Add an RFC for ShapeCastable.from_bits and lib.data.Const.

[wanda-phi]

Rendered

[whitequark]

Would from_bits ever need to know the size of the original value?

[wanda-phi]

Would from_bits ever need to know the size of the original value?

It is called on ShapeCastable, which has the length available (since it has to implement as_shape()).

[whitequark]

Possible naming for LiteralView: lib.data.Const.

[whitequark]

We have discussed this RFC on the 2024-03-04 weekly meeting. The disposition was to merge, with the following changes:

• lib.data.LiteralView is renamed to lib.data.Const and made immutable;
• ShapeCastable.from_bits is allowed to return any type as long as ShapeCastable.const can turn it back into a const-castable value;
  • Additional unresolved question: With the same bit pattern or equivalent bit pattern? This is important if we ever try to roundtrip a NaN through Python's float.
• lib.data.Layout.const is changed to return a lib.data.Const;
• Mentioning that instead of mutating a lib.data.Const in simulation, one can set the field directly.

[whitequark]

Thank you!