Make choice work for alternatives without parameters #4815
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danakj
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This adds support for choice types at a similar level to that of a C enum, where each alternative has a name but no additional data/parameters attached to it. We generate a TODO diagnostic if parameters are specified. Because there's no extra data, the storage is a simple unsigned integer discriminant of the smallest possible size. A choice without any alternatives is not constructible. A choice with a single alternative is, and has an empty tuple in place of a discriminant since it has only one state. The empty tuple is used to make the class non-constructible. This can be improved. Each alternative is turned into a let binding on the choice that is a value of the choice with that alternative set as the active one in the discriminant. This isn't possible to write in user code with a class right now, since the let binding has the same type as the choice (which is a class) it is within. It's possible to generate it in semir however by adding the binding after the class is marked complete.
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This adds support for choice types at a similar level to that of a C
enum, where each alternative has a name but no additional
data/parameters attached to it. We generate a TODO diagnostic if
parameters are specified.
Because there's no extra data, the storage is a simple unsigned integer
discriminant of the smallest possible size.
A choice without any alternatives is not constructible. A choice with a
single alternative is, and has an empty tuple in place of a discriminant
since it has only one state. The empty tuple is used to make the class
non-constructible. This can be improved.
Each alternative is turned into a let binding on the choice that is a
value of the choice with that alternative set as the active one in the
discriminant. This isn't possible to write in user code with a class
right now, since the let binding has the same type as the choice
(which is a class) it is within. It's possible to generate it in semir
however by adding the binding after the class is marked complete.