Improve parsing error on EOF

[pocke]

This pull request improves parsing error on EOF.

Problem

Currently rbs parse command raises an error when the target file has a syntax error that is caused by an EOF.
For example:

# test.rbs
class Foo

$ rbs parse test.rbs
/path/to/lib/rbs/cli.rb:741:in `rescue in block (2 levels) in run_parse': undefined method `location' for "$":String (NoMethodError)
	from /path/to/lib/rbs/cli.rb:739:in `block (2 levels) in run_parse'
	from /path/to/lib/rbs/environment_loader.rb:93:in `each_file'
	from /path/to/lib/rbs/cli.rb:738:in `block in run_parse'
	from /path/to/lib/rbs/cli.rb:736:in `each'
	from /path/to/lib/rbs/cli.rb:736:in `run_parse'
	from /path/to/lib/rbs/cli.rb:113:in `run'
	from /path/to/exe/rbs:7:in `<top (required)>'
	from /path2/to/bin/rbs:23:in `load'
	from /path2/to/bin/rbs:23:in `<main>'
parser.y:1380:in `on_error': parse error on value: "$" ($end) (RBS::Parser::SyntaxError)
	from (eval):3:in `_racc_do_parse_c'
	from (eval):3:in `do_parse'
	from parser.y:1110:in `parse_signature'
	from /path/to/lib/rbs/cli.rb:739:in `block (2 levels) in run_parse'
	from /path/to/lib/rbs/environment_loader.rb:93:in `each_file'
	from /path/to/lib/rbs/cli.rb:738:in `block in run_parse'
	from /path/to/lib/rbs/cli.rb:736:in `each'
	from /path/to/lib/rbs/cli.rb:736:in `run_parse'
	from /path/to/lib/rbs/cli.rb:113:in `run'
	from /path/to/exe/rbs:7:in `<top (required)>'
	from /path2/to/bin/rbs:23:in `load'
	from /path2/to/bin/rbs:23:in `<main>'

Because RBS::Parser::SyntaxError expects a LocatedValue as error_value, but actually, it receives a String, which is "$", on this case.

Solution

Handle EOF properly in next_token method.

Currently it just returns a nil on EOF, so Racc can't be aware of the LocatedValue for EOF.

next_token can returns [false, any_value] on EOF.

The final symbol (End of file) must be false.

https://github.com/ruby/racc/blob/aff6613fb00e350864dc80cbe513cf4a70d3744b/lib/racc/parser.rb#L276

So I changed the returned value of next_token to [false, LocatedValue].

rbs parse command will display the following.

$ rbs parse test.rbs
test.rbs:2:9: parse error on value: ($end)

[soutaro]

Great improvement! 👏