Reduce object creation during substitution

The `#sub` method generates many objects, but in most classes, substitution does not occur at all.
By modifying it so that no objects are created when there are no variables to substitute, we will optimize performance.

lib/rbs/ast/members.rb

@@ -30,6 +30,8 @@ def update(annotations: self.annotations, method_type: self.method_type)
           end
 
           def sub(subst)
+            return self if subst.empty?
+
             update(method_type: self.method_type.sub(subst))
           end
 

lib/rbs/definition.rb

@@ -14,6 +14,8 @@ def initialize(parent_variable:, type:, declared_in:)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           parent_variable: parent_variable,
           type: type.sub(s),
@@ -142,6 +144,8 @@ def private?
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           super_method: super_method&.sub(s),
           defs: defs.map {|defn| defn.update(type: defn.type.sub(s)) },
@@ -347,6 +351,8 @@ def type_params_decl
     end
 
     def sub(s)
+      return self if s.empty?
+
       definition = self.class.new(type_name: type_name, self_type: _ = self_type.sub(s), ancestors: ancestors, entry: entry)
 
       definition.methods.merge!(methods.transform_values {|method| method.sub(s) })

lib/rbs/method_type.rb

@@ -33,6 +33,8 @@ def to_json(state = _ = nil)
     def sub(s)
       sub = s.without(*type_param_names)
 
+      return self if sub.empty?
+
       self.class.new(
         type_params: type_params.map do |param|
           param.map_type do |bound|

lib/rbs/types.rb

@@ -329,6 +329,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(name: name,
                        args: args.map {|ty| ty.sub(s) },
                        location: location)
@@ -371,6 +373,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(name: name,
                        args: args.map {|ty| ty.sub(s) },
                        location: location)
@@ -413,6 +417,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         Alias.new(name: name, args: args.map {|ty| ty.sub(s) }, location: location)
       end
 
@@ -469,6 +475,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(types: types.map {|ty| ty.sub(s) },
                        location: location)
       end
@@ -574,6 +582,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           all_fields: all_fields.transform_values {|ty, required| [ty.sub(s), required] },
           location: location
@@ -664,6 +674,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(type: type.sub(s), location: location)
       end
 
@@ -752,6 +764,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(types: types.map {|ty| ty.sub(s) },
                        location: location)
       end
@@ -841,6 +855,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(types: types.map {|ty| ty.sub(s) },
                        location: location)
       end
@@ -1093,6 +1109,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         map_type {|ty| ty.sub(s) }
       end
 
@@ -1272,6 +1290,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(subst)
+        return self if subst.empty?
+
         map_type { _1.sub(subst) }
       end
 
@@ -1346,6 +1366,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           type: type.sub(s),
           required: required,
@@ -1415,6 +1437,8 @@ def to_json(state = _ = nil)
       end
 
       def sub(s)
+        return self if s.empty?
+
         self.class.new(
           type: type.sub(s),
           block: block&.sub(s),