[ruby-core:115533] [Ruby master Feature#20024] SyntaxError subclasses

Issue #20024 has been updated by yui-knk (Kaneko Yuichiro).


# SyntaxError includes multiple errors

`SyntaxError` includes multiple errors like below, in this case two errors are included into one `SyntaxError`. Therefore it needs to consider how to handle such cases.

```ruby
begin
  eval <<~CODE
    def m
      retry
    
  CODE
rescue SyntaxError => e
  puts e.message
end

# (eval at test.rb:2):2: Invalid retry without rescue
#   retry
#   ^~~~~
# (eval at test.rb:2):3: syntax error, unexpected end-of-input, expecting `end' or dummy end
```

We need to avoid losing information to provide rich information to `SyntaxError` users. Therefore these are not options for this problem:
* Merge multiple errors into one `SyntaxError` subclass. Because this is misleading if these errors are different types of errors.
* Use only a single error and ignore following erorrs. Because information is lost.

# irb use case and error tolerance parser

Regarding the [irb use case](https://github.com/ruby/irb/blob/f86d9dbe2fc05ed62332069a27f4aacc59ba9634/lib/irb/ruby-lex.rb#L220-L267), it categorizes error , which is recoverable by adding tokens to the end, as `recoverable_error` and error, which is recoverable by deleting tokens, as `unrecoverable_error`, so that irb can determine to require more input or not.

When irb was created, error tolerance parser didn't exist. Therefore irb needs to take care of how to recover the input using `SyntaxError#message`. However it's parser responsibility to recover errors.

irb can categorize syntax error if error tolerance parser provides information how the parser recover errors.
If only single error and recovery requires only token insert operations for the last of input, it's `recoverable_error`.

By the way, does the parser always raise `SyntaxError` for invalid input? For this use case, it's better to not raise `SyntaxError` but users ask to parser to have syntax errors or not after parsing.

# Proposal

Considering these points, my proposal is defining parser interface.

* Parser can run without SyntaxError mode
* Parser provides a method to get syntax errors
* Each syntax error includes
  * message: "syntax error, unexpected end-of-input, expecting `end' or dummy end"
  * location: (1,0)-(1,1), first/last & line/column
  * operations for recovery:
    * insert / delete
    * location of recovery

----------------------------------------
Feature #20024: SyntaxError subclasses
https://bugs.ruby-lang.org/issues/20024#change-105468

* Author: kddnewton (Kevin Newton)
* Status: Open
* Priority: Normal
----------------------------------------
There are many places around the Ruby ecosystem that handle syntax errors in different ways. Some provide highlighting, others provide recovery of some form, still more provide LSP metadata. In order to provide more rich information, most of them switch on the message of the error being returned, as in:

https://github.com/ruby/irb/blob/f86d9dbe2fc05ed62332069a27f4aacc59ba9634/lib/irb/ruby-lex.rb#L220-L267

Within ruby/spec, specific error messages are required for these kinds of messages in order to support this implicit interface that syntax errors have a hidden type, which is only expressed through their message. For example:

https://github.com/ruby/spec/blob/c3206f644325c026fc5b700f0ea75ce9bd2e9d02/language/if_spec.rb#L323
https://github.com/ruby/spec/blob/c3206f644325c026fc5b700f0ea75ce9bd2e9d02/language/numbered_parameters_spec.rb#L31
https://github.com/ruby/spec/blob/c3206f644325c026fc5b700f0ea75ce9bd2e9d02/language/pattern_matching_spec.rb#L210
https://github.com/ruby/spec/blob/c3206f644325c026fc5b700f0ea75ce9bd2e9d02/language/rescue_spec.rb#L262
https://github.com/ruby/spec/blob/c3206f644325c026fc5b700f0ea75ce9bd2e9d02/language/yield_spec.rb#L196

It's not clear from these specs or from the parser itself which error messages are permanent/guaranteed versus which are changeable. Either way, relying on the error message itself as opposed to the type of the error is brittle at best.

I would like to suggest instead we implement subclasses on `SyntaxError` that would allow tools that depend on specific syntax errors to rescue those subclasses instead of parsing the message. In addition to alleviating the need to parse error messages with regex, this would also allow for the possibility that the error messages could change in the future without breaking external tooling.

Allowing these to change would allow them to be potentially enhanced or changed by other tools - for example by providing recovery information or translating them.

This is particularly important for Prism since we are getting down to individual spec failures and some of the failures are related to the fact that we have messages like `"Numbered parameter is already used in outer scope"` where the spec requires `/numbered parameter is already used in/`. Even this case-sensitivity is causing failures, which seems like we're testing the wrong thing.



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