nix: switch to Nix Flake

This modernizes the Nix files of this project. Nix Flakes are the
de-facto standard way in Nix to package projects and specify their
inputs. Inputs can be any Nix or non-Nix project.

Updating Nix dependencies now is as easy as `nix flake update` and
a corresponding commit.

As they are still not stabilized yet, you need the line

    experimental-features = nix-command flakes

in /etc/nix/nix.conf to use the flake. By usage, I mean entering
the Nix development shell using `$ nix develop .` or if you want
to run `nix flake update`.

The package manager niv, we used to have, is basically dead.

Author: phip1611

.envrc

@@ -1,2 +1,3 @@
-# If your shell has direnv support, it will automatically open a "nix-shell".
-use nix
+# If your shell has direnv support, it will automatically open a Nix shell with
+# all relevant dependencies to develop this project.
+use flake

.github/workflows/developer_productivity.yml

@@ -4,7 +4,7 @@ on:
   push:
   pull_request:
 jobs:
-  # Job to run change detection
+  # Job to run change detection for Nix-related files
   changes:
     runs-on: ubuntu-latest
     # Set job outputs to values from filter step.
@@ -19,13 +19,14 @@ jobs:
           filters: |
             nix-src:
               - 'nix/**'
-              - 'shell.nix'
+              - 'flake.nix'
+              - 'flake.lock'
   # This is a convenience test to verify that the toolchain provided by
   # shell.nix is valid and can build + run the integration test.
   #
   # It only runs if the "nix-src" output of the "changes" job is true.
   nix_shell_toolchain:
-    name: "Nix toolchain: `cargo xtask run` works"
+    name: "Nix shell toolchain: `cargo xtask run` works"
     needs: changes
     if: ${{ needs.changes.outputs.nix-src == 'true' }}
     runs-on: ubuntu-latest
@@ -34,17 +35,13 @@ jobs:
         uses: actions/checkout@v4
       - uses: Swatinem/rust-cache@v2
       - uses: cachix/install-nix-action@v30
-        with:
-          # This channel is only required to invoke "nix-shell".
-          # Everything inside that nix-shell will use a pinned version of nixpkgs.
-          nix_path: nixpkgs=channel:nixpkgs-unstable
       # Dedicated step to separate all the
       # "copying path '/nix/store/...' from 'https://cache.nixos.org'."
-      # messages from the actual build output. This job takes ~60secs.
+      # messages from the actual build output.
       - name: Prepare Nix Store
-        run: nix-shell --pure --run "cargo --version"
+        run: nix develop --command bash -c "cargo --version"
       - name: Run VM tests
         run: |
           COMMAND="cargo xtask run --target x86_64 --headless --ci --tpm=v1"
-          echo "Executing in nix shell: $COMMAND"
-          nix-shell --pure --run "$COMMAND"
+          echo "Executing in Nix shell: $COMMAND"
+          nix develop --command bash -c "$COMMAND"

flake.lock

@@ -0,0 +1,61 @@
+{
+  description = "uefi-rs";
+
+  inputs = {
+    # We follow the latest stable release of nixpkgs
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-24.11";
+    rust-overlay.url = "github:oxalica/rust-overlay";
+  };
+
+  outputs =
+    inputs@{ self, nixpkgs, ... }:
+    let
+      # Systems definition for dev shells and exported packages,
+      # independent of the NixOS configurations and modules defined here. We
+      # just use "every system" here to not restrict any user. However, it
+      # likely happens that certain packages don't build for/under certain
+      # systems.
+      systems = nixpkgs.lib.systems.flakeExposed;
+      forAllSystems =
+        function: nixpkgs.lib.genAttrs systems (system: function nixpkgs.legacyPackages.${system});
+
+      # We directly instantiate the functionality, without using an
+      # nixpkgs overlay.
+      # https://github.com/oxalica/rust-overlay/blob/f4d5a693c18b389f0d58f55b6f7be6ef85af186f/docs/reference.md?plain=1#L26
+      rustToolchain =
+        pkgs:
+        let
+          rust-bin = (inputs.rust-overlay.lib.mkRustBin { }) pkgs;
+          rustToolchainBuilder = import ./nix/rust-toolchain.nix;
+        in
+        rustToolchainBuilder { inherit rust-bin; };
+    in
+    {
+      devShells = forAllSystems (pkgs: {
+        default = pkgs.mkShell {
+          packages = with pkgs; [
+            nixfmt-rfc-style
+
+            # Integration test dependencies
+            swtpm
+            qemu
+
+            # Rust toolchain
+            (rustToolchain pkgs)
+
+            # Other
+            cargo-llvm-cov
+            mdbook
+            yamlfmt
+            which # used by "cargo xtask fmt"
+          ];
+
+          # Set ENV vars.
+          # OVMF_CODE="${pkgs.OVMF.firmware}";
+          # OVMF_VARS="${pkgs.OVMF.variables}";
+          # OVMF_SHELL="${pkgs.edk2-uefi-shell}";
+        };
+      });
+      formatter = forAllSystems (pkgs: pkgs.nixfmt-rfc-style);
+    };
+}

flake.nix

@@ -1,5 +1,4 @@
-# Returns the Rust toolchain for Nix compliant to the rust-toolchain.toml file
-# but without rustup.
+# Returns a Rust toolchain for Nix that matches the one from the toolchain file.
 
 {
   # Comes from rust-overlay