Refactor and document BuildEnvironment.

Library/Homebrew/build_environment.rb

@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+# Settings for the build environment.
+#
+# @api private
 class BuildEnvironment
   def initialize(*settings)
     @settings = Set.new(*settings)
@@ -23,19 +26,15 @@ class BuildEnvironment
     @settings.include? :userpaths
   end
 
+  # DSL for specifying build environment settings.
   module DSL
     def env(*settings)
       @env ||= BuildEnvironment.new
       @env.merge(settings)
     end
   end
-end
 
-module Homebrew
-  module_function
-
-  def build_env_keys(env)
-    %w[
+  KEYS = %w[
     CC CXX LD OBJC OBJCXX
     HOMEBREW_CC HOMEBREW_CXX
     CFLAGS CXXFLAGS CPPFLAGS LDFLAGS SDKROOT MAKEFLAGS
@@ -47,11 +46,15 @@ module Homebrew
     MAKE GIT CPP
     ACLOCAL_PATH PATH CPATH
     LD_LIBRARY_PATH LD_RUN_PATH LD_PRELOAD LIBRARY_PATH
-    ].select { |key| env.key?(key) }
+  ].freeze
+  private_constant :KEYS
+
+  def self.keys(env)
+    KEYS & env.keys
   end
 
-  def dump_build_env(env, f = $stdout)
-    keys = build_env_keys(env)
+  def self.dump(env, f = $stdout)
+    keys = self.keys(env)
     keys -= %w[CC CXX OBJC OBJCXX] if env["CC"] == env["HOMEBREW_CC"]
 
     keys.each do |key|

Library/Homebrew/cmd/--env.rb

@@ -43,9 +43,9 @@ module Homebrew
       Utils::Shell.from_path(args.shell)
     end
 
-    env_keys = build_env_keys(ENV)
+    env_keys = BuildEnvironment.keys(ENV)
     if shell.nil?
-      dump_build_env ENV
+      BuildEnvironment.dump ENV
     else
       env_keys.each do |key|
         puts Utils::Shell.export_value(key, ENV[key], shell)

Library/Homebrew/exceptions.rb

@@ -369,7 +369,7 @@ class BuildError < RuntimeError
       ohai "Configuration"
       SystemConfig.dump_verbose_config
       ohai "ENV"
-      Homebrew.dump_build_env(env)
+      BuildEnvironment.dump env
       puts
       onoe "#{formula.full_name} #{formula.version} did not build"
       unless (logs = Dir["#{formula.logs}/*"]).empty?

Library/Homebrew/formula.rb

@@ -2020,7 +2020,7 @@ class Formula
 
         SystemConfig.dump_verbose_config(log)
         log.puts
-        Homebrew.dump_build_env(env, log)
+        BuildEnvironment.dump env, log
 
         raise BuildError.new(self, cmd, args, env)
       end