Better support source builds under API mode

Library/Homebrew/formula.rb

@@ -1954,6 +1954,11 @@ class Formula
     ohai "#{verb} #{name} from #{tap}"
   end
 
+  # @private
+  def tap_git_head
+    tap&.git_head
+  end
+
   # @private
   delegate env: :"self.class"
 
@@ -2125,7 +2130,7 @@ class Formula
       "disabled"                 => disabled?,
       "disable_date"             => disable_date,
       "disable_reason"           => disable_reason,
-      "tap_git_head"             => tap&.git_head,
+      "tap_git_head"             => tap_git_head,
     }
 
     if stable

Library/Homebrew/formula_installer.rb

@@ -1181,14 +1181,12 @@ class FormulaInstaller
 
     if pour_bottle?(output_warning: true)
       formula.fetch_bottle_tab
-    elsif formula.core_formula? && !formula.tap.installed? && Homebrew::EnvConfig.install_from_api?
+    elsif formula.core_formula? && Homebrew::EnvConfig.install_from_api?
-      odie <<~EOS
+      url = "https://raw.githubusercontent.com/#{formula.tap.full_name}/#{formula.tap_git_head}/Formula/#{formula.name}.rb"
-        Unable to build #{formula.name} from source while Homebrew/homebrew-core is
+      @formula = Formulary.factory(url, formula.active_spec_sym,
-        untapped and HOMEBREW_NO_INSTALL_FROM_API is unset! To resolve please run:
+                                   alias_path: formula.alias_path,
-          export HOMEBREW_NO_INSTALL_FROM_API=1
+                                   flags:      formula.class.build_flags,
-          brew tap Homebrew/core
+                                   from:       :formula_installer)
-        and retry.
-      EOS
     else
       formula.fetch_patches
       formula.resources.each(&:fetch)

Library/Homebrew/formulary.rb

@@ -220,6 +220,11 @@ module Formulary
       def caveats
         self.class.instance_variable_get(:@caveats_string)
       end
+
+      @tap_git_head_string = json_formula["tap_git_head"]
+      def tap_git_head
+        self.class.instance_variable_get(:@tap_git_head_string)
+      end
     end
 
     klass.loaded_from_api = true
@@ -395,24 +400,27 @@ module Formulary
 
     attr_reader :url
 
-    sig { params(url: T.any(URI::Generic, String)).void }
+    sig { params(url: T.any(URI::Generic, String), from: T.nilable(Symbol)).void }
-    def initialize(url)
+    def initialize(url, from: nil)
       @url = url
+      @from = from
       uri = URI(url)
       formula = File.basename(uri.path, ".rb")
       super formula, HOMEBREW_CACHE_FORMULA/File.basename(uri.path)
     end
 
     def load_file(flags:, ignore_errors:)
-      if %r{githubusercontent.com/[\w-]+/[\w-]+/[a-f0-9]{40}(?:/Formula)?/(?<formula_name>[\w+-.@]+).rb} =~ url
+      if @from != :formula_installer
-        raise UnsupportedInstallationMethod,
+        if %r{githubusercontent.com/[\w-]+/[\w-]+/[a-f0-9]{40}(?:/Formula)?/(?<formula_name>[\w+-.@]+).rb} =~ url
-              "Installation of #{formula_name} from a GitHub commit URL is unsupported! " \
+          raise UnsupportedInstallationMethod,
-              "`brew extract #{formula_name}` to a stable tap on GitHub instead."
+                "Installation of #{formula_name} from a GitHub commit URL is unsupported! " \
-      elsif url.match?(%r{^(https?|ftp)://})
+                "`brew extract #{formula_name}` to a stable tap on GitHub instead."
-        raise UnsupportedInstallationMethod,
+        elsif url.match?(%r{^(https?|ftp)://})
-              "Non-checksummed download of #{name} formula file from an arbitrary URL is unsupported! " \
+          raise UnsupportedInstallationMethod,
-              "`brew extract` or `brew create` and `brew tap-new` to create a formula file in a tap " \
+                "Non-checksummed download of #{name} formula file from an arbitrary URL is unsupported! " \
-              "on GitHub instead."
+                "`brew extract` or `brew create` and `brew tap-new` to create a formula file in a tap " \
+                "on GitHub instead."
+        end
       end
       HOMEBREW_CACHE_FORMULA.mkpath
       FileUtils.rm_f(path)
@@ -664,7 +672,7 @@ module Formulary
     when HOMEBREW_BOTTLES_EXTNAME_REGEX
       return BottleLoader.new(ref)
     when URL_START_REGEX
-      return FromUrlLoader.new(ref)
+      return FromUrlLoader.new(ref, from: from)
     when HOMEBREW_TAP_FORMULA_REGEX
       if ref.start_with?("homebrew/core/") && Homebrew::EnvConfig.install_from_api?
         name = ref.split("/", 3).last

Library/Homebrew/install.rb

@@ -113,15 +113,6 @@ module Homebrew
         EOS
       end
 
-      if head && Homebrew::EnvConfig.install_from_api?
-        raise UsageError, <<~EOS
-          --HEAD is not supported with HOMEBREW_NO_INSTALL_FROM_API unset! To resolve please run:
-            export HOMEBREW_NO_INSTALL_FROM_API=1
-            brew tap Homebrew/core
-          and retry this command.
-        EOS
-      end
-
       # --HEAD, fail with no head defined
       odie "No head is defined for #{f.full_name}" if head && f.head.nil?