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Tweak wording for consistency #5

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Feb 20, 2025
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Tweak wording for consistency #5

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19 changes: 10 additions & 9 deletions proposals/custom-rtts/Overview.md
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@ This proposal primarily provides mechanisms to:

1. Save memory in WasmGC structs by allowing data associated with source types
(e.g. static fields, vtables, itables, etc.)
to be stored alongside the engine-managed runtime type information (RTT)
to be accessed alongside the engine-managed runtime type information (RTT)
in a custom descriptor object for their corresponding WebAssembly types.
Rather than using a user-controlled reference field
to point to this type-associated information,
Expand Down Expand Up @@ -34,7 +34,7 @@ not the custom descriptor itself.
## Custom Descriptor Definitions

Custom descriptor types are defined struct types with a `describes` clause saying
what other defined struct type they contain runtime type information for.
what other defined struct type they provide runtime type information for.
Similarly, struct types that use custom descriptors have a `descriptor` clause saying
what type their custom descriptors have.

Expand Down Expand Up @@ -152,8 +152,9 @@ The latter two rules,
governing types with or without `describes` clauses,
are necessary to ensure subtypes have layouts compatible with their supertypes.
Custom descriptor types (i.e. those with `describes` clauses)
have different layouts than other structs because their user-controlled fields
are laid out after the engine-managed RTT for the type they describe.
may have different layouts than other structs because their user-controlled fields
might be laid out after the engine-managed RTT for the type they describe.
(But this is just one possible implementation that we specifically want to allow.)

```wasm
(rec
Expand Down Expand Up @@ -267,16 +268,16 @@ To facilitate that we introduce exact reference types,
which are inhabited by references to a particular heap type (and possibly the null value),
but not any of the heap type's strict subtypes.

Exact reference types can be nullable with the form `(ref exact null ht)`
Exact reference types can be nullable with the form `(ref null exact ht)`
or non-nullable with the form `(ref exact ht)`.

For any heap type `ht`:

```
(ref ht) <: (ref null ht)
(ref exact null ht) <: (ref null ht)
(ref null exact ht) <: (ref null ht)
(ref exact ht) <: (ref ht)
(ref exact ht) <: (ref exact null ht)
(ref exact ht) <: (ref null exact ht)
```

Furthermore, to ensure that each type hierarchy remains a lattice,
Expand All @@ -300,8 +301,8 @@ as having "hidden" subtypes of `any` and `extern`,
respectively.
This preserves our ability to assign them more specific types in the future.

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

Expand Down