Add support for TypedValue, a more strongly typed value

[Maaarcocr]

why?

often when working with magnus I'd like to have more information about what a Value is. When writing code that can accept and handle any value, sometimes I have the need, as an example, to implement a generic get_str_slice. Now I could just call to_s on the value, but it incurs performance overhead, as it calls back into ruby and makes a copy to save into a rust string ,even when I the value is an RString, on which I could call as_str.

how?

Introduce a new TypedValue which can be constructed from a Value. It's an enum containing all primitive types, so that developers can dispatch on the enum kind and choose to do different things depending on what the TypedValue is. I think this is better than a chain of:

if let Some(s) = RString::from_value(val) {
  ...
} else if let Some(a) = RArray::from_value(val) {
 ...
}
...

as I think:

• we access the rb_type only once and we match on it, rather than having a long list of if statements
• it's also a bit more user friendly, it's clear what primitives you can get from a TypedValue

All in all I think this would be nice to have!

why can't it be done outside of magnus?

all the from_rb_value_unchecked are pub(crate) so building this outside of magnus is impossible.

[matsadler]

Hey, thanks for this, sorry for taking a while to get back to you. I have thought about implementing something like this, and so far I've not been convinced that it's that useful, but I'm not committed to that view.

I have not actually checked it, but I have been working with the assumption that a chain of if let Some(s) = Something::from_value(val) will compile down to the same as a match. The from_value functions should all be #[inline] so the compiler should be able to see and eliminate the duplicate checks of rb_type.

I think I prefer the chain of ifs as it easily expands nicely to types like Time, Range, or Enumerator which don't have specific basic types.

I'm also unsure the best way to handle RUBY_T_UNDEF, RUBY_T_IMEMO, etc.

[matsadler]

This won't ever match, the Enumerator class's basic type is RObject.

[matsadler]

I think this should be separate Fixnum/Bignum variants to match the Ruby API.

[matsadler]

This won't ever match, the Range class's basic type is RStruct.

[matsadler]

I know they are constants, but I think these should contain Qtrue, Qfalse, Qnil as it would make it API more consistent.

I don't think there's any downside, the size of TypedValue will be the same regardless, and while containing Qfalse would prevent niche optimisation (as it is represented as 0/null), the Value variant will already prevent that optimisation (as type wise Value can be false).

[matsadler]

TypedValue is Copy so this can take self.