`DOCS_RS=1 cargo build` fails

[wtdcode]

This crate fails to build with DOCS_RS=1:

error[E0277]: the trait bound `proc_macro2::Span: From<proc_macro::Span>` is not satisfied
   --> src/lib.rs:554:32
    |
554 |                 first: (*self).into(),
    |                                ^^^^ the trait `From<proc_macro::Span>` is not implemented for `proc_macro2::Span`
    |
    = note: required for `proc_macro::Span` to implement `Into<proc_macro2::Span>`

error[E0277]: the trait bound `proc_macro2::Span: From<proc_macro::Span>` is not satisfied
   --> src/lib.rs:555:31
    |
555 |                 last: (*self).into(),
    |                               ^^^^ the trait `From<proc_macro::Span>` is not implemented for `proc_macro2::Span`
    |
    = note: required for `proc_macro::Span` to implement `Into<proc_macro2::Span>`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `proc-macro-error2` (lib) due to 2 previous errors

Funnily, if cargo build and then DOCS_RS=1 cargo build would pass. Therefore, to reproduce this, ensure doing:

cargo clean && DOCS_RS=1 cargo build

[wtdcode]

The root cause is that when DOCS_RS=1 is enabled, proc-marco2 will enable semver_exempt:

https://github.com/dtolnay/proc-macro2/blob/master/build.rs#L33

This further disables wrap_proc_macro:

https://github.com/dtolnay/proc-macro2/blob/master/build.rs#L108

And therefore an alternative implementation is used:

https://github.com/dtolnay/proc-macro2/blob/master/src/lib.rs#L156

Unfortunately, From<proc_macro::Span> is exactly not implemented here.