dprint: add shell completions

The completion feature was introduced in version 0.39.0

- https://github.com/dprint/dprint/releases/tag/0.39.0
- https://github.com/dprint/dprint/commit/717546b0d7268f3b95b0047ca4a941cc94125dc2

pkgs/by-name/dp/dprint/package.nix

@@ -3,6 +3,7 @@
   stdenv,
   fetchCrate,
   rustPlatform,
+  installShellFiles,
   testers,
   nix-update-script,
   dprint,
@@ -23,6 +24,18 @@ rustPlatform.buildRustPackage rec {
   # every commit upstream on GitHub Actions
   doCheck = false;
 
+  nativeBuildInputs = lib.optionals (stdenv.buildPlatform.canExecute stdenv.hostPlatform) [
+    installShellFiles
+  ];
+
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    export DPRINT_CACHE_DIR="$(mktemp -d)"
+    installShellCompletion --cmd dprint \
+      --bash <($out/bin/dprint completions bash) \
+      --zsh <($out/bin/dprint completions zsh) \
+      --fish <($out/bin/dprint completions fish)
+  '';
+
   passthru = {
     tests.version = testers.testVersion {
       inherit version;