- [x] Avoid copying buffers.
https://encoding.spec.whatwg.org/#dom-textdecoder-decode
> Implementations are strongly encouraged to use an implementation
strategy that avoids this copy. When doing so they will have to make
sure that changes to input do not affect future calls to
[decode()](https://encoding.spec.whatwg.org/#dom-textdecoder-decode).
- [x] Special op to avoid string label deserialization and parsing.
(Ideally we should map labels to integers in JS)
- [x] Avoid webidl `Object.assign` when options is undefined.
Welcome to better optimised op calls! Currently opSync is called with parameters of every type and count. This most definitely makes the call megamorphic. Additionally, it seems that spread params leads to V8 not being able to optimise the calls quite as well (apparently Fast Calls cannot be used with spread params).
Monomorphising op calls should lead to some improved performance. Now that unwrapping of sync ops results is done on Rust side, this is pretty simple:
```
opSync("op_foo", param1, param2);
// -> turns to
ops.op_foo(param1, param2);
```
This means sync op calls are now just directly calling the native binding function. When V8 Fast API Calls are enabled, this will enable those to be called on the optimised path.
Monomorphising async ops likely requires using callbacks and is left as an exercise to the reader.
In the `transform` function to `TextEncoderStream`'s internal
`TransformStream`, if `chunk` is the empty string and
`this.#pendingHighSurrogate` is null, then `lastCodeUnit` will be NaN.
As it turns out, this does not cause a bug because the comparison to
check for lone surrogates turns out to be false for NaN, but to rely on
it makes the code brittle.
The Web IDL conversion to `BufferSource` and similar types shouldn't
check whether the buffer is detached.
In the case of `TextDecoder`, our implementation would still throw after
the Web IDL conversions because we're creating a new `Uint8Array` from
the buffer source's buffer, which throws if it's detached. This change
also fixes this bug.
The implementation of `TextDecoder` had a bug where it was copying the
input data in every case. This change removes that copy in
non-`SharedArrayBuffer` cases.
Since passing a shared buffer source to Rust would fail, this copy of
the input data was making `TextDecoder` work in cases where the input
is shared. In order to avoid a breaking change, the copy is retained in
those cases.
