[Fix #948] Avoid auto-correction changing code meaning in Blocks

Re-parse and check that the AST hasn't changed when doing auto-
correct in Blocks cop.

CHANGELOG.md

@@ -14,6 +14,7 @@
 * [#952](https://github.com/bbatsov/rubocop/pull/952): Handle implicit receiver in `StringConversionInInterpolation`. ([@bbatsov][])
 * [#956](https://github.com/bbatsov/rubocop/pull/956): Apply `ClassMethods` check only on `class`/`module` bodies. ([@bbatsov][])
 * [#945](https://github.com/bbatsov/rubocop/issues/945): Fix SpaceBeforeFirstArg cop for multiline argument and exclude assignments. ([@cschramm][])
+* [#948](https://github.com/bbatsov/rubocop/issues/948): `Blocks` cop avoids auto-correction if it would introduce a semantic change. ([@jonas054][])
 
 ## 0.20.0 (02/04/2014)
 

lib/rubocop/cop/style/blocks.rb

@@ -36,7 +36,23 @@ def on_block(node)
         end
 
         def autocorrect(node)
-          @corrections << lambda do |corrector|
+          c = correction(node)
+          new_source = rewrite_node(node, c)
+
+          # Make the correction only if it doesn't change the AST.
+          @corrections << c if node == SourceParser.parse(new_source).ast
+        end
+
+        private
+
+        def rewrite_node(node, correction)
+          processed_source = SourceParser.parse(node.loc.expression.source)
+          c = correction(processed_source.ast)
+          Corrector.new(processed_source.buffer, [c]).rewrite
+        end
+
+        def correction(node)
+          lambda do |corrector|
             b, e = node.loc.begin, node.loc.end
             if b.is?('{')
               # If the left brace is immediately preceded by a word character,
@@ -53,8 +69,6 @@ def autocorrect(node)
           end
         end
 
-        private
-
         def get_block(node)
           case node.type
           when :block

spec/rubocop/cop/style/blocks_spec.rb

@@ -27,6 +27,12 @@
     expect(new_source).to eq('block { |x| }')
   end
 
+  it 'does not auto-correct do-end if {} would change the meaning' do
+    src = "s.subspec 'Subspec' do |sp| end"
+    new_source = autocorrect_source(cop, src)
+    expect(new_source).to eq(src)
+  end
+
   context 'when there are braces around a multi-line block' do
     it 'registers an offense in the simple case' do
       inspect_source(cop, ['each { |x|',