Load installed formulae from kegs

Author: Rylan12

Library/Homebrew/formulary.rb

@@ -738,6 +738,15 @@ def self.try_new(ref, from: nil, warn: false)
       return unless path.expand_path.exist?
       return unless ::Utils::Path.loadable_package_path?(path, :formula)
 
+      # If the path is for an installed keg, use FromKegLoader instead
+      begin
+        keg = Keg.for(path)
+        loader = FromKegLoader.try_new(keg.name, from:, warn:)
+        return T.cast(loader, T.attached_class)
+      rescue NotAKegError
+        # Not a keg path, continue
+      end
+
       if (tap = Tap.from_path(path))
         # Only treat symlinks in taps as aliases.
         if path.symlink?
@@ -936,9 +945,15 @@ class FromKegLoader < FormulaLoader
     def self.try_new(ref, from: nil, warn: false)
       ref = ref.to_s
 
-      return unless (keg_formula = HOMEBREW_PREFIX/"opt/#{ref}/.brew/#{ref}.rb").file?
+      keg_directory = HOMEBREW_PREFIX/"opt/#{ref}"
+      return unless keg_directory.directory?
 
-      new(ref, keg_formula)
+      # The formula file in `.brew` will use the canonical name, whereas `ref` can be an alias.
+      # Use `Keg#name` to get the canonical name.
+      keg = Keg.new(keg_directory)
+      return unless (keg_formula = HOMEBREW_PREFIX/"opt/#{ref}/.brew/#{keg.name}.rb").file?
+
+      new(keg.name, keg_formula, tap: keg.tab.tap)
     end
   end
 
@@ -1054,7 +1069,12 @@ def klass(flags:, ignore_errors:)
 
     sig { overridable.params(flags: T::Array[String]).void }
     def load_from_api(flags:)
-      json_formula = Homebrew::API::Formula.all_formulae[name]
+      json_formula = if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Formula.formula_json(name)
+      else
+        Homebrew::API::Formula.all_formulae[name]
+      end
+
       raise FormulaUnavailableError, name if json_formula.nil?
 
       Formulary.load_formula_from_json!(name, json_formula, flags:)
@@ -1223,7 +1243,16 @@ def self.from_keg(
       flags:,
     }.compact
 
-    f = if tap.nil?
+    loader = FromKegLoader.try_new(keg.name, warn: false)
+    f = if loader.present?
+      begin
+        loader.get_formula(spec, alias_path:, force_bottle:, flags:, ignore_errors: true)
+      rescue FormulaUnreadableError
+        nil
+      end
+    end
+
+    f ||= if tap.nil?
       factory(formula_name, spec, **options)
     else
       begin