[ruby-core:120635] [Ruby master Bug#21031] Incompatibility with prism and parse.y when eval'ing unnamed forwarding variables

Issue #21031 has been updated by jeremyevans0 (Jeremy Evans).


Eregon (Benoit Daloze) wrote in #note-1:
> @jeremyevans0 Isn't this parse.y behavior breaking the optimization to forward `*/**/&` efficiently (and maybe `...` too)?
> It means static analysis can't reason about where the `*/**/&/...` are used.

For `*` and `**`, it shouldn't break the allocationless anonymous splat forwarding optimization.  The optimization just makes the parameter not allocate an array/hash, regardless of the content of the method, since you can only splat the arguments, not access them directly.

For `&`, there is no optimization for anonymous block arguments.  The block argument optimization to avoid Proc allocation was `getblockparamproxy` in Ruby 2.5, and applies to both named and anonymous blocks, and I believe works fine in `eval` (maybe @ko1 can confirm).

For `...`, that never used allocationless anonymous splat forwarding in a shipped version of Ruby, only temporarily during the 3.4 development cycle.  @tenderlovemaking applied a separate generic argument forwarding optimization (commit:cdf33ed5f37f9649c482c3ba1d245f0d80ac01ce).  I assume it works inside `eval`, but @tenderlovemaking would know better than I.


----------------------------------------
Bug #21031: Incompatibility with prism and parse.y when eval'ing unnamed forwarding variables
https://bugs.ruby-lang.org/issues/21031#change-111464

* Author: ksss (Yuki Kurihara)
* Status: Open
* ruby -v: ruby 3.5.0dev (2025-01-13T03:12:28Z master 384e6945ac) +PRISM [arm64-darwin23]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
## Case 1

```rb
# t.rb
def foo(*)
  eval("p(*)")
end

foo(1)
```

```
$ ruby --parser=prism t.rb
t.rb:4:in 'Kernel#eval': (eval at t.rb:4):1: syntax error found (SyntaxError)
> 1 | p(*)
    |   ^ unexpected `*`; no anonymous rest parameter

	from t.rb:4:in 'Object#foo'
	from t.rb:7:in '<main>'
```

```
$ ruby --parser=parse.y t.rb
1
```

## Case 2

```rb
# t.rb
def foo(**)
  eval("p(**)")
end

foo(a: 1)
```

```
$ ruby --parser=prism t.rb
t.rb:4:in 'Kernel#eval': (eval at t.rb:4):1: syntax error found (SyntaxError)
> 1 | p(**)
    |   ^~ unexpected `**`; no anonymous keyword rest parameter

	from t.rb:4:in 'Object#foo'
	from t.rb:7:in '<main>'
```

```
$ ruby --parser=parse.y t.rb
{a: 1}
```

## Case 3

```rb
# t.rb
def foo(&)
  eval("p(&)")
end

foo{}
```

```
$ ruby --parser=prism t.rb
t.rb:4:in 'Kernel#eval': (eval at t.rb:4):1: syntax error found (SyntaxError)
> 1 | p(&)
    |   ^ unexpected `&`; no anonymous block parameter

	from t.rb:4:in 'Object#foo'
	from t.rb:7:in '<main>'
```

```
$ ruby --parser=parse.y t.rb

```

## Case 4

```rb
# t.rb
def foo(...)
  eval("p(...)")
end

foo(1, 2, 3)
```

```
$ ruby --parser=prism t.rb
t.rb:4:in 'Kernel#eval': (eval at t.rb:4):1: syntax error found (SyntaxError)
> 1 | p(...)
    |   ^~~ unexpected ... when the parent method is not forwarding

	from t.rb:4:in 'Object#foo'
	from t.rb:7:in '<main>'
```

```
$ ruby --parser=parse.y t.rb
1
2
3
```

I expect to the parse.y behavior.



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