--prefix: use CLI::Parser.

Library/Homebrew/cmd/--prefix.rb

@@ -1,13 +1,25 @@
-#:  * `--prefix`:
-#:    Display Homebrew's install path. *Default:* `/usr/local` on macOS and `/home/linuxbrew/.linuxbrew` on Linux
-#:
-#:  * `--prefix` <formula>:
-#:    Display the location in the cellar where <formula> is or would be installed.
+require "cli_parser"
 
 module Homebrew
   module_function
 
+  def __prefix_args
+    Homebrew::CLI::Parser.new do
+      usage_banner <<~EOS
+        `--prefix` [<formula>]
+
+        Display Homebrew's install path. *Default:* `/usr/local` on macOS and
+        `/home/linuxbrew/.linuxbrew` on Linux.
+
+        If <formula> is provided, display the location in the cellar where <formula>
+        is or would be installed.
+      EOS
+    end
+  end
+
   def __prefix
+    __prefix_args.parse
+
     if ARGV.named.empty?
       puts HOMEBREW_PREFIX
     else