Span for FnArg::Receiver in proc_macro seems to point to first token not whole receiver

[illicitonion]

See this repo for a repro.

I tried to point an error message at a function receiver. Parsing the code as a string, this seems to work fine. Parsing it from a TokenStream, in a proc_macro, the Span for the FnArg::Receiver appears to point to the first token of the receiver, rather than the whole thing:

error: Saw receiver with span #0 bytes(63..64)
 --> example/src/lib.rs:7:19
  |
7 |     fn by_mut_ref(&mut self) -> bool {
  |                   ^

error: Saw receiver with span #0 bytes(126..130)
  --> example/src/lib.rs:12:12
   |
12 |     fn own(self) -> bool {
   |            ^^^^

error: Saw receiver with span #0 bytes(188..191)
  --> example/src/lib.rs:17:16
   |
17 |     fn own_mut(mut self) -> bool {
   |                ^^^

error: Saw receiver with span #0 bytes(253..254)
  --> example/src/lib.rs:22:15
   |
22 |     fn by_ref(&self) -> bool {
   |               ^

If I parse the same code from a str, I get the spans I expect (or at least, :

bytes(21..30)
bytes(86..90)

[dtolnay]

Hi @illicitonion, this matches the documented behavior -- please see https://docs.rs/syn/1.0.38/syn/spanned/index.html#limitations.