e.g. this irlo thread was likely the result of someone translating a void-returning C function to a c_void-returning Rust function.

Given the following:

pub fn example() -> core::ffi::c_void {
}

The current output is:

error[E0308]: mismatched types
 --> src/lib.rs:1:21
  |
1 | pub fn example() -> core::ffi::c_void {
  |        -------      ^^^^^^^^^^^^^^^^^ expected enum `c_void`, found `()`
  |        |
  |        implicitly returns `()` as its body has no tail or `return` expression

Ideally, this should also contain a help message along the lines of

help = returning void in C is the same as returning () in Rust

and a suggestion to remove the -> c_void.

It might be reasonable to have this as a dedicated warning lint for -> c_void either instead of or in addition to help on the type mismatch error, as writing bindings to extern "C" { fn foo() -> c_void; } is wrong but won't get a type mismatch error. This probably should go straight to a rustc lint rather than in clippy due to the practically-100% applicability.

It might also be beneficial to do more thorough linting of misuse of c_void (basically any use that isn't *mut c_void or *const c_void (or maybe ptr::NonNull<c_void>), but that is a separate issue from teaching in this error.