Implement streaming support for TextDecoder

[Ms2ger]

Blocks #5601.

• Spec: https://encoding.spec.whatwg.org/#dom-textdecoder-decode.
• Code: components/script/dom/textdecoder.rs, components/script/dom/webidls/TextDecoder.webidl.
• Test: /encoding/textdecoder-fatal-streaming.html, /encoding/textdecoder-streaming.html.

[yodalee]

I wanna make sure that my thought is correct. To implement the streaming support, I think we need to add a RawDecoder, which is provided by encoding, to the TextDecoder
Currently, the decode method will process the input buffer to the end by calling encoding.decode(), thus we are unable to do incremental decoding. To allow stream decode, we need to add a RawDecoder and use raw_feed method instead.

[jdm]

That looks right to me.

[yodalee]

I cannot get it to work in the end. Here is the branch I tried so far.
master...yodalee:add-streaming-support-for-textdecoder
I add the stream options, Box<RawDecoder>, do not flush flag and BOMseen flags.
However I cannot get this work, since Decode method take &self instead of &mut self as arguments. So I cannot modify any flag or RawDecoder in TextDecoder since it is not mutable.
No idea how to implement this.

[jdm]

@yodalee You need to store mutable fields inside Cell or DOMRefCell in order to mutate them. Does that help?

[yodalee]

I got an idea that: I can add a RawDecoder into TextDecoder to support streaming. To achieve that, I need to make customized change to Rust-Encoding, adding a function to reset the unprocessed buffer in RawDecoder. However I cannot get it worked.
I download rust-encoding and modify Cargo.toml in component/script, changing encoding setting to

{path="localpath", version="0.2"}

However, the compiler error when compiling dom/htmlformelement.rs. Here is part of error message:

   --> /home/yodalee/mozilla/servo/components/script/dom/htmlformelement.rs:384:42
    |
384 |                  .encoding_override(Some(self.pick_encoding()))
    |                                          ^^^^^^^^^^^^^^^^^^^^ expected trait `html5ever::encoding::Encoding`, found trait `encoding::Encoding`
    |
    = note: expected type `&html5ever::encoding::Encoding + Send + Sync`
    = note:    found type `&'static encoding::Encoding + Send + Sync + 'static`

which I cannot understand how to fix it.

[jdm]

You should follow http://doc.crates.io/specifying-dependencies.html#overriding-with-local-dependencies or http://doc.crates.io/specifying-dependencies.html#overriding-dependencies instead.

[jdm]

This requires adding the TextDecodeOptions dictionary from the spec and implementing the steps of the encode method that relate to streaming.

[talklittle]

I'd like to give this a shot.