sizelint: add shell completions

pkgs/by-name/si/sizelint/package.nix

@@ -1,7 +1,9 @@
 {
   lib,
+  stdenv,
   rustPlatform,
   fetchFromGitHub,
+  installShellFiles,
   git,
 }:
 
@@ -16,10 +18,19 @@ rustPlatform.buildRustPackage (finalAttrs: {
     hash = "sha256-1k1+7fVWhflEKyhOlb7kMn2xqeAM6Y5N9uHtOJvVn4A=";
   };
 
+  nativeBuildInputs = [ installShellFiles ];
+
   nativeCheckInputs = [ git ];
 
   cargoHash = "sha256-Z+pmlp/0LlKfc4QLosePw7TdLFYe6AnAVOJSw2DzlfI=";
 
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd sizelint \
+      --bash <($out/bin/sizelint completions bash) \
+      --fish <($out/bin/sizelint completions fish) \
+      --zsh <($out/bin/sizelint completions zsh)
+  '';
+
   meta = {
     description = "Lint your file tree based on file sizes";
     homepage = "https://github.com/a-kenji/sizelint";