Add MutationVisitor#remove API to remove nodes from the tree

Author: nvasilevski

README.md

@@ -578,14 +578,17 @@ visitor.mutate("IfNode[predicate: Assign | OpAssign]") do |node|
   node.copy(predicate: predicate)
 end
 
-source = "if a = 1; end"
+# remove `do_more_work` method call node 
+visitor.remove("SyntaxTree::VCall[value: SyntaxTree::Ident[value: 'do_more_work']]")
+
+source = "if a = 1; perform_work; do_more_work; end"
 program = SyntaxTree.parse(source)
 
 SyntaxTree::Formatter.format(source, program)
-# => "if a = 1\nend\n"
+# => "if a = 1\n  perform_work\n  do_more_work\nend\n"
 
 SyntaxTree::Formatter.format(source, program.accept(visitor))
-# => "if (a = 1)\nend\n"
+# => "if (a = 1)\n  perform_work\nend\n"
 ```
 
 ### WithEnvironment

lib/syntax_tree/visitor/mutation_visitor.rb

@@ -5,10 +5,11 @@ class Visitor
     # This visitor walks through the tree and copies each node as it is being
     # visited. This is useful for mutating the tree before it is formatted.
     class MutationVisitor < BasicVisitor
-      attr_reader :mutations
+      attr_reader :mutations, :removals
 
       def initialize
         @mutations = []
+        @removals = []
       end
 
       # Create a new mutation based on the given query that will mutate the node
@@ -20,6 +21,10 @@ def mutate(query, &block)
         mutations << [Pattern.new(query).compile, block]
       end
 
+      def remove(query)
+        @removals << Pattern.new(query).compile
+      end
+
       # This is the base visit method for each node in the tree. It first
       # creates a copy of the node using the visit_* methods defined below. Then
       # it checks each mutation in sequence and calls it if it finds a match.
@@ -31,6 +36,10 @@ def visit(node)
           result = mutation.call(result) if pattern.call(result)
         end
 
+        removals.each do |removal_pattern|
+          return if removal_pattern.call(result)
+        end
+
         result
       end
 
@@ -712,7 +721,7 @@ def visit_sclass(node)
 
       # Visit a Statements node.
       def visit_statements(node)
-        node.copy(body: visit_all(node.body))
+        node.copy(body: visit_all(node.body).compact)
       end
 
       # Visit a StringContent node.

test/mutation_test.rb

@@ -21,6 +21,35 @@ def test_mutates_based_on_patterns
       assert_equal(expected, SyntaxTree::Formatter.format(source, program))
     end
 
+    def test_removes_node
+      source = <<~RUBY
+        App.configure do |config|
+          config.config_value_a = 1
+          config.config_value_b = 2
+          config.config_value_c = 2
+        end
+      RUBY
+
+      expected = <<~RUBY
+        App.configure do |config|
+          config.config_value_a = 1
+
+          config.config_value_c = 2
+        end
+      RUBY
+
+      mutation_visitor = SyntaxTree.mutation do |mutation|
+        mutation.remove("SyntaxTree::Assign[
+          target: SyntaxTree::Field[
+            name: SyntaxTree::Ident[value: 'config_value_b']
+          ],
+        ]")
+      end
+
+      program = SyntaxTree.parse(source).accept(mutation_visitor)
+      assert_equal(expected, SyntaxTree::Formatter.format(source, program))
+    end
+
     private
 
     def build_mutation