Uninitialized variables/fields
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Simply "zero-initializing" does not work for some types of data:
- non-nullable refs
- integers with ranges
- other kinds of constraints?

Simplest solution: Always require initialization:
- in scalars
- in structs/arrays
- in return values (e.g. returning a struct)

BUT: we want to:
- not require large arrays to be initialized
- track uninitialized data, rather than just setting it to a default value