Use encoding cache

Library/Homebrew/extend/blank.rb

@@ -133,6 +133,12 @@ end
 
 class String
   BLANK_RE = /\A[[:space:]]*\z/.freeze
+  # This is a cache that is intentionally mutable
+  # rubocop:disable Style/MutableConstant
+  ENCODED_BLANKS_ = T.let(Hash.new do |h, enc|
+    h[enc] = Regexp.new(BLANK_RE.source.encode(enc), BLANK_RE.options | Regexp::FIXEDENCODING)
+  end, T::Hash[Encoding, Regexp])
+  # rubocop:enable Style/MutableConstant
 
   # A string is blank if it's empty or contains whitespaces only:
   #
@@ -153,7 +159,7 @@ class String
       begin
         BLANK_RE.match?(self)
       rescue Encoding::CompatibilityError
-        Regexp.new(BLANK_RE.source.encode(encoding), BLANK_RE.options | Regexp::FIXEDENCODING).match?(self)
+        T.must(ENCODED_BLANKS_[encoding]).match?(self)
       end
   end