nix: get rust toolchain from rust-overlay

Now, a nix-shell always has the latest rust stable version, compliant to our
rust-toolchain.toml file.

nix/nixpkgs.nix

@@ -0,0 +1,6 @@
+let
+  sources = import ./sources.nix { };
+  rust-overlay = import sources.rust-overlay;
+in
+import sources.nixpkgs { overlays = [ rust-overlay ]; }
+

nix/rust-toolchain.nix

@@ -0,0 +1,10 @@
+# Returns the Rust toolchain for Nix compliant to the rust-toolchain.toml file
+# but without rustup.
+
+{
+  # Comes from rust-overlay
+  rust-bin
+}:
+
+# Includes rustc, cargo, rustfmt, etc
+rust-bin.fromRustupToolchainFile ../rust-toolchain.toml

nix/sources.json

@@ -1,14 +1,26 @@
 {
-  "nixpkgs": {
-    "branch": "nixos-23.05",
-    "description": "Nix Packages collection",
-    "homepage": null,
-    "owner": "NixOS",
-    "repo": "nixpkgs",
-    "rev": "6b0edc9c690c1d8a729f055e0d73439045cfda55",
-    "sha256": "1fdsnmkcz1h5wffjci29af4jrd68pzdvrhbs083niwqfd6ssm633",
-    "type": "tarball",
-    "url": "https://github.com/NixOS/nixpkgs/archive/6b0edc9c690c1d8a729f055e0d73439045cfda55.tar.gz",
-    "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
-  }
+    "nixpkgs": {
+        "branch": "nixos-23.05",
+        "description": "Nix Packages collection",
+        "homepage": null,
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "6b0edc9c690c1d8a729f055e0d73439045cfda55",
+        "sha256": "1fdsnmkcz1h5wffjci29af4jrd68pzdvrhbs083niwqfd6ssm633",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/6b0edc9c690c1d8a729f055e0d73439045cfda55.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "rust-overlay": {
+        "branch": "master",
+        "description": "Pure and reproducible nix overlay of binary distributed rust toolchains",
+        "homepage": "",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "e485313fc485700a9f1f9b8b272ddc0621d08357",
+        "sha256": "1v1gq022rnni6mm42pxmw6c5yy9il4jb2l92irh154ax616x2rzd",
+        "type": "tarball",
+        "url": "https://github.com/oxalica/rust-overlay/archive/e485313fc485700a9f1f9b8b272ddc0621d08357.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
 }

rust-toolchain.toml

@@ -1,2 +1,5 @@
 [toolchain]
+channel = "stable"
+# cargo, clippy, rustc, rust-docs, rustfmt, rust-std
+profile = "default"
 targets = ["aarch64-unknown-uefi", "i686-unknown-uefi", "x86_64-unknown-uefi"]

shell.nix

@@ -2,10 +2,11 @@
 # "cargo xtask run|test|clippy". It uses rustup rather than a pinned rust
 # toolchain.
 
-{ sources ? import ./nix/sources.nix { }
-, pkgs ? import sources.nixpkgs { }
-}:
-
+let
+  sources = import ./nix/sources.nix;
+  pkgs = import ./nix/nixpkgs.nix;
+  rustToolchain = pkgs.callPackage ./nix/rust-toolchain.nix {};
+in
 pkgs.mkShell {
   nativeBuildInputs = with pkgs; [
     # nix related stuff (such as dependency management)
@@ -17,7 +18,7 @@ pkgs.mkShell {
     qemu
 
     # Rust toolchain
-    rustup
+    rustToolchain
 
     # Other
     yamlfmt