linkage_checker: deprecate linkage to libcrypt.so.1

Library/Homebrew/extend/os/linux/linkage_checker.rb

@@ -26,9 +26,47 @@ class LinkageChecker
     libstdc++.so.6
   ].freeze
 
+  def display_deprecated_warning(strict: false)
+    return unless @libcrypt_found
+
+    # Steps when moving this to `odisabled`:
+    # - Remove `libcrypt.so.1` from SYSTEM_LIBRARY_ALLOWLIST above.
+    # - Remove the `disable` and `disable_for_developer` kwargs here.
+    # - Remove `broken_library_linkage?` override below and the generic alias in HOMEBREW_LIBRARY/linkage_checker.rb.
+    # - Remove `fail_on_libcrypt1?`.
+    # Steps when removing this entirely (assuming the above has already been done):
+    # - Remove the `display_` overrides here and the associated generic aliases in HOMEBREW_LIBRARY/linkage_checker.rb
+    # - Remove the setting of `@libcrypt_found` in `check_dylibs` below.
+    odeprecated "linkage to libcrypt.so.1", "libcrypt.so.2 in the libxcrypt formula",
+                disable:                fail_on_libcrypt1?(strict: strict),
+                disable_for_developers: false
+  end
+
+  def display_normal_output
+    generic_display_normal_output
+    display_deprecated_warning
+  end
+
+  def display_test_output(puts_output: true, strict: false)
+    generic_display_test_output(puts_output: puts_output, strict: strict)
+    display_deprecated_warning(strict: strict)
+  end
+
+  def broken_library_linkage?(strict: false)
+    generic_broken_library_linkage?(strict: strict) || (fail_on_libcrypt1?(strict: strict) && @libcrypt_found)
+  end
+
+  private
+
+  def fail_on_libcrypt1?(strict:)
+    strict || ENV["HOMEBREW_DISALLOW_LIBCRYPT1"].present?
+  end
+
   def check_dylibs(rebuild_cache:)
     generic_check_dylibs(rebuild_cache: rebuild_cache)
 
+    @libcrypt_found = true if @system_dylibs.any? { |s| File.basename(s) == "libcrypt.so.1" }
+
     # glibc and gcc are implicit dependencies.
     # No other linkage to system libraries is expected or desired.
     @unwanted_system_dylibs = @system_dylibs.reject do |s|

Library/Homebrew/linkage_checker.rb

@@ -49,6 +49,8 @@ class LinkageChecker
     display_items "Unwanted system libraries", @unwanted_system_dylibs
     display_items "Files with missing rpath", @files_missing_rpaths
   end
+  alias generic_display_normal_output display_normal_output
+  private :generic_display_normal_output
 
   def display_reverse_output
     return if @reverse_links.empty?
@@ -75,6 +77,8 @@ class LinkageChecker
     display_items "Undeclared dependencies with linkage", @undeclared_deps, puts_output: puts_output
     display_items "Files with missing rpath", @files_missing_rpaths, puts_output: puts_output
   end
+  alias generic_display_test_output display_test_output
+  private :generic_display_test_output
 
   sig { params(strict: T::Boolean).returns(T::Boolean) }
   def broken_library_linkage?(strict: false)
@@ -82,6 +86,8 @@ class LinkageChecker
     issues += [@undeclared_deps, @files_missing_rpaths] if strict
     [issues, unexpected_broken_dylibs, unexpected_present_dylibs].flatten.any?(&:present?)
   end
+  alias generic_broken_library_linkage? broken_library_linkage?
+  private :generic_broken_library_linkage?
 
   def unexpected_broken_dylibs
     return @unexpected_broken_dylibs if @unexpected_broken_dylibs