refactor(rustup): generate completion file in the background (#10392)

Closes #10392

plugins/rustup/rustup.plugin.zsh

-if (( $+commands[rustup] )); then
-  # remove old generated completion file
-  command rm -f "${0:A:h}/_rustup"
-
-  ver="$(rustup --version 2>/dev/null)"
-  ver_file="$ZSH_CACHE_DIR/rustup_version"
-  comp_file="$ZSH_CACHE_DIR/completions/_rustup"
+if (( ! $+commands[rustup] )); then
+  return
+fi
 
-  mkdir -p "${comp_file:h}"
-  (( ${fpath[(Ie)${comp_file:h}]} )) || fpath=("${comp_file:h}" $fpath)
+# Remove old generated completion file
+# TODO: 2021-09-15: remove this line
+command rm -f "${0:A:h}/_rustup"
 
-  if [[ ! -f "$comp_file" || ! -f "$ver_file" || "$ver" != "$(< "$ver_file")" ]]; then
-    rustup completions zsh >| "$comp_file"
-    echo "$ver" >| "$ver_file"
-  fi
+# Add completions/ folder in $ZSH_CACHE_DIR
+comp_file="$ZSH_CACHE_DIR/completions/_rustup"
+command mkdir -p "${comp_file:h}"
+(( ${fpath[(Ie)"$ZSH_CACHE_DIR/completions"]} )) || fpath=("$ZSH_CACHE_DIR/completions" $fpath)
 
-  declare -A _comps
+# 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 "$comp_file" ]]; then
   autoload -Uz _rustup
+  declare -A _comps
   _comps[rustup]=_rustup
-
-  unset ver ver_file comp_file
 fi
+
+# Generate completion file in the background
+rustup completions zsh >| "$comp_file" &|
+unset comp_file