Implement TextDecoder.

[Ms2ger]

Spec: https://encoding.spec.whatwg.org/#interface-textdecoder.

Most of the actual decoding infrastructure should already be implemented in rust-encoding.

This includes enabling the relevant tests by adding

[encoding]
  skip: false

to tests/wpt/include.ini.

This may also include updating the mozjs and rust-mozjs dependencies to ensure we can work with typed arrays.

[KiChjang]

Alright, time to actually move from easy to less easy, lemme try and flex my muscles on this one.
I'm assuming I'll have to create new files, yes? They should be located in the components/script/dom directory?

[jdm]

Yep! See https://github.com/servo/servo/wiki/Adding-a-new-WebIDL-binding for reference.

[KiChjang]

Question... in the HTML spec it says that the TextDecoder::decode function must accept an optional BufferSource. What would the be the equivalent of BufferSource in rust-speak?

[jdm]

Good question! I've filed a spec bug about that: https://www.w3.org/Bugs/Public/show_bug.cgi?id=27979 . Googling around, I found http://heycam.github.io/webidl/#common-BufferSource, so I suggest adding that definition to the file for now.

[KiChjang]

Okay, so the question becomes "what is an ArrayBuffer in rust?"
Looking at the Servo documentation, I found std::io::Buffer. Would that suffice as an ArrayBuffer?

[jdm]

Nope - ArrayBuffer actually corresponds to a *mut JSObject, as it's a type that the JS engine provides. See #4639 for another example of a DOM implementation that uses them.

[jdm]

Well, it's uses Uint8ClampedArray, but that's just a a specialized form of ArrayBuffer.

[KiChjang]

Oh, big problem - the python script that generates the bindings threw an exception saying TypeError:: Can't handle SpiderMonkey Interface arguments yet whenever I tried to use ArrayBuffer or ArrayBufferView. Uint8Array gives me the same TypeError as well.

[KiChjang]

Slight issue... now when JavaScript does new TextDecoder(), Rust throws a SyntaxError. The default label argument to the constructor is utf-8, but encoding_from_whatwg_labeldoesn't like the default utf-8 string generated by the python bindings. Instead, if you do new TextDecoder('utf-8') in JavaScript, then encoding_from_whatwg_label doesn't complain.

I tried trim() on the &str that gets passed as an argument to encoding_from_whatwg_label to no avail.

[Ms2ger]

Pre-existing bug; I'll deal with it.

[KiChjang]

Is there an issue number that tracks the bug that @Ms2ger mentioned? The SpiderMonkey interface arguments for the WebIDL also needs to be implemented (Uint8Array, ArrayBuffer, ...).

[yodalee]

I see the the sample code posted by jdm using Uint8ClampedArray, I try to add

use js::jsfriendapi::bindgen::{JS_NewArrayBuffer, JS_GetArrayBufferData};

as depict in file: components/bindings/codegen/TypedArray.h
But compiler cannot resolve this include: unresolved import
I trace the code in components/servo/target/build/mozjs-sys-6b64ab2c5b5d63e7/out/dist/include, only Array of uint8clampedArray got JS_BEGIN_EXTERN_C and JS_END_EXTERN_C around them. I think mozjs doesn't support declaration of JS_BufferArray yet.

[Ms2ger]

You'll need changes like servo/mozjs#25 and servo/rust-mozjs#128, and then use ./mach update-cargo -p js to pull in those changes.

[yodalee]

See, I will do the changes to servo/mozjs first.

[yodalee]

@KiChjang Thanks Ms2ger, now we can use all TypedArray.
Sorry wait... I don't know why my servo still use mozjs with clamparray only.

[jdm]

Oh, #4998 only updated rust-mozjs, not mozjs. Use -p js-sys for that one.

[yodalee]

It says js-sys matched no packages

[yodalee]

OK I found, use -p mozjs-sys

[KiChjang]

I'm not entirely sure how to use these yet... I did ./mach update-cargo -p js and ./mach update-cargo -p mozjs-sys, but it still gave me TypeError: Can't handle SpiderMonkey interface arguments yet. I even rebased my branch to the lastest commit on master, but that didn't work.

[Ms2ger]

I would suggest using object in the IDL for now.

[KiChjang]

That wouldn't work either; it would just tell me TypeError: Can't handle object arguments yet. I think this issue is going to be blocked until CodegenRust.py can generate either Uint8Array/ArrayBuffer/BufferSource arguments or object arguments