• Timothy Gu's avatar
    Reland "[builtins] Clean up the use of class_name / ES5 [[Class]]" · 1aa51b49
    Timothy Gu authored
    This is a reland of 29c1eab9
    
    Original change's description:
    > [builtins] Clean up the use of class_name / ES5 [[Class]]
    >
    > Before ES2015, the ES spec had a [[Class]] internal slot for all
    > objects, which Object.prototype.toString() would use to figure the
    > returned string. Post-ES2015, the [[Class]] slot was removed in spec for
    > all objects, with the @@toStringTag well-known symbol the proper way to
    > change Object.prototype.toString() output.
    >
    > At the time, spec-identical handling without the use of [[Class]] was
    > implemented in V8 for all objects other than API objects, where issues
    > with the Web IDL spec [1] prevented Blink, and hence V8, to totally
    > migrate to @@toStringTag. However, since 2016 [2] Blink has been setting
    > @@toStringTag on API class prototypes to manage the
    > Object.prototype.toString() output, so the legacy [[Class]] handling in
    > V8 has not been necessary for the past couple of years.
    >
    > This CL removes the remaining legacy [[Class]] handling in
    > Object.prototype.toString(), JSReceiver::class_name(), and
    > GetConstructorName(). However, it does not remove the class_name field
    > in FunctionTemplateInfo, as it is still used for the `name` property of
    > created functions.
    >
    > This CL also cleans up other places in the codebase that still reference
    > [[Class]].
    >
    > This change should have minimal impact on web-compatibility. For the
    > change to be observable, a script must do one of the following:
    >
    > 1. delete APIConstructor.prototype[Symbol.toStringTag];
    > 2. Object.setPrototypeOf(apiObject, somethingElse);
    >
    > Before this CL, these changes will not change the apiObject.toString()
    > output. But after this CL, they will make apiObject.toString() show
    > "[object Object]" (in the first case) or the @@toStringTag of the other
    > prototype (in the latter case).
    >
    > However, both are deemed unlikely. @@toStringTag is not well-known
    > feature of JavaScript, nor does it get tampered much on API
    > constructors. In the second case, setting the prototype of an API object
    > would effectly render the object useless, as all its methods (including
    > property getters/setters) would no longer be accessible.
    >
    > Currently, @@toStringTag-based API object branding is not yet
    > implemented by other browsers. This V8 bug in particular has been an
    > impediment to standardizing toString behavior. Fixing this bug will
    > unblock [3] and lead to a better Web IDL spec, and better toString()
    > compatibility for all.
    >
    > [1]: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244
    > [2]: https://crrev.com/909c0d7d5a53c8526ded351683c65ea7d17531d4
    > [3]: https://github.com/heycam/webidl/pull/357
    >
    > Bug: chromium:793406
    > Cq-Include-Trybots: luci.chromium.try:linux-rel
    > Change-Id: Iceded24e37afa2646ec385d5018909f55b177f93
    > Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2146996
    > Commit-Queue: Timothy Gu <timothygu@chromium.org>
    > Reviewed-by: Toon Verwaest <verwaest@chromium.org>
    > Cr-Commit-Position: refs/heads/master@{#67327}
    
    Bug: chromium:793406
    Change-Id: Ia5d97bd4e1c44cadc6f18a17ffc9d06b038cf8f1
    Cq-Include-Trybots: luci.chromium.try:linux-rel
    Cq-Include-Trybots: luci.v8.try:v8_linux_blink_rel
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2163881
    Auto-Submit: Timothy Gu <timothygu@chromium.org>
    Commit-Queue: Toon Verwaest <verwaest@chromium.org>
    Reviewed-by: 's avatarToon Verwaest <verwaest@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#67361}
    1aa51b49
object-tostring-builtins.js 2.26 KB