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Elaborate on habitability of exact references #14

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Mar 13, 2025
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Elaborate on habitability of exact references #14

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10 changes: 10 additions & 0 deletions proposals/custom-descriptors/Overview.md
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Expand Up @@ -301,6 +301,16 @@ as having "hidden" subtypes of `any` and `extern`,
respectively.
This preserves our ability to assign them more specific types in the future.

In fact, non-nullable exact references to most abstract heap types are uninhabited.
The only exceptions are:

- `(ref exact i31)` and `(ref exact (shared i31))`, which are inhabited by i31 references.
- `(ref exact exn)`, which is inhabited by exception references.
- `(ref exact (shared waitqueue))`,
which is inhabited by waitqueue references from the shared-everything threads proposal

Note that this will prevent us from ever introducing more specific subtypes of `i31`, `exn`, and `waitqueue`.

When allocating types with custom descriptors,
`struct.new` and `struct.new_default` take exact references to the descriptors
as their first operands.
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