[ruby-core:116876] [Ruby master Feature#19979] Allow methods to declare that they don't accept a block via `&nil`

Issue #19979 has been updated by Eregon (Benoit Daloze).


Is there any reason not to do #15554 instead?
This proposal requires manual effort to annotate methods, which feels to me very un-Ruby-like. #15554 is automatic.

Regarding `yield in eval` that's already discussed in #15554 and requiring to use a named block seems reasonable, no?

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Feature #19979: Allow methods to declare that they don't accept a block via `&nil`
https://bugs.ruby-lang.org/issues/19979#change-106911

* Author: ufuk (Ufuk Kayserilioglu)
* Status: Open
* Priority: Normal
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## Abstract

This feature proposes new syntax to allow methods to explicitly declare that they don't accept blocks, and makes passing of a block to such methods an error.

## Background

In #15554, it was proposed to automatically detect methods that do not use the block passed to them, and to error if a block was passed to such methods. As far as I can tell, it was later on closed since #10499 solved a large part of the problem.

That proposal has, as part of [a dev meeting discussion](https://github.com/ruby/dev-meeting-log/blob/b4357853c03dfe71b6eab320d5642d463854f50f/2019/DevMeeting-2019-01-10.md?plain=1#L110-L120), a proposal from @matz to allow methods to use `&nil` to explicitly declare that they don't accept a block. At the time, the proposal was trying to solve a bigger problem, so this sub-proposal was never considered seriously. However, notes in the proposal say:
> It is explicit, but it is tough to add this `&nil` parameter declaration to all of methods (do you want to add it to `def []=(i, e, &nil)`?). (I agree `&nil` is valuable on some situations)

This proposal extracts that sub-proposal to make this a new language feature.

## Proposal

In Ruby, it is always valid for the caller to pass a block to a method call, even if the callee is not expecting a block to be passed. This leads to subtle user errors, where the author of some code assumes a method call uses a block, but the block passed to the method call is silently ignored.

The proposal is to introduce `&nil` at method declaration sites to mean "This method does not accept a block". This is symmetric to the ability to pass `&nil` at call sites to mean "I am not passing a block to this method call", which is sometimes useful when making `super` calls (since blocks are always implicitly passed).

Explicitly, the proposal is to make the following behaviour be a part of Ruby:
```ruby
def find(item = nil, &nil)
  # some implementation that doesn't call `yield` or `block_given?`
end

find { |i| i == 42 }
# => ArgumentError: passing block to the method `find' that does not accept a block.
```

## Implementation

I assume the implementation would be a grammar change to make `&nil` valid at method declaration sites, as well as raising an `ArgumentError` for methods that are called with a block but are declared with `&nil`.

## Evaluation

Since I don't have an implementation, I can't make a proper evaluation of the feature proposal. However, I would expect the language changes to be minimal with no runtime costs for methods that don't use the `&nil` syntax.

## Discussion

This proposal has much smaller scope than #15554 so that the Ruby language can start giving library authors the ability to explicitly mark their methods as not accepting a block. This is fully backward compatible, since it is an opt-in behaviour and not an opt-out one.

Future directions after this feature proposal could be a way to signal to the VM that any method in a file that doesn't explicitly use `yield`/`block_given?` or explicitly declared a block parameter should be treated as not accepting a block. This can be done via some kind of pragma similar to `frozen_string_literal`, or through other means. However, such future directions are beyond the scope of this proposal.

## Summary

Adding the ability for methods to declare that they don't accept a block will make writing code against such methods safer and more resilient, and will prevent silently ignored behaviour that is often hard to catch or troubleshoot.



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