Incorrect error output for mistakes in HTML fragments

[ilyvion]

Problem
The macro parser doesn't correctly handle errors in HTML fragments.

Steps To Reproduce
Steps to reproduce the behavior:

1. Put the following into a html! invocation:

       html! {
           <key="key" key="key">
               <div>{ "Hello World!" }</div>
           </>
       };

2. Attempt to compile the code
3. See that the error you get is "error: unexpected token"

Expected behavior
If you look at the code in yew-macro\src\html_tree\html_list.rs,

yew/yew-macro/src/html_tree/html_list.rs

Lines 148 to 156 in 9aff041

	impl Parse for ParseKey {
	fn parse(input: ParseStream) -> ParseResult<Self> {
	let key = input.parse::<HtmlProp>()?;
	if !input.is_empty() {
	input.error("Only a single key element is allowed on a <></>");
	}
	Ok(ParseKey { key })
	}
	}

the intended error message here is clearly meant to be

Only a single key element is allowed on a <></>

Environment:

• Yew version: master
• Rust version: 1.44

Questionnaire

• I'm interested in fixing this myself but don't know where to start
• I would like to fix and I have a solution
• I don't have time to fix this right now, but maybe later

[ilyvion]

My guess is that this happens because something in here says "I'll handle this one" from their peek() calls, but then shouldn't actually have done so.

yew/yew-macro/src/html_tree/mod.rs

Lines 41 to 72 in 9aff041

	impl Parse for HtmlTree {
	fn parse(input: ParseStream) -> Result<Self> {
	let html_type = HtmlTree::peek(input.cursor())
	.ok_or_else(|| input.error("expected a valid html element"))?;
	let html_tree = match html_type {
	HtmlType::Empty => HtmlTree::Empty,
	HtmlType::Component => HtmlTree::Component(Box::new(input.parse()?)),
	HtmlType::Tag => HtmlTree::Tag(Box::new(input.parse()?)),
	HtmlType::Block => HtmlTree::Block(Box::new(input.parse()?)),
	HtmlType::List => HtmlTree::List(Box::new(input.parse()?)),
	};
	Ok(html_tree)
	}
	}
	impl PeekValue<HtmlType> for HtmlTree {
	fn peek(cursor: Cursor) -> Option<HtmlType> {
	if cursor.eof() {
	Some(HtmlType::Empty)
	} else if HtmlComponent::peek(cursor).is_some() {
	Some(HtmlType::Component)
	} else if HtmlTag::peek(cursor).is_some() {
	Some(HtmlType::Tag)
	} else if HtmlBlock::peek(cursor).is_some() {
	Some(HtmlType::Block)
	} else if HtmlList::peek(cursor).is_some() {
	Some(HtmlType::List)
	} else {
	None
	}
	}
	}

[ilyvion]

Also, while it's only bad for compilation times, due to all the peeking, this code effectively parses everything twice, doesn't it? Plus it may also partially parse things that aren't even the thing in need of parsing.

[siku2]

You're right, unfortunately Yew performs a lot of duplicate parsing. This is something I've thought about a lot and I think our best bet is to get rid of the Peek traits entirely. It should be possible implement the parsing using only a few lookaheads to distinguish between multiple variants.
This is a huge endeavour though and deserves its own issue.