Handle encoding error from the parser gem

This specific exception has been added only in the most recent release.
Previously it threw an `ArgumentError`.

changelog/fix_error_for_magic_encoding_comment.md

@@ -0,0 +1 @@
+* [#289](https://github.com/rubocop/rubocop-ast/pull/289): Fix an error during parsing when encountering unknown encodings in the encoding magic comment. ([@Earlopain][])

lib/rubocop/ast/processed_source.rb

@@ -209,7 +209,7 @@ def parse(source, ruby_version, parser_engine)
 
         begin
           @buffer.source = source
-        rescue EncodingError => e
+        rescue EncodingError, Parser::UnknownEncodingInMagicComment => e
           @parser_error = e
           @ast = nil
           @comments = []

rubocop-ast.gemspec

@@ -33,7 +33,7 @@ Gem::Specification.new do |s|
     'rubygems_mfa_required' => 'true'
   }
 
-  s.add_runtime_dependency('parser', '>= 3.3.0.4')
+  s.add_runtime_dependency('parser', '>= 3.3.1.0')
 
   ##### Do NOT add `rubocop` (or anything depending on `rubocop`) here. See Gemfile
 end

spec/rubocop/ast/processed_source_spec.rb

@@ -28,6 +28,16 @@ def some_method
       end
     end
 
+    context 'when parsing code with an invalid encoding comment' do
+      let(:source) { '# encoding: foobar' }
+
+      it 'returns a parser error' do
+        expect(processed_source.parser_error).to be_a(Parser::UnknownEncodingInMagicComment)
+        expect(processed_source.parser_error.message)
+          .to include('unknown encoding name - foobar')
+      end
+    end
+
     shared_examples 'invalid parser_engine' do
       it 'raises ArgumentError' do
         expect { processed_source }.to raise_error(ArgumentError) do |e|