Use Nix built-ins to extract the build revision

default.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> { } }:
+{ pkgs ? import <nixpkgs> { }, gitSha ? "unknown" }:
   pkgs.rustPlatform.buildRustPackage {
     pname = "amp";
     version = "0.7.0";
@@ -13,6 +13,13 @@
     # Packages needed during the build
     nativeBuildInputs = with pkgs; [ git ];
 
+    # Make the build command explicit so we can
+    # provide the commit SHA for the splash screen
+    buildPhase = ''
+      export BUILD_REVISION=${gitSha}
+      cargo build --release
+    '';
+
     # Amp creates files and directories in $HOME/.config/amp when run.
     # Since the checkPhase of the build process runs the test suite, we
     # need a writable location to avoid permission error test failures.

flake.nix

@@ -5,14 +5,16 @@
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
   };
 
-  outputs = { self, nixpkgs }:
+  outputs = { self, nixpkgs, ampSha ? "unknown" }:
     let
       supportedSystems = [ "x86_64-linux" ];
       forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
       pkgsFor = nixpkgs.legacyPackages;
     in {
       packages = forAllSystems (system: {
-        default = pkgsFor.${system}.callPackage ./. { };
+        default = pkgsFor.${system}.callPackage ./. {
+          gitSha = ampSha;
+        };
       });
     };
 }