[ruby-core:113991] [Ruby master Feature#19708] Support `attr_reader :foo?`

Issue #19708 has been updated by AMomchilov (Alexander Momchilov).


Hey Jean & Benoit, thanks for finding those prior discussions. I tried searching for them before posting, but the question mark makes it really hard to find anything relevant, haha

I think the use case is clear: defining reader methods that are both fast (`VM_METHOD_TYPE_IVAR`) _and_ idiomatic (`#foo?` over `#is_foo`).

Today you're forced to choose one or the other:
* Either you get idomatic naming with `def foo? = @foo`, but a slower `VM_METHOD_TYPE_ISEQ` method,
* Or you get a faster method with `attr_reader :is_foo`, but lose the idomatic naming.

With the proposed change, you can get the best of both worlds.

There's also the case for convenience: you no longer have to split your attribute readers into two groups:

```ruby
class C
  attr_reader :a, :c, :d, :f # Group 1: non-predicate methods can use attr_reader

  def initialize
    # ...
  end

  # ... other stuff

  # Group 2: predicate methods need explicit `def`s
  def b? = @b
  def e? = @e
end
```

> I think a PR/patch for this would help to make progress (it might not be trivial).

I took a cursory look: I think it'll definitely be a challenge, but I think it's something to take on.

Ideally, I'd like to first get some sort of tentative interest, to make sure I don't put effort into a PR that's dead-on-arrival :D

----------------------------------------
Feature #19708: Support `attr_reader :foo?`
https://bugs.ruby-lang.org/issues/19708#change-103649

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
* Priority: Normal
----------------------------------------
Creating reader methods with `attr_reader` is preferable over defining them by hand, not only because it's more convenient, but also because it hits a fast path in MRI (see `VM_METHOD_TYPE_IVAR`).

Since local and instance variables can't end with `?`, you can't use `attr_reader` to define predicate-style methods for boolean values, like:

```ruby
class Person
  attr_reader :name, :age, :active? # invalid attribute name `active?' (NameError)
  
  def initialize
    @name = "Bob"
    @age = 30
    @active = true
  end
end
```

It would be nice if `attr_reader` (and friends) could behave like:

```ruby
def active? = @active
```

(`attr_writer` and `attr_accessor` couldn't do the same, since `def active?=(new_value)` isn't valid, so they'd probably stick with `def active=(new_value)`)



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