Improve error message for invalid attribute values

[lambda-fairy]

Currently, when the user types in:

input type="number" min=0 max=100;

or

script src="pinkie.js" defer=true;

or

.col-sm-2 {}

they get the error "expected string". This is technically correct but I think we can do better.

• For number literals (integer or float), suggest wrapping the number in quotes.
  • Alternatively, we can accept the number literal as-is, but I think that can be confusing because adding parentheses to the value can change the result. See Improve error message for invalid attribute values #208 (comment)
• For boolean literals, suggest removing the =true bit altogether.

[CreepySkeleton]

For example, 0x2a and 0.99999999999999999999999 will be parsed as 42 and 1.0 respectively.

Actually, while Literal::* methods do lose precision, impl FromStr for TokenStream and Display seem to be able to round-trip number literals loselessly. There rest is pretty easy.

[lambda-fairy]

Ah, that's interesting. I'm still against allowing that, though, as it means that factoring out a variable can change the result:

html! { 0x2a }  // "0x2a"

let ponk = 0x2a;
html! { (ponk) }  // "42"

[CreepySkeleton]

But ponk is not a literal, it's an identifier. It shouldn't be a subject for suggestions in the first place.

[lambda-fairy]

To clarify, I mean that so far it's been true that adding parentheses () to a literal doesn't change it:

html! { "ponk" }  // "ponk"
html! { ("ponk") }  // "ponk"

But allowing bare number literals will break this property:

html! { 0x2a }  // "0x2a"
html! { (0x2a) }  // "42"

[lambda-fairy]

#265 improves the error message for non-string literals in element bodies and attribute values.

But it doesn't handle the class/ID shorthand case (.col-sm-2). I'll file a follow-up issue to address that.