2 files changed, 17 insertions, 0 deletions
diff --git a/Library/Homebrew/diagnostic.rb b/Library/Homebrew/diagnostic.rb
index a5adfeb63..ceb6ad4d1 100644
--- a/Library/Homebrew/diagnostic.rb
+++ b/Library/Homebrew/diagnostic.rb
@@ -98,6 +98,17 @@ module Homebrew
         EOS
       end
 
+      def check_build_from_source
+        return unless ENV["HOMEBREW_BUILD_FROM_SOURCE"]
+
+        <<-EOS.undent
+          You have HOMEBREW_BUILD_FROM_SOURCE set. This environment variable is
+          intended for use by Homebrew developers. If you are encountering errors,
+          please try unsetting this. Please do not file issues if you encounter
+          errors when using this environment variable.
+        EOS
+      end
+
       # See https://github.com/Homebrew/legacy-homebrew/pull/9986
       def check_path_for_trailing_slashes
         bad_paths = PATH.new(ENV["HOMEBREW_PATH"]).select { |p| p.end_with?("/") }
diff --git a/Library/Homebrew/test/diagnostic_spec.rb b/Library/Homebrew/test/diagnostic_spec.rb
index 058b9823c..8a25413ed 100644
--- a/Library/Homebrew/test/diagnostic_spec.rb
+++ b/Library/Homebrew/test/diagnostic_spec.rb
@@ -12,6 +12,12 @@ describe Homebrew::Diagnostic::Checks do
       .to match("Some directories in your path end in a slash")
   end
 
+  specify "#check_build_from_source" do
+    ENV["HOMEBREW_BUILD_FROM_SOURCE"] = "1"
+    expect(subject.check_build_from_source)
+      .to match("You have HOMEBREW_BUILD_FROM_SOURCE set.")
+  end
+
   specify "#check_for_anaconda" do
     mktmpdir do |path|
       anaconda = "#{path}/anaconda"