Fix Rust Analyzer

Previously we had warnings about sysroot, and limited feedback. This fix is from the Crane docs section about building standard library crates.

flake.lock

@@ -609,7 +609,28 @@
         "nixpkgs": [
           "haskellNix",
           "nixpkgs-unstable"
+        ],
+        "rust-overlay": "rust-overlay"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
         ]
+      },
+      "locked": {
+        "lastModified": 1764816035,
+        "narHash": "sha256-F0IQSmSj4t2ThkbWZooAhkCTO+YpZSd2Pqiv2uoYEHo=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "74d9abb7c5c030469f90d97a67d127cc5d76c238",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
       }
     },
     "stackage": {

flake.nix

@@ -3,7 +3,11 @@
   inputs.nixpkgs.follows = "haskellNix/nixpkgs-unstable";
   inputs.flake-utils.url = "github:numtide/flake-utils";
   inputs.crane.url = "github:ipetkov/crane";
-  outputs = { self, nixpkgs, flake-utils, haskellNix, crane }:
+  inputs.rust-overlay = {
+    url = "github:oxalica/rust-overlay";
+    inputs.nixpkgs.follows = "nixpkgs";
+  };
+  outputs = { self, nixpkgs, flake-utils, haskellNix, crane, rust-overlay }:
     flake-utils.lib.eachSystem [ "x86_64-linux" ] (system:
       let
         overlays = [
@@ -15,10 +19,16 @@
                 evalSystem = "x86_64-linux";
               };
           })
+          (import rust-overlay)
         ];
         pkgs = import nixpkgs { inherit system overlays; inherit (haskellNix) config; };
         haskell = pkgs.hixProject.flake { };
-        rust = crane.mkLib pkgs;
+        rust = (crane.mkLib pkgs).overrideToolchain (p: p.rust-bin.selectLatestNightlyWith (
+          toolchain: toolchain.default.override {
+            extensions = [ "rust-src" ];
+            targets = [ "x86_64-unknown-linux-gnu" ];
+          }
+        ));
       in
       {
         devShells.default = pkgs.mkShell {