Deprecate installing casks/formulae from paths.

We've already disabled installing casks/formulae from URLs and we
regularly tell people not to install from paths so let's just deprecate
this behaviour entirely.

Even Homebrew developers do not need to work this way.

Library/Homebrew/brew.rb

@@ -211,8 +211,13 @@
     $stderr.puts "If reporting this issue please do so at (not Homebrew/brew or Homebrew/homebrew-core):"
     $stderr.puts "  #{Formatter.url(issues_url)}"
   elsif internal_cmd
-    $stderr.puts "#{Tty.bold}Please report this issue:#{Tty.reset}"
-    $stderr.puts "  #{Formatter.url(OS::ISSUES_URL)}"
+    if e.is_a?(MethodDeprecatedError) && (first_backtrace = e.backtrace&.first.presence) &&
+       (first_backtrace.include?("/formulary.rb") || first_backtrace.include?("/cask_loader.rb"))
+      $stderr.puts Utils::Backtrace.clean(e) if ARGV.include?("--debug") || args.debug?
+    else
+      $stderr.puts "#{Tty.bold}Please report this issue:#{Tty.reset}"
+      $stderr.puts "  #{Formatter.url(OS::ISSUES_URL)}"
+    end
   end
 
   exit 1

Library/Homebrew/cask/cask_loader.rb

@@ -105,10 +105,14 @@ def self.try_new(ref, warn: false)
         end
 
         return if %w[.rb .json].exclude?(path.extname)
+        return if path.to_s.include?("/Formula/")
         return unless path.expand_path.exist?
 
-        return if Homebrew::EnvConfig.forbid_packages_from_paths? &&
-                  !path.realpath.to_s.start_with?("#{Caskroom.path}/", "#{HOMEBREW_LIBRARY}/Taps/")
+        unless path.realpath.to_s.start_with?("#{Caskroom.path}/", "#{HOMEBREW_LIBRARY}/Taps/",
+                                              "#{HOMEBREW_LIBRARY_PATH}/test/support/fixtures/")
+          odeprecated "installing formulae from paths or URLs", "installing formulae from taps"
+          return if Homebrew::EnvConfig.forbid_packages_from_paths?
+        end
 
         new(path)
       end
@@ -195,6 +199,8 @@ def initialize(url)
       def load(config:)
         path.dirname.mkpath
 
+        odeprecated "installing casks from paths or URLs", "installing casks from taps"
+
         if ALLOWED_URL_SCHEMES.exclude?(url.scheme)
           raise UnsupportedInstallationMethod,
                 "Non-checksummed download of #{name} formula file from an arbitrary URL is unsupported! " \

Library/Homebrew/formulary.rb

@@ -646,8 +646,12 @@ def self.try_new(ref, from: T.unsafe(nil), warn: false)
 
       return unless path.expand_path.exist?
 
-      return if Homebrew::EnvConfig.forbid_packages_from_paths? &&
-                !path.realpath.to_s.start_with?("#{HOMEBREW_CELLAR}/", "#{HOMEBREW_LIBRARY}/Taps/")
+      unless path.realpath.to_s.start_with?("#{HOMEBREW_CELLAR}/", "#{HOMEBREW_TAP_DIRECTORY}/",
+                                            "#{HOMEBREW_LIBRARY_PATH}/test/support/fixtures/")
+        odeprecated "installing formulae from paths or URLs", "installing formulae from taps"
+
+        return if Homebrew::EnvConfig.forbid_packages_from_paths?
+      end
 
       options = if (tap = Tap.from_path(path))
         # Only treat symlinks in taps as aliases.
@@ -716,6 +720,8 @@ def self.try_new(ref, from: T.unsafe(nil), warn: false)
       return unless uri.path
       return unless uri.scheme.present?
 
+      odeprecated "installing formulae from paths or URLs", "installing formulae from taps"
+
       new(uri, from:)
     end
 

Library/Homebrew/test/cask/cask_loader/from_uri_loader_spec.rb

@@ -29,28 +29,28 @@
       loader = described_class.new("https://brew.sh/foo.rb")
       expect do
         loader.load(config: nil)
-      end.to raise_error(UnsupportedInstallationMethod)
+      end.to raise_error(MethodDeprecatedError)
     end
 
     it "raises an error when given an ftp URL" do
       loader = described_class.new("ftp://brew.sh/foo.rb")
       expect do
         loader.load(config: nil)
-      end.to raise_error(UnsupportedInstallationMethod)
+      end.to raise_error(MethodDeprecatedError)
     end
 
     it "raises an error when given an sftp URL" do
       loader = described_class.new("sftp://brew.sh/foo.rb")
       expect do
         loader.load(config: nil)
-      end.to raise_error(UnsupportedInstallationMethod)
+      end.to raise_error(MethodDeprecatedError)
     end
 
-    it "does not raise an error when given a file URL" do
+    it "raises an error when given a file URL" do
       loader = described_class.new("file://#{TEST_FIXTURE_DIR}/cask/Casks/local-caffeine.rb")
       expect do
         loader.load(config: nil)
-      end.not_to raise_error(UnsupportedInstallationMethod)
+      end.to raise_error(MethodDeprecatedError)
     end
   end
 end

Library/Homebrew/test/cask/cask_spec.rb

@@ -46,16 +46,10 @@
       expect(c.token).to eq("caffeine")
     end
 
-    it "returns an instance of the Cask from a URL", :needs_utils_curl, :no_api do
-      c = Cask::CaskLoader.load("file://#{tap_path}/Casks/local-caffeine.rb")
-      expect(c).to be_a(described_class)
-      expect(c.token).to eq("local-caffeine")
-    end
-
     it "raises an error when failing to download a Cask from a URL", :needs_utils_curl, :no_api do
       expect do
         Cask::CaskLoader.load("file://#{tap_path}/Casks/notacask.rb")
-      end.to raise_error(Cask::CaskUnavailableError)
+      end.to raise_error(MethodDeprecatedError)
     end
 
     it "returns an instance of the Cask from a relative file location" do