LGTM to experiment from M141 to M147 inclusive.
On 8/13/25 4:40 p.m., 'Thomas Lively' via blink-dev wrote:
Good question! The first toolchain to support custom descriptors is
j2cl <https://github.com/google/j2cl>, which is an open source
Java-to-JS/Wasm compiler developed by Google. Developers will enable
the feature via an option
<https://github.com/google/j2cl/commit/a8cdfa7aa36d4c46e8b794139a38ccefcd99e5fb#diff-be4e99d1c21ff16eff4ab47ef7d35d297118d455e283f292745218a53d853553R147-R151> passed
to the compiler. We hope that once the feature is available via an
origin trial, other toolchains will be motivated to start
experimenting with it as well.
Note that in the initial experiments we don't expect toolchains to
expose any new developer-facing functionality based on this feature.
The use of custom descriptors will only be an internal implementation
detail in the compiler that results in less memory usage at runtime.
Later experimentation will additionally enable the JS interop features
built on top of custom descriptors, and it will vary from toolchain to
toolchain whether this enables new developer-facing functionality or
enables a performance improvement in existing functionality.
Also note that this feature will generally only be usable from
managed-memory languages that compile to WasmGC such as Java, Kotlin,
and Dart. It's unlikely that C/C++/Rust and similar languages will
ever make use of custom descriptors.
On Wed, Aug 13, 2025 at 12:24 PM Daniel Bratell <bratel...@gmail.com>
wrote:
I might have missed it, but how is this new feature going to be
used by web developers? Do you have custom C compiler for the
experiment? And do you need to do something in C to enable it? Or
whatever language people prefer.
/Daniel
On 2025-08-13 20:04, Thomas Lively wrote:
Yes, apologies. The explainer link should be the same as the
specification link:
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-descriptors/Overview.md
On Wed, Aug 13, 2025 at 7:58 AM Daniel Bratell
<bratel...@gmail.com> wrote:
The explainer link is dead, but maybe it was meant to be the
same as the Specification link?
/Daniel
On 2025-08-12 00:50, 'Thomas Lively' via blink-dev wrote:
Contact emails
tliv...@google.com, ecmzieg...@chromium.org,
jkumme...@chromium.org
Explainer
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-rtts/Overview.md
<https://github.com/WebAssembly/custom-rtts/blob/main/proposals/custom-rtts/Overview.md>
Specification
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-descriptors/Overview.md
Summary
Allows WebAssembly to store data associated with
source-level types more efficiently in new "custom
descriptor" objects. These custom descriptors can be
configured with prototypes for the WebAssembly objects of
that source-level type. This allows methods to be installed
on a WebAssembly object's prototype chain and called
directly from JS using normal method call syntax. The
prototypes and methods can be configured declaratively using
an imported builtin function.
Blink component
Blink>JavaScript>WebAssembly
<https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3EJavaScript%3EWebAssembly%22>
TAG review
None
TAG review status
Pending
Origin Trial documentation link
https://github.com/WebAssembly/custom-descriptors/blob/main/proposals/custom-descriptors/Overview.md
Risks
Interoperability and Compatibility
None
/Gecko/: No signal
/WebKit/: No signal
/Web developers/: No signals
/Other signals/:
WebView application risks
Does this intent deprecate or change behavior of existing
APIs, such that it has potentially high risk for Android
WebView-based applications?
None
Goals for experimentation
Initially we are looking to measure real-world performance,
heap usage, and OOM rate impact of just the core Wasm part
of the proposal. We want to verify that heap usage and OOM
rates will decrease as expected and that the performance
impact is negligible. If we do measure a performance impact,
that will inform additional optimization work we would need
to do in the implementation. Notably, the JS interop
features of the proposal are out of scope for this initial
experimentation. They will likely be evaluated in follow-on
experiments.
Ongoing technical constraints
None.
Debuggability
None
Will this feature be supported on all six Blink
platforms (Windows, Mac, Linux, ChromeOS, Android,
and Android WebView)?
No
Is this feature fully tested by web-platform-tests
<https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?
No
Flag name on about://flags
None
Finch feature name
None
Non-finch justification
None
Requires code in //chrome?
False
Estimated milestones
Origin trial desktop first 141
Origin trial desktop last 147
Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/6024844719947776?gate=5062088692858880
This intent message was generated by Chrome Platform Status
<https://chromestatus.com/>.
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