Add From<Span> for all tokens

[kdy1]

Fixes #278.

[dtolnay]

Thanks for the PR!

As a general rule in the standard library we have tried to always associate conversions with the most specific type involved -- whichever one provides some additional invariant or interpretation that is not present in the other type. For example str is more specific then &[u8] since it is a UTF-8 encoded sequence of bytes. Thus str provides both the as_bytes method and the from_utf8 constructor for converting to and from &[u8] values.

In syn we have token which is more specific than Span, and Ident which is more specific than Span, so I would prefer to associate constructing tokens and idents with those types rather than associating it with Span.

The keyword tokens are already easy to create from a Span: for example Else(span). For the punctuation tokens which hold an array of spans I added a new constructor: for example AddEq::new(span). And for Ident there is Ident::new("value", span). Could you take another look and let me know whether anything still needs to be improved?

[kdy1]

I'm happy with Ident::new(), but I'm not sure if Else(span) is better.
Currently span from procedural macros work in strange way.
In my experience, which span is used is important than which token is I'm currently creating.

For example, rustc fails to resolve module if span from function body is used in ::crate_name (first part) of return type.

[dtolnay]

@mystor could you take a look and recommend a path forward? Thanks!

[kdy1]

IMHO, it's similar to From<Span> and Into<token::XX>. Implementing From<Span> for tokens and using span.into() would be better than span.as_token().

While creating pr, I completely forgot why I chose span.as_token() instead of span.into().

[dtolnay]

I would be on board with providing From<Span> for all the token types. Good idea! Could you rebase the PR and change it to From impls?

[kdy1]

I'll do it soon.

[kdy1]

I did it.
(I made a mistake because ide didn't show it...)

[dtolnay]

Thanks!

[dtolnay]

I published this in syn 0.12.5.