dev-cmd/cat: auto-install bat when running brew cat ... with HOMEBREW_BAT set

Library/Homebrew/dev-cmd/cat.rb

@@ -2,6 +2,7 @@
 # frozen_string_literal: true
 
 require "cli/parser"
+require "formula"
 
 module Homebrew
   extend T::Sig
@@ -31,8 +32,16 @@ module Homebrew
 
     cd HOMEBREW_REPOSITORY
     pager = if Homebrew::EnvConfig.bat?
+      unless Formula["bat"].any_version_installed?
+        # The user might want to capture the output of `brew cat ...`
+        # Redirect stdout to stderr
+        redirect_stdout($stderr) do
+          ohai "Installing `bat` for displaying <formula>/<cask> source..."
+          safe_system HOMEBREW_BREW_FILE, "install", "bat"
+        end
+      end
       ENV["BAT_CONFIG_PATH"] = Homebrew::EnvConfig.bat_config_path
-      "#{HOMEBREW_PREFIX}/bin/bat"
+      Formula["bat"].opt_bin/"bat"
     else
       "cat"
     end

Library/Homebrew/utils.rb

@@ -437,21 +437,23 @@ module Kernel
     $stderr = old
   end
 
-  def nostdout
+  def nostdout(&block)
     if verbose?
       yield
     else
-      begin
-        out = $stdout.dup
-        $stdout.reopen(File::NULL)
-        yield
-      ensure
-        $stdout.reopen(out)
-        out.close
-      end
+      redirect_stdout(File::NULL, &block)
     end
   end
 
+  def redirect_stdout(file)
+    out = $stdout.dup
+    $stdout.reopen(file)
+    yield
+  ensure
+    $stdout.reopen(out)
+    out.close
+  end
+
   def paths
     @paths ||= PATH.new(ENV["HOMEBREW_PATH"]).map do |p|
       File.expand_path(p).chomp("/")