Get the span of an Expr

[antoyo]

Hi.
Is there an easy way to get the Span of an Expr?
That is without having to use pattern matching on node and getting the span manually.
Thank you.

[mystor]

No, currently there is no easy way to extract the span of an individual AST node, as spans are only stored on the tokens at the leafs of the AST. It might be possible in the future to add a fn span() -> Option<Span> method which uses the Span.join() method to try to make a span for the full node, but we don't support it right now. (We'd also need dtolnay/proc-macro2#36 to merge before we could do that).

[antoyo]

I started to write such a function here even though it is not completely accurate (I don't merge Spans yet).

[mystor]

I think we would probably want to generate that function, if we decided we wanted it, behind a feature flag and using syn_codegen so we can avoid manually writing large error-prone match statements.

cc @dtolnay to see if we'd want to have something like that in syn behind a feature gate

(I'm imagining we'd add some sort of #span attributes to some fields, or something like that, which would tell the codegen which fields to pull spans out of)

[dtolnay]

Seems reasonable! I like the idea of the implementations being generated.

[dtolnay]

This has been added in syn::spanned::Spanned.

[da-x]

Hi, I have a question regarding this issue -

If I run a.b.c through syn::parse_str, a Debug print of the result yields: Field(ExprField { attrs: [], base: Field(ExprField { attrs: [], base: Path(ExprPath { attrs: [], qself: None, path: Path { leading_colon: None, segments: [PathSegment { ident: Ident(a), arguments: None }] } }), dot_token: Dot, member: Named(Ident(b)) }), dot_token: Dot, member: Named(Ident(c)) })

So far so good, however it's not clear to me how to use syn::spanned::Spanned in order to obtain Span information by pattern matching on that result. I'd expect it would be in attrs, no? But these vectors seem empty.