[ruby/syntax_suggest] Fix

https://github.com/ruby/syntax_suggest/pull/187 Handle if/else with
empty/comment
line

Reported in #187 this code:

```
class Foo
  def foo
    if cond?
      foo
    else
      # comment
    end
  end

  # ...

  def bar
    if @recv
    end_is_missing_here
  end
end
```

Triggers an incorrect suggestion:

```
Unmatched keyword, missing `end' ?
   1  class Foo
   2    def foo
>  3      if cond?
>  5      else
   8    end
  16  end
```

Part of the issue is that while scanning we're using newlines to determine when to stop and pause. This is useful for determining logically smaller chunks to evaluate but in this case it causes us to pause before grabbing the "end" that is right below the newline. This problem is similar to https://github.com/ruby/syntax_suggest/pull/179.

However in the case of expanding same indentation "neighbors" I want to always grab all empty values at the end of the scan. I don't want to do that when changing indentation levels as it affects scan results.

There may be some way to normalize this behavior between the two, but the tests really don't like that change.

To fix this issue for expanding against different indentation I needed a way to first try and grab any additional newlines with the ability to rollback that guess. In #192 I experimented with decoupling scanning from the AroundBlockScan logic. It also added the ability to take a snapshot of the current scanner and rollback to prior changes.

With this ability in place now we:

- Grab extra empties before looking at the above/below line for the matching keyword/end statement
- If there's a match, grab it
- If there's no match, discard the newlines we picked up in the evaluation

That solves the issue.

https://github.com/ruby/syntax_suggest/commit/513646b912

lib/syntax_suggest/around_block_scan.rb

@@ -181,7 +181,7 @@ module SyntaxSuggest
           lines << line if line.is_kw? || line.is_end?
         }
       )
-      @scanner.try_rollback
+      @scanner.stash_changes
       lines
     end
 
@@ -240,7 +240,7 @@ module SyntaxSuggest
           true
         }
       )
-      @scanner.try_rollback
+      @scanner.stash_changes
       self
     end
 
@@ -275,26 +275,34 @@ module SyntaxSuggest
 
       return self if kw_count == end_count # nothing to balance
 
+      @scanner.commit_if_changed
+      scan_while { |line| line.empty? }
+
       # More ends than keywords, check if we can balance expanding up
       next_up = @scanner.next_up
       next_down = @scanner.next_down
       if (end_count - kw_count) == 1 && next_up
-        if next_up.is_kw? && next_up.indent >= @orig_indent
+        if next_up.is_kw? && next_up.indent >= @target_indent
           @scanner.scan(
             up: ->(line, _, _) { line == next_up },
             down: ->(line, _, _) { false }
           )
+          @scanner.commit_if_changed
         end
 
       # More keywords than ends, check if we can balance by expanding down
       elsif (kw_count - end_count) == 1 && next_down
-        if next_down.is_end? && next_down.indent >= @orig_indent
+        if next_down.is_end? && next_down.indent >= @target_indent
           @scanner.scan(
             up: ->(line, _, _) { false },
-            down: ->(line) { line == next_down }
+            down: ->(line, _, _) { line == next_down }
           )
+          @scanner.commit_if_changed
         end
       end
+
+      @scanner.stash_changes
+
       self
     end
 

lib/syntax_suggest/block_expand.rb

@@ -61,11 +61,14 @@ module SyntaxSuggest
     # they can expand to capture more code up and down). It does this conservatively
     # as there's no undo (currently).
     def expand_indent(block)
-      AroundBlockScan.new(code_lines: @code_lines, block: block)
+      now = AroundBlockScan.new(code_lines: @code_lines, block: block)
         .force_add_hidden
         .stop_after_kw
         .scan_adjacent_indent
-        .code_block
+
+      now.lookahead_balance_one_line
+
+      now.code_block
     end
 
     # A neighbor is code that is at or above the current indent line.

lib/syntax_suggest/scan_history.rb

@@ -17,13 +17,20 @@ module SyntaxSuggest
     def commit_if_changed
       if changed?
         @history << CodeBlock.new(lines: @code_lines[before_index..after_index])
-        refresh_index
       end
 
       self
     end
 
-    def try_rollback
+    # Discards any changes that have not been committed
+    def stash_changes
+      refresh_index
+    end
+
+    # Discard changes that have not been committed and revert the last commit
+    #
+    # Cannot revert the first commit
+    def revert_last_commit
       if @history.length > 1
         @history.pop
         refresh_index

spec/syntax_suggest/integration/syntax_suggest_spec.rb

@@ -204,5 +204,36 @@ module SyntaxSuggest
         > 4  end
       EOM
     end
+
+    it "empty else" do
+      source = <<~'EOM'
+        class Foo
+          def foo
+            if cond?
+              foo
+            else
+
+            end
+          end
+
+          # ...
+
+          def bar
+            if @recv
+            end_is_missing_here
+          end
+        end
+      EOM
+
+      io = StringIO.new
+      SyntaxSuggest.call(
+        io: io,
+        source: source
+      )
+      out = io.string
+      expect(out).to include(<<~EOM)
+        end_is_missing_here
+      EOM
+    end
   end
 end