link: refactor, reorder and fix bugs.

The change in #4441 broke the handling of the `elsif`s due to the
change in logic. As every block here has a `next` there's no need to do
an `elsif` in here at all. Additionally, reorder the conditions in here
so you get an appropriate message depending on what you're trying to do.
Finally, tweak some of the messaging to remove things that are ignored
and tell people correct commands to run to link things.

Library/Homebrew/cmd/link.rb

@@ -27,7 +27,43 @@ module Homebrew
 
     ARGV.kegs.each do |keg|
       keg_only = Formulary.keg_only?(keg.rack)
-      if HOMEBREW_PREFIX.to_s == "/usr/local" && keg_only
+
+      if keg.linked?
+        opoo "Already linked: #{keg}"
+        name_and_flag = if keg_only
+          "--force #{keg.name}"
+        else
+          keg.name
+        end
+        puts "To relink: brew unlink #{keg.name} && brew link #{name_and_flag}"
+        next
+      end
+
+      if mode.dry_run
+        if mode.overwrite
+          puts "Would remove:"
+        else
+          puts "Would link:"
+        end
+        keg.link(mode)
+        puts_keg_only_path_message(keg) if keg_only
+        next
+      end
+
+      if keg_only
+        if HOMEBREW_PREFIX.to_s == "/usr/local"
+          if keg.to_formula.keg_only_reason.reason == :provided_by_macos &&
+             (MacOS.version >= :mojave ||
+              MacOS::Xcode.version >= "10.0" ||
+              MacOS::CLT.version >= "10.0")
+            opoo <<~EOS
+              Refusing to link macOS-provided software: #{keg.name}
+              Instead, pass the full include/library paths to your compiler e.g.:
+                -I#{HOMEBREW_PREFIX}/opt/#{keg.name}/include -L#{HOMEBREW_PREFIX}/opt/#{keg.name}/lib
+            EOS
+            next
+          end
+
           if keg.name.start_with?("openssl", "libressl")
             opoo <<~EOS
               Refusing to link: #{keg.name}
@@ -37,37 +73,14 @@ module Homebrew
                 -I#{HOMEBREW_PREFIX}/opt/#{keg.name}/include -L#{HOMEBREW_PREFIX}/opt/#{keg.name}/lib
             EOS
             next
-        elsif (MacOS.version >= :mojave ||
-               MacOS::Xcode.version >= "10.0" ||
-               MacOS::CLT.version >= "10.0") &&
-              keg.to_formula.keg_only_reason.reason == :provided_by_macos
-          opoo <<~EOS
-            Refusing to link macOS-provided software: #{keg.name}
-            Instead, pass the full include/library paths to your compiler e.g.:
-              -I#{HOMEBREW_PREFIX}/opt/#{keg.name}/include -L#{HOMEBREW_PREFIX}/opt/#{keg.name}/lib
-          EOS
-          next
           end
-      elsif keg.linked?
+        end
-        opoo "Already linked: #{keg}"
+
-        puts "To relink: brew unlink #{keg.name} && brew link #{keg.name}"
+        unless ARGV.force?
-        next
-      elsif keg_only && !ARGV.force?
           opoo "#{keg.name} is keg-only and must be linked with --force"
-        puts "Note that doing so can interfere with building software."
           puts_keg_only_path_message(keg)
           next
-      elsif mode.dry_run && mode.overwrite
+        end
-        puts "Would remove:"
-        keg.link(mode)
-
-        next
-      elsif mode.dry_run
-        puts "Would link:"
-        keg.link(mode)
-        puts_keg_only_path_message(keg) if keg_only
-
-        next
       end
 
       keg.lock do

Library/Homebrew/test/cmd/link_spec.rb

@@ -37,9 +37,8 @@ describe "brew link", :integration_test do
 
     expect { brew "link", "testball1", "SHELL" => "/bin/zsh" }
       .to output(/testball1 is keg-only/).to_stderr
-      .and output(a_string_matching(/Note that doing so can interfere with building software\./)
+      .and output(a_string_matching(/If you need to have this software first in your PATH instead consider running:/)
-        .and(matching("If you need to have this software first in your PATH instead consider running:")
+        .and(including("echo 'export PATH=\"#{HOMEBREW_PREFIX}/opt/testball1/bin:$PATH\"' >> ~/.zshrc"))).to_stdout
-        .and(including("echo 'export PATH=\"#{HOMEBREW_PREFIX}/opt/testball1/bin:$PATH\"' >> ~/.zshrc")))).to_stdout
       .and be_a_success
   end
 end