add dry-run option to cask#install

Library/Homebrew/cask/cmd/install.rb

@@ -56,7 +56,8 @@ module Cask
         require_sha: nil,
         quarantine: nil,
         quiet: nil,
-        zap: nil
+        zap: nil,
+        dry_run: nil
       )
         odie "Installing casks is supported only on macOS" unless OS.mac?
 
@@ -69,6 +70,7 @@ module Cask
           quarantine:     quarantine,
           quiet:          quiet,
           zap:            zap,
+          dry_run:        dry_run,
         }.compact
 
         options[:quarantine] = true if options[:quarantine].nil?

Library/Homebrew/cask/installer.rb

@@ -24,7 +24,7 @@ module Cask
                    skip_cask_deps: false, binaries: true, verbose: false,
                    zap: false, require_sha: false, upgrade: false,
                    installed_as_dependency: false, quarantine: true,
-                   verify_download_integrity: true, quiet: false)
+                   verify_download_integrity: true, quiet: false, dry_run: false)
       @cask = cask
       @command = command
       @force = force
@@ -39,11 +39,12 @@ module Cask
       @quarantine = quarantine
       @verify_download_integrity = verify_download_integrity
       @quiet = quiet
+      @dry_run = dry_run
     end
 
     attr_predicate :binaries?, :force?, :skip_cask_deps?, :require_sha?,
                    :reinstall?, :upgrade?, :verbose?, :zap?, :installed_as_dependency?,
-                   :quarantine?, :quiet?
+                   :quarantine?, :quiet?, :dry_run?
 
     def self.caveats(cask)
       odebug "Printing caveats"
@@ -96,6 +97,10 @@ module Cask
       check_conflicts
 
       print caveats
+      if dry_run?
+        puts "#{Formatter.identifier(@cask)} would be installed"
+        exit
+      end
       fetch
       uninstall_existing_cask if reinstall?
 

Library/Homebrew/cmd/install.rb

@@ -45,6 +45,8 @@ module Homebrew
                           "(binaries and symlinks are excluded, unless originally from the same cask)."
       switch "-v", "--verbose",
              description: "Print the verification and postinstall steps."
+      switch "-n", "--dry-run",
+             description: "Show what would be installed, but do not actually install anything."
       [
         [:switch, "--formula", "--formulae", {
           description: "Treat all named arguments as formulae.",
@@ -193,6 +195,7 @@ module Homebrew
         skip_cask_deps: args.skip_cask_deps?,
         quarantine:     args.quarantine?,
         quiet:          args.quiet?,
+        dry_run:        args.dry_run?,
       )
     end