"Expected fn item, found a different fn item" when working with function pointers

Question

I have the following code (Playground):

// Two dummy functions, both with the signature `fn(u32) -> bool` fn foo(x: u32) -> bool {     x % 2 == 0 } fn bar(x: u32) -> bool {     x == 27 }   fn call_both<F>(a: F, b: F) where     F: Fn(u32) -> bool, {     a(5);     b(5); }  fn main() {     call_both(foo, bar);  // <-- error } 

To me, it seems like this should compile as foo and bar have the same signature: fn(u32) -> bool. Yet, I get the following error:

error[E0308]: mismatched types   --> src/main.rs:20:20    | 20 |     call_both(foo, bar);    |                    ^^^ expected fn item, found a different fn item    |    = note: expected type `fn(u32) -> bool {foo}`               found type `fn(u32) -> bool {bar}` 

The same error can be triggered with this code:

let mut x = foo; x = bar;  // <-- error 

I also tried to cast bar to the function pointer type again:

let mut x = foo; x = bar as fn(u32) -> bool;  // <-- error 

This resulted in a slightly different error:

error[E0308]: mismatched types   --> src/main.rs:20:9    | 20 |     x = bar as fn(u32) -> bool;    |         ^^^^^^^^^^^^^^^^^^^^^^ expected fn item, found fn pointer    |    = note: expected type `fn(u32) -> bool {foo}`               found type `fn(u32) -> bool` 

I don't understand these errors at all. What are fn items vs. fn pointers and why are foo and bar different fn items?

Answer 1

Each named function has a distinct type since Rust PR #19891 was merged. You can, however, cast the functions to the corresponding function pointer type with the as operator.

call_both(foo as fn(u32) -> bool, bar as fn(u32) -> bool); 

It's also valid to cast only the first of the functions: the cast will be inferred on the second, because both functions must have the same type.

call_both(foo as fn(u32) -> bool, bar);