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Shared string builtins #108

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Shared string builtins
tlively Jan 16, 2026
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Apply suggestion from @jakobkummerow
tlively Jan 16, 2026
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Add paragraph about imported strings
tlively Jan 16, 2026
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17 changes: 17 additions & 0 deletions proposals/shared-everything-threads/Overview.md
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Expand Up @@ -562,6 +562,23 @@ Waitqueue references are converted to `WebAssembly.WaitQueue` objects, described

[proposal-structs]: https://github.com/tc39/proposal-structs

#### Shared String Builtins

As mentioned above, JS strings will be able to be converted to `(ref extern)` or
`(ref (shared extern))` when entering WebAssembly. To facilitate working with shared strings,
the [string builtins][string-builtins] will each be updated to be importable with two function types:
the current function type using `extern` and a new function type that is equivalent except that 1) it
is shared, and 2) uses `(shared extern)` in place of `extern` (and also shared versions of any other
heap types, e.g. the array parameter in `fromCharCodeArray`). Mixing shared and unshared heap types
in the same signature is not allowed.

Similarly, imported string constants can also be shared. They are imported from the same configured
`importedStringModule` as non-shared string constants, but are imported as globals with type
`(ref (shared extern))` or `(ref null (shared extern))`. Notably, no spec changes are required for
this to work once strings are allowed to enter Wasm as shared extern references.

[string-builtins]: https://webassembly.github.io/spec/js-api/#builtins-js-string

#### Shared Annotations

Just like the original threads proposal exposed shared memories in the JS API, this proposal exposes
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