Sync SyntaxSuggest

```
$ tool/sync_default_gems.rb syntax_suggest
```

1 files changed, 103 insertions, 0 deletions

diff --git a/lib/syntax_suggest/explain_syntax.rb b/lib/syntax_suggest/explain_syntax.rb
new file mode 100644
index 0000000000..142ed2e269
--- /dev/null
+++ b/lib/syntax_suggest/explain_syntax.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+require_relative "left_right_lex_count"
+
+module SyntaxSuggest
+  # Explains syntax errors based on their source
+  #
+  # example:
+  #
+  #   source = "def foo; puts 'lol'" # Note missing end
+  #   explain ExplainSyntax.new(
+  #     code_lines: CodeLine.from_source(source)
+  #   ).call
+  #   explain.errors.first
+  #   # => "Unmatched keyword, missing `end' ?"
+  #
+  # When the error cannot be determined by lexical counting
+  # then ripper is run against the input and the raw ripper
+  # errors returned.
+  #
+  # Example:
+  #
+  #   source = "1 * " # Note missing a second number
+  #   explain ExplainSyntax.new(
+  #     code_lines: CodeLine.from_source(source)
+  #   ).call
+  #   explain.errors.first
+  #   # => "syntax error, unexpected end-of-input"
+  class ExplainSyntax
+    INVERSE = {
+      "{" => "}",
+      "}" => "{",
+      "[" => "]",
+      "]" => "[",
+      "(" => ")",
+      ")" => "(",
+      "|" => "|"
+    }.freeze
+
+    def initialize(code_lines:)
+      @code_lines = code_lines
+      @left_right = LeftRightLexCount.new
+      @missing = nil
+    end
+
+    def call
+      @code_lines.each do |line|
+        line.lex.each do |lex|
+          @left_right.count_lex(lex)
+        end
+      end
+
+      self
+    end
+
+    # Returns an array of missing elements
+    #
+    # For example this:
+    #
+    #   ExplainSyntax.new(code_lines: lines).missing
+    #   # => ["}"]
+    #
+    # Would indicate that the source is missing
+    # a `}` character in the source code
+    def missing
+      @missing ||= @left_right.missing
+    end
+
+    # Converts a missing string to
+    # an human understandable explanation.
+    #
+    # Example:
+    #
+    #   explain.why("}")
+    #   # => "Unmatched `{', missing `}' ?"
+    #
+    def why(miss)
+      case miss
+      when "keyword"
+        "Unmatched `end', missing keyword (`do', `def`, `if`, etc.) ?"
+      when "end"
+        "Unmatched keyword, missing `end' ?"
+      else
+        inverse = INVERSE.fetch(miss) {
+          raise "Unknown explain syntax char or key: #{miss.inspect}"
+        }
+        "Unmatched `#{inverse}', missing `#{miss}' ?"
+      end
+    end
+
+    # Returns an array of syntax error messages
+    #
+    # If no missing pairs are found it falls back
+    # on the original ripper error messages
+    def errors
+      if missing.empty?
+        return RipperErrors.new(@code_lines.map(&:original).join).call.errors
+      end
+
+      missing.map { |miss| why(miss) }
+    end
+  end
+end