air-formatter: add shell completions (#537602)

pkgs/by-name/ai/air-formatter/package.nix

@@ -4,6 +4,7 @@
   fetchFromGitHub,
   versionCheckHook,
   nix-update-script,
+  installShellFiles,
 }:
 rustPlatform.buildRustPackage (finalAttrs: {
   pname = "air-formatter";
@@ -32,6 +33,16 @@ rustPlatform.buildRustPackage (finalAttrs: {
     updateScript = nix-update-script { };
   };
 
+  nativeBuildInputs = [ installShellFiles ];
+  # TODO: Upstream also provides Elvish and PowerShell completions,
+  # but `installShellCompletion` only has support for Bash, Zsh and Fish at the moment.
+  postInstall = ''
+    installShellCompletion --cmd air-formatter \
+      --bash <($out/bin/air generate-shell-completion bash) \
+      --fish <($out/bin/air generate-shell-completion fish) \
+      --zsh  <($out/bin/air generate-shell-completion zsh)
+  '';
+
   meta = {
     description = "Extremely fast R code formatter";
     homepage = "https://posit-dev.github.io/air";