diff --git a/Changelog.md b/Changelog.md
index c167eede4..56611dc49 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -1,3 +1,19 @@
+# v0.13.0 unreleased
+
+  Significant unparser upgrade. Mutant now:
+
+  * Avoids emitting mutations that do not round trip against unparsers API.
+  * Does not emit mutations anymore that are prone to not round tripping.
+
+  This change generates less mutations, and currently slightly increases boot time.
+  But the number of mutations that are hitting the test suite is much lower so it should balance.
+
+  Also its a good step towards parallel mutation generation reducing boot times.
+
+  This release removes all mutations that modify the local variable scope, mostly as these are
+  prone to not round trip via unparser. Unused local variables are now also reported by ruby / rubocop
+  so mutant does not have to spend dynamic analysis capabilty on these anymore.
+
 # v0.12.5 unreleased
 
 * [#1458](https://github.com/mbj/mutant/pull/1458)
diff --git a/lib/mutant/mutation.rb b/lib/mutant/mutation.rb
index 12f8228d5..4ef5765e8 100644
--- a/lib/mutant/mutation.rb
+++ b/lib/mutant/mutation.rb
@@ -75,10 +75,6 @@ def insert(kernel)
       end
     end
 
-    # Rendered mutation diff
-    #
-    # @return [String, nil]
-    #   the diff, if present
     def diff
       Unparser::Diff.build(original_source, source)
     end
diff --git a/lib/mutant/mutator/node/argument.rb b/lib/mutant/mutator/node/argument.rb
index 8126a012d..d4ed4d022 100644
--- a/lib/mutant/mutator/node/argument.rb
+++ b/lib/mutant/mutator/node/argument.rb
@@ -14,14 +14,7 @@ class Argument < self
 
       private
 
-        def dispatch
-          emit_name_mutation
-        end
-
-        def emit_name_mutation
-          return if skip?
-          emit_name(:"#{UNDERSCORE}#{name}")
-        end
+        def dispatch; end
 
         def skip?
           name.start_with?(UNDERSCORE)
@@ -42,7 +35,6 @@ class Optional < self
         private
 
           def dispatch
-            emit_name_mutation
             emit_required_mutation
             emit_default_mutations
           end
diff --git a/lib/mutant/mutator/node/arguments.rb b/lib/mutant/mutator/node/arguments.rb
index 04f336fa1..f76e77d30 100644
--- a/lib/mutant/mutator/node/arguments.rb
+++ b/lib/mutant/mutator/node/arguments.rb
@@ -13,24 +13,11 @@ class Arguments < self
       private
 
         def dispatch
-          emit_argument_presence
           emit_argument_mutations
           emit_mlhs_expansion
           emit_procarg0_removal
         end
 
-        def emit_argument_presence
-          emit_type unless removed_block_arg?(EMPTY_ARRAY) || forward_arg?
-
-          children.each_with_index do |removed, index|
-            new_arguments = children.dup
-            new_arguments.delete_at(index)
-            unless forward_type?(removed) || removed_block_arg?(new_arguments) || only_mlhs?(new_arguments)
-              emit_type(*new_arguments)
-            end
-          end
-        end
-
         def forward_type?(removed)
           n_forward_arg?(removed) || n_restarg?(removed) || n_kwrestarg?(removed)
         end
diff --git a/lib/mutant/mutator/node/block.rb b/lib/mutant/mutator/node/block.rb
index 9e461fc3f..53df93a00 100644
--- a/lib/mutant/mutator/node/block.rb
+++ b/lib/mutant/mutator/node/block.rb
@@ -25,7 +25,9 @@ def mutate_body
           emit_body(N_RAISE)
 
           return unless body
-          emit(body) unless body_has_control?
+
+          emit_body_promotion
+
           emit_body_mutations do |node|
             !(n_nil?(node) && unconditional_loop?)
           end
@@ -45,6 +47,10 @@ def body_has_control?
           false
         end
 
+        def emit_body_promotion
+          emit(body) if !body_has_control? && fix_me_argument_need_to_be_empty_on_block)
+        end
+
         def mutate_body_receiver
           return if n_lambda?(send) || !n_send?(body)
 
diff --git a/lib/mutant/subject.rb b/lib/mutant/subject.rb
index 25d1f7a40..9d128cb34 100644
--- a/lib/mutant/subject.rb
+++ b/lib/mutant/subject.rb
@@ -19,7 +19,7 @@ def mutations
           Mutation::Evil
             .from_node(subject: self, node: wrap_node(mutant))
             .either(
-              ->(validation) { $stderr.puts("generated invalid mutation: #{validation.report}") },
+              ->(validation) { report_violation(mutant, validation) },
               aggregate.method(:<<)
             )
         end
@@ -27,6 +27,14 @@ def mutations
     end
     memoize :mutations
 
+    def report_violation(mutant, validation)
+      mutation = Mutation::Evil.new(subject: self, node: wrap_node(mutant), source: validation.original_source.from_right)
+      puts mutation.diff.colorized_diff
+      puts validation.report
+      fail
+    end
+    private :report_violation
+
     def inline_disabled?
       config.inline_disable
     end
diff --git a/meta/block.rb b/meta/block.rb
index 9e6ba6c61..ecbcd71b8 100644
--- a/meta/block.rb
+++ b/meta/block.rb
@@ -20,26 +20,16 @@
   singleton_mutations
   mutation 'foo'
   mutation 'foo { |a, b| raise }'
-  mutation 'foo { |a, _b| }'
-  mutation 'foo { |_a, b| }'
-  mutation 'foo { |a, | }'
-  mutation 'foo { |b, | }'
-  mutation 'foo { || }'
 end
 
 Mutant::Meta::Example.add :block do
   source 'foo { |(a, b), c| }'
 
   singleton_mutations
-  mutation 'foo { || }'
   mutation 'foo { |a, b, c| }'
   mutation 'foo { |(a, b), c| raise }'
   mutation 'foo { |(a), c| }'
   mutation 'foo { |(b), c| }'
-  mutation 'foo { |c, | }'
-  mutation 'foo { |(_a, b), c| }'
-  mutation 'foo { |(a, _b), c| }'
-  mutation 'foo { |(a, b), _c| }'
   mutation 'foo'
 end
 
@@ -56,22 +46,11 @@
   mutation 'foo {}'
 end
 
-Mutant::Meta::Example.add :block do
-  source 'foo { |_a| }'
-
-  singleton_mutations
-  mutation 'foo { || }'
-  mutation 'foo { |_a| raise }'
-  mutation 'foo'
-end
-
 Mutant::Meta::Example.add :block do
   source 'foo { |a| }'
 
   singleton_mutations
-  mutation 'foo { || }'
   mutation 'foo { |a| raise }'
-  mutation 'foo { |_a| }'
   mutation 'foo'
 end
 
diff --git a/meta/block_pass.rb b/meta/block_pass.rb
index e28e3310c..368913177 100644
--- a/meta/block_pass.rb
+++ b/meta/block_pass.rb
@@ -44,8 +44,6 @@
   Mutant::Meta::Example.add :block_pass do
     source 'def foo(a, &); foo(&); end'
 
-    mutation 'def foo(&); foo(&); end'
-    mutation 'def foo(_a, &); foo(&); end'
     mutation 'def foo(a, &); end'
     mutation 'def foo(a, &); foo; end'
     mutation 'def foo(a, &); nil; end'
diff --git a/meta/blockarg.rb b/meta/blockarg.rb
index 03044e7b2..0966d15bb 100644
--- a/meta/blockarg.rb
+++ b/meta/blockarg.rb
@@ -5,6 +5,5 @@
 
   singleton_mutations
   mutation 'foo { |&bar| raise }'
-  mutation 'foo {}'
   mutation 'foo'
 end
diff --git a/meta/def.rb b/meta/def.rb
index 01c3ae9c5..ad4a35a4a 100644
--- a/meta/def.rb
+++ b/meta/def.rb
@@ -68,17 +68,6 @@
 Mutant::Meta::Example.add :def do
   source 'def foo(a, b); end'
 
-  # Deletion of each argument
-  mutation 'def foo(a); end'
-  mutation 'def foo(b); end'
-
-  # Deletion of all arguments
-  mutation 'def foo; end'
-
-  # Rename each argument
-  mutation 'def foo(_a, b); end'
-  mutation 'def foo(a, _b); end'
-
   # Mutation of body
   mutation 'def foo(a, b); raise; end'
   mutation 'def foo(a, b); super; end'
@@ -87,51 +76,23 @@
 Mutant::Meta::Example.add :def do
   source 'def foo(a, b = nil); true; end'
 
-  mutation 'def foo(_a, b = nil); true; end'
   mutation 'def foo(a, b = nil); end'
-  mutation 'def foo; true; end'
   mutation 'def foo(a, b = nil); raise; end'
   mutation 'def foo(a, b = nil); false; end'
-  mutation 'def foo(a); true; end'
   mutation 'def foo(a, b = nil); b = nil; true; end'
-  mutation 'def foo(b = nil); true; end'
-  mutation 'def foo(a, _b = nil); true; end'
   mutation 'def foo(a, b); true; end'
   mutation 'def foo(a, b = nil); super; end'
 end
 
-Mutant::Meta::Example.add :def, :arg do
-  source 'def foo(_unused); end'
-
-  mutation 'def foo(_unused); raise; end'
-  mutation 'def foo; end'
-  mutation 'def foo(_unused); super; end'
-end
-
-Mutant::Meta::Example.add :def do
-  source 'def foo(_unused = true); end'
-
-  mutation 'def foo(_unused = false); end'
-  mutation 'def foo(_unused = true); raise; end'
-  mutation 'def foo(_unused); end'
-  mutation 'def foo; end'
-  mutation 'def foo(_unused = true); super; end'
-end
-
 Mutant::Meta::Example.add :def do
   source 'def foo(a = 0, b = 0); end'
-  mutation 'def foo(a = 0, _b = 0); end'
-  mutation 'def foo(_a = 0, b = 0); end'
   mutation 'def foo(a = 0, b = 1); end'
   mutation 'def foo(a = 0, b = -1); end'
   mutation 'def foo(a = 0, b = nil); end'
   mutation 'def foo(a = -1, b = 0); end'
   mutation 'def foo(a = nil, b = 0); end'
   mutation 'def foo(a = 1, b = 0); end'
-  mutation 'def foo(a = 0); end'
-  mutation 'def foo(b = 0); end'
   mutation 'def foo(a, b = 0); end'
-  mutation 'def foo; end'
   mutation 'def foo(a = 0, b = 0); a = 0; end'
   mutation 'def foo(a = 0, b = 0); b = 0; end'
   mutation 'def foo(a = 0, b = 0); raise; end'
@@ -142,9 +103,7 @@
   source 'def foo(a = true); end'
 
   mutation 'def foo(a); end'
-  mutation 'def foo(); end'
   mutation 'def foo(a = false); end'
-  mutation 'def foo(_a = true); end'
   mutation 'def foo(a = true); raise; end'
   mutation 'def foo(a = true); a = true; end'
   mutation 'def foo(a = true); super; end'
@@ -172,17 +131,6 @@
 Mutant::Meta::Example.add :def do
   source 'def self.foo(a, b); end'
 
-  # Deletion of each argument
-  mutation 'def self.foo(a); end'
-  mutation 'def self.foo(b); end'
-
-  # Deletion of all arguments
-  mutation 'def self.foo; end'
-
-  # Rename each argument
-  mutation 'def self.foo(_a, b); end'
-  mutation 'def self.foo(a, _b); end'
-
   # Mutation of body
   mutation 'def self.foo(a, b); raise; end'
   mutation 'def self.foo(a, b); super; end'
@@ -278,10 +226,8 @@
 Mutant::Meta::Example.add :def do
   source 'def foo(a, ...); end'
 
-  mutation 'def foo(_a, ...); end'
   mutation 'def foo(a, ...); raise; end'
   mutation 'def foo(a, ...); super; end'
-  mutation 'def foo(...); end'
 end
 
 Mutant::Meta::Example.add :def, :send do
diff --git a/meta/kwarg.rb b/meta/kwarg.rb
index 2a4f81463..9a4e7f95d 100644
--- a/meta/kwarg.rb
+++ b/meta/kwarg.rb
@@ -3,8 +3,6 @@
 Mutant::Meta::Example.add :kwarg do
   source 'def foo(bar:); end'
 
-  mutation 'def foo; end'
   mutation 'def foo(bar:); raise; end'
   mutation 'def foo(bar:); super; end'
-  mutation 'def foo(_bar:); end'
 end
diff --git a/meta/kwoptarg.rb b/meta/kwoptarg.rb
index 7c729e6c6..c8fdf0b48 100644
--- a/meta/kwoptarg.rb
+++ b/meta/kwoptarg.rb
@@ -3,10 +3,8 @@
 Mutant::Meta::Example.add :kwarg do
   source 'def foo(bar: baz); end'
 
-  mutation 'def foo; end'
   mutation 'def foo(bar: baz); raise; end'
   mutation 'def foo(bar: baz); super; end'
   mutation 'def foo(bar: nil); end'
   mutation 'def foo(bar:); end'
-  mutation 'def foo(_bar: baz); end'
 end
diff --git a/meta/procarg_zero.rb b/meta/procarg_zero.rb
index ffd7fee18..18356ec94 100644
--- a/meta/procarg_zero.rb
+++ b/meta/procarg_zero.rb
@@ -5,17 +5,6 @@
 
   singleton_mutations
   mutation 'foo { |a| raise }'
-  mutation 'foo { |_a| }'
-  mutation 'foo { }'
-  mutation 'foo'
-end
-
-Mutant::Meta::Example.add :procarg0 do
-  source 'foo { |_a| }'
-
-  singleton_mutations
-  mutation 'foo { |_a| raise }'
-  mutation 'foo { }'
   mutation 'foo'
 end
 
@@ -24,8 +13,6 @@
 
   singleton_mutations
   mutation 'foo { |(a)| raise }'
-  mutation 'foo { |(_a)| }'
-  mutation 'foo { }'
   mutation 'foo'
 end
 
@@ -35,9 +22,6 @@
   singleton_mutations
   mutation 'foo { |a, b| }'
   mutation 'foo { |(a, b)| raise }'
-  mutation 'foo { |(_a, b)| }'
-  mutation 'foo { |(a, _b)| }'
-  mutation 'foo { }'
   mutation 'foo'
 end
 
@@ -46,7 +30,6 @@
 
   singleton_mutations
   mutation 'foo { |(*)| raise }'
-  mutation 'foo { }'
   mutation 'foo'
 end
 
@@ -56,7 +39,5 @@
   singleton_mutations
   mutation 'foo { |a, (*)| }'
   mutation 'foo { |(a, (*))| raise }'
-  mutation 'foo { |(_a, (*))| }'
-  mutation 'foo { }'
   mutation 'foo'
 end