Spread Attributes / Functional Update Syntax

[vldm]

Original issue: stoically#27

• JSX is <div {...props} /> (https://zhenyong.github.io/react/docs/jsx-spread.html#spread-attributes)

  • Yew went with <div ..props />, inspired by Rust's functional update syntax (https://doc.rust-lang.org/stable/reference/expressions/struct-expr.html#functional-update-syntax)

This might already be possible with the transform_block configuration, but not exactly sure yet.

[vldm]

The proposed solution:

Use syntax ...spread as in JS. But don't enforce user to use braces {}.
Expect to have spread syntax only at end of attributes declaration.

Motivation:

The syntax ... was used in rust before, as inclusive range, but for now it is kept unused.
So it is impossible to find ... in valid rust expressions, thats why using braces is not necessary.
Also, if someone want to implement spread syntax as {...foo} it will be available using transform_block feature.
The yew syntax ..foo is conflicting with our attribute parsing, and can't be supported atm.

On other hand syn@2.0 at its current form detect .. as part of range expression.
We using syn to parse attribute value, so using spread syntax after attribute value is imposible (example: foo="bar" ...spread is invalid)
Thats why we limit ...spread syntax to only apear at end of attributes declaration.

Examples:

Examples of some cases that would and wouldn't parse as spread syntax

❌ attribute x=y is not expected after spread syntax

<div foo="bar" ...spread x=y />

❌ spread syntax should contain 3 dots, this syntax would be treated as range expression

<div foo="bar"..spread />

✅ Spread syntax at end is valid, even if it is formatted incorrectly

<div foo="bar" some_flag...spread />
<Baz ...spread />

Limitations:

At MVP, i propose to only support one apearance of spread syntax.