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Updated validation rules for struct.new #17

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Mar 18, 2025
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Updated validation rules for struct.new #17

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21 changes: 19 additions & 2 deletions proposals/custom-descriptors/Overview.md
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Expand Up @@ -305,10 +305,27 @@ As a general rule, non-nullable exact references to abstract heap types are unin

When allocating types with custom descriptors,
`struct.new` and `struct.new_default` take exact references to the descriptors
as their first operands.
as their last operands.
This makes the unsound program above invalid.

> TODO: Provide new validation rules for `struct.new` and `struct.new_default`.
```
struct.new x
C |- struct.new x : t* (ref null exact y) -> (ref exact x)
-- C.types[x] ~ descriptor y (struct (field t)*)
```

```
struct.new_default x
C |- struct.new_default x : (ref null exact y) -> (ref exact x)
-- C.types[x] ~ descriptor y (struct (field t)*)
-- defaultable(t)*
```

> Note: The descriptors could alternatively be the first operands.
> They are chosen to be the last operands here because in a hypothetical future
> where we have variants of these instructions that do not take type immediates,
> the descriptors would have to be on top of the stack to determine the type of
> the allocation. This is consistent with GC accessor instructions.

## New Instructions

Expand Down