Span::resolved_at and Span::located_at to combine behavior of two spans
Proc macro spans serve two mostly unrelated purposes: controlling name
resolution and controlling error messages. It can be useful to mix the
name resolution behavior of one span with the line/column error message
locations of a different span.
In particular, consider the case of a trait brought into scope within
the def_site of a custom derive. I want to invoke trait methods on the
fields of the user's struct. If the field type does not implement the
right trait, I want the error message to underline the corresponding
struct field.
Generating the method call with the def_site span is not ideal -- it
compiles and runs but error messages sadly always point to the derive
attribute like we saw with Macros 1.1.
```
|
4 | #[derive(HeapSize)]
| ^^^^^^^^
```
Generating the method call with the same span as the struct field's
ident or type is not correct -- it shows the right underlines but fails
to resolve to the trait in scope at the def_site.