formula: make prefix usually return opt_prefix.

Return `opt_prefix` if it exists and `prefix` is not called from within
the same formula's `install` or `post_install` methods. Otherwise, fall
back to the existing functionality.

This avoids the need to use `opt_prefix` etc. everywhere and generally
means we don't expose an implementation detail (i.e. the full Cellar
path) to dependents that have a habit of hard-coding it.

Library/Homebrew/cmd/--prefix.rb

@@ -11,7 +11,7 @@ module Homebrew
     if ARGV.named.empty?
       puts HOMEBREW_PREFIX
     else
-      puts ARGV.resolved_formulae.map { |f| f.opt_prefix.exist? ? f.opt_prefix : f.installed_prefix }
+      puts ARGV.resolved_formulae.map(&:installed_prefix)
     end
   end
 end

Library/Homebrew/formula.rb

@@ -198,6 +198,7 @@ class Formula
     @build = active_spec.build
     @pin = FormulaPin.new(self)
     @follow_installed_alias = true
+    @versioned_prefix = false
   end
 
   # @private
@@ -548,9 +549,16 @@ class Formula
   end
 
   # The directory in the cellar that the formula is installed to.
-  # This directory contains the formula's name and version.
+  # This directory points to {#opt_prefix} if it exists and if #{prefix} is not
+  # called from within the same formula's {#install} or {#post_install} methods.
+  # Otherwise, return the full path to the formula's versioned cellar.
   def prefix(v = pkg_version)
-    Pathname.new("#{HOMEBREW_CELLAR}/#{name}/#{v}")
+    prefix = rack/v
+    if !@versioned_prefix && prefix.directory? && Keg.new(prefix).optlinked?
+      opt_prefix
+    else
+      prefix
+    end
   end
 
   # Is the formula linked?
@@ -579,7 +587,7 @@ class Formula
   # installed versions of this software
   # @private
   def rack
-    prefix.parent
+    Pathname.new("#{HOMEBREW_CELLAR}/#{name}")
   end
 
   # All currently installed prefix directories.
@@ -994,6 +1002,7 @@ class Formula
 
   # @private
   def run_post_install
+    @versioned_prefix = true
     build = self.build
     self.build = Tab.for_formula(self)
     old_tmpdir = ENV["TMPDIR"]
@@ -1008,6 +1017,7 @@ class Formula
     ENV["TMPDIR"] = old_tmpdir
     ENV["TEMP"] = old_temp
     ENV["TMP"] = old_tmp
+    @versioned_prefix = false
   end
 
   # Tell the user about any caveats regarding this package.
@@ -1110,6 +1120,7 @@ class Formula
   # where staging is a Mktemp staging context
   # @private
   def brew
+    @versioned_prefix = true
     stage do |staging|
       staging.retain! if ARGV.keep_tmp?
       prepare_patches
@@ -1123,6 +1134,8 @@ class Formula
         cp Dir["config.log", "CMakeCache.txt"], logs
       end
     end
+  ensure
+    @versioned_prefix = false
   end
 
   # @private
@@ -1624,6 +1637,7 @@ class Formula
 
   # @private
   def run_test
+    @versioned_prefix = true
     old_home = ENV["HOME"]
     old_curl_home = ENV["CURL_HOME"]
     old_tmpdir = ENV["TMPDIR"]
@@ -1655,6 +1669,7 @@ class Formula
     ENV["TEMP"] = old_temp
     ENV["TMP"] = old_tmp
     ENV["TERM"] = old_term
+    @versioned_prefix = false
   end
 
   # @private

Library/Homebrew/formula_installer.rb

@@ -546,7 +546,7 @@ class FormulaInstaller
   def summary
     s = ""
     s << "#{Emoji.install_badge}  " if Emoji.enabled?
-    s << "#{formula.prefix}: #{formula.prefix.abv}"
+    s << "#{formula.prefix.resolved_path}: #{formula.prefix.abv}"
     s << ", built in #{pretty_duration build_time}" if build_time
     s
   end

Library/Homebrew/keg.rb

@@ -147,6 +147,7 @@ class Keg
   protected :path
 
   def initialize(path)
+    path = path.resolved_path if path.to_s.start_with?("#{HOMEBREW_PREFIX}/opt/")
     raise "#{path} is not a valid keg" unless path.parent.parent.realpath == HOMEBREW_CELLAR.realpath
     raise "#{path} is not a directory" unless path.directory?
     @path = path