[ruby-core:78115] [Ruby trunk Feature#12931] Add support for Binding#instance_eval

Issue #12931 has been updated by Nobuyoshi Nakada.

Description updated

Since local variables and constants depend on the context, it won't be able to be "cached," as a `Proc` compiled other place.
So I think it isn't worth.

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Feature #12931: Add support for Binding#instance_eval
https://bugs.ruby-lang.org/issues/12931#change-61480

* Author: Samuel Williams
* Status: Open
* Priority: Normal
* Assignee: 
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Many people would probably like to use `binding.instance_eval` when executing templates. The reason for this is because `binding.eval` is slow.

The use case is template rendering, e.g. systems like ERB.

In my template renderer, I did have

```ruby
			if Binding === scope
				# Slow code path, evaluate the code string in the given binding (scope).
				scope.eval(code, @buffer.path)
			else
				# Faster code path, use instance_eval on a compiled Proc.
				scope.instance_eval(&to_proc)
			end
```

The binding path is several orders of magnitude slower because the code is a string and must be parsed, compiled, etc. But, `to_proc` path can be cached.

In terms of duck typing, it would be nice if binding implemented `#instance_eval` and would allow passing a block.



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