug";
+              cargoCheckType = newAttrs.cargoBuildType;
+
+              dontStrip = true;
+            }
+          );
+
+          default = niri;
+        }
+      );
+
+      overlays.default = final: _: {
+        niri = final.callPackage niri-package { };
       };
     };
 }