Inconsistency in quoted and mustache attribute value AST

[kwangure]

Describe the bug

Given the following element:

<div class='a' value={a}/>

Svelte outputs the following in the AST.
For 'a':

value: [{
	start: 12,
	end: 13,
	type: "Text",
	...
}]

For {a}:

value: [{
	start: 21,
	end: 24,
	type: "MustacheTag",
	...
}]

Note how according the AST one attribute value is one character long and other is three characters long. They should both be three characters long, where for 'a', the quotes are included, resulting in:

value: [{
	start: 11,
	end: 14,
	type: "Text",
	...
}]

This aligns with how other parsers like Acorn JSX and Babel parse attribute values too.

Reproduction

Compare Svelte, Acorn JSX, Babel JSX and friends here. https://astexplorer.net/#/gist/43f92f399852b126ac6d21755f65cd6f/317b9a09a9af33f93fbbbed822d59b0429d852d4

Logs

No response

System Info

N/A

Severity

annoyance

[dummdidumm]

Before merging a related PR we should check if prettier, eslint and language-tools don't rely on the behavior in some way, or work around it (and fixing it breaking the workaround). Maybe it would be safer to only do this for the modern AST, if we can reliably detect these cases to undo the fixes for the legacy AST.

[dummdidumm]

This would be a pretty substantial AST change for little practical benefit. I don't think we should change this.

[Rich-Harris]

Recapping a discussion from today: the current behaviour is correct, so closing this as not planned. A non-boolean attribute is a sequence of one or more Text or ExpressionTag chunks, and that sequence may or may not be wrapped in quotes (though we intend to be stricter in future about what must and must not be quoted — it's weird that x={y}{z} is permitted, and conversely per #7925 it's confusing that x="{y}" doesn't stringify y).

So in the example above it's natural that the surrounding quotes aren't included in the Text chunk. Put differently, it would be very strange indeed if in a case like x="a{b}c{d}e" the start of the a and the end of the e were adjusted to include the opening and closing quote marks, giving them both a length of 2, but the c had a length of 1.