feat: add the rustup completion script

plugins/rustup/README.md

@@ -0,0 +1,9 @@
+# Buf plugin
+
+This plugin adds completion for [rustup](https://github.com/rust-lang/rustup), the Rust toolchain installer.
+
+To use it, add `rustup` to the plugins array in you rustup file:
+
+```zsh
+plugins=(... rustup)
+```

plugins/rustup/rustup.plugin.zsh

@@ -0,0 +1,14 @@
+# Autocompletion for the rustup CLI (rustup).
+if (( !$+commands[rustup] )); then
+  return
+fi
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `rustup`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_rustup" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _rustup
+  _comps[rustup]=_rustup
+fi
+
+# Generate and load rustup completion
+rustup completions zsh >! "$ZSH_CACHE_DIR/completions/_rustup" &|