error[E0277]: expected a `Fn<(proc_macro::TokenStream,)>` closure, found `unsafe extern "C" fn(i32, u32) -> u32 {foo}` --> $DIR/signature.rs:10:1 | LL | / pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 { LL | | LL | | loop {} LL | | } | | ^ | | | | |_call the function in a closure: `|| unsafe { /* code */ }` | required by a bound introduced by this call | = help: the trait `Fn<(proc_macro::TokenStream,)>` is not implemented for fn item `unsafe extern "C" fn(i32, u32) -> u32 {foo}` = note: unsafe function cannot be called generically without an unsafe block note: required by a bound in `ProcMacro::custom_derive` --> $SRC_DIR/proc_macro/src/bridge/client.rs:LL:COL | LL | expand: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy, | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ProcMacro::custom_derive` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.