Module abi_stable::docs::troubleshooting

Here are some problems and their solutions

Opaque compiletime errors

As of writing this section,having extern fn in a type definition causes compile-time errors for #[derive(StableAbi)] to look like this

error: unknown lifetime

ẁhere it doesn't point at what the cause of the error is.

To fix this,replace extern fn with extern "C" fn and the error message will look like this:

error: unknown lifetime
   --> abi_stable/src/lib.rs:313:6
    |
313 |   a:&'b (),
    |      ^^