Code change:
<ins> // check-pass #![feature(c_variadic)] #![warn(function_item_references)] use std::fmt::Pointer; use std::fmt::Formatter; fn nop() { } fn foo() -> u32 { 42 } fn bar(x: u32) -> u32 { x } fn baz(x: u32, y: u32) -> u32 { x + y } unsafe fn unsafe_fn() { } extern "C" fn c_fn() { } unsafe extern "C" fn unsafe_c_fn() { } unsafe extern fn variadic(_x: u32, _args: ...) { } //function references passed to these functions should never lint fn call_fn(f: &dyn Fn(u32) -> u32, x: u32) { f(x); } fn parameterized_call_fn<F: Fn(u32) -> u32>(f: &F, x: u32) { f(x); } //function references passed to these functions should lint fn print_ptr<F: Pointer>(f: F) { println!("{:p}", f); } fn bound_by_ptr_trait<F: Pointer>(_f: F) { } fn bound_by_ptr_trait_tuple<F: Pointer, G: Pointer>(_t: (F, G)) { } fn implicit_ptr_trait<F>(f: &F) { println!("{:p}", f); } //case found in tinyvec that triggered a compiler error in an earlier version of the lint checker trait HasItem { type Item; fn assoc_item(&self) -> Self::Item; } fn _format_assoc_item<T: HasItem>(data: T, f: &mut Formatter) -> std::fmt::Result where T::Item: Pointer { //when the arg type bound by `Pointer` is an associated type, we shouldn't attempt to normalize Pointer::fmt(&data.assoc_item(), f) } //simple test to make sure that calls to `Pointer::fmt` aren't double counted fn _call_pointer_fmt(f: &mut Formatter) -> std::fmt::Result { let zst_ref = &foo; Pointer::fmt(&zst_ref, f) //~^ WARNING taking a reference to a function item does not give a function pointer } fn main() { //`let` bindings with function references shouldn't lint let _ = &foo; let _ = &mut foo; let zst_ref = &foo; let fn_item = foo; let indirect_ref = &fn_item; let _mut_zst_ref = &mut foo; let mut mut_fn_item = foo; let _mut_indirect_ref = &mut mut_fn_item; let cast_zst_ptr = &foo as *const _; let coerced_zst_ptr: *const _ = &foo; let _mut_cast_zst_ptr = &mut foo as *mut _; let _mut_coerced_zst_ptr: *mut _ = &mut foo; let _cast_zst_ref = &foo as &dyn Fn() -> u32; let _coerced_zst_ref: &dyn Fn() -> u32 = &foo; let _mut_cast_zst_ref = &mut foo as &mut dyn Fn() -> u32; let _mut_coerced_zst_ref: &mut dyn Fn() -> u32 = &mut foo; //the suggested way to cast to a function pointer let fn_ptr = foo as fn() -> u32; //correct ways to print function pointers println!("{:p}", foo as fn() -> u32); println!("{:p}", fn_ptr); //potential ways to incorrectly try printing function pointers println!("{:p}", &foo); //~^ WARNING taking a reference to a function item does not give a function pointer print!("{:p}", &foo); //~^ WARNING taking a reference to a function item does not give a function pointer format!("{:p}", &foo); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &foo as *const _); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", zst_ref); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", cast_zst_ptr); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", coerced_zst_ptr); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &fn_item); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", indirect_ref); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &nop); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &bar); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &baz); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &unsafe_fn); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &c_fn); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &unsafe_c_fn); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &variadic); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p}", &std::env::var::<String>); //~^ WARNING taking a reference to a function item does not give a function pointer println!("{:p} {:p} {:p}", &nop, &foo, &bar); //~^ WARNING taking a reference to a function item does not give a function pointer //~^^ WARNING taking a reference to a function item does not give a function pointer //~^^^ WARNING taking a reference to a function item does not give a function pointer //using a function reference to call a function shouldn't lint (&bar)(1); //passing a function reference to an arbitrary function shouldn't lint call_fn(&bar, 1); parameterized_call_fn(&bar, 1); std::mem::size_of_val(&foo); unsafe { //potential ways to incorrectly try transmuting function pointers std::mem::transmute::<_, usize>(&foo); //~^ WARNING taking a reference to a function item does not give a function pointer std::mem::transmute::<_, (usize, usize)>((&foo, &bar)); //~^ WARNING taking a reference to a function item does not give a function pointer //~^^ WARNING taking a reference to a function item does not give a function pointer //the correct way to transmute function pointers std::mem::transmute::<_, usize>(foo as fn() -> u32); std::mem::transmute::<_, (usize, usize)>((foo as fn() -> u32, bar as fn(u32) -> u32)); } //function references as arguments required to be bound by std::fmt::Pointer should lint print_ptr(&bar); //~^ WARNING taking a reference to a function item does not give a function pointer bound_by_ptr_trait(&bar); //~^ WARNING taking a reference to a function item does not give a function pointer bound_by_ptr_trait_tuple((&foo, &bar)); //~^ WARNING taking a reference to a function item does not give a function pointer //~^^ WARNING taking a reference to a function item does not give a function pointer implicit_ptr_trait(&bar); // ignore //correct ways to pass function pointers as arguments bound by std::fmt::Pointer print_ptr(bar as fn(u32) -> u32); bound_by_ptr_trait(bar as fn(u32) -> u32); bound_by_ptr_trait_tuple((foo as fn() -> u32, bar as fn(u32) -> u32)); } </ins>

Related discussion:
text: I tried this code: I expected to see this happen: The compilation succeed or guide me to (1) cast the to with . (2) borrow the : . But in this case the addresses printed are the same (what?). Instead, this happened*: Cryptic error message: There is : . It took me minutes before I found the workaround. But it still surprised me. : <!-- TRIAGEBOTSTART --<!-- TRIAGEBOTASSIGNSTART --<!-- TRIAGEBOTASSIGNDATASTARTTRIAGEBOTASSIGNDATAEND --<!-- TRIAGEBOTASSIGNEND --<!-- TRIAGEBOT_END --
This creates a reference-to-function-pointer (type ), a double indirection. So it is not what you want. The error could possibly be improved, and the entire situation documented better, but I think getting an error is expected here. is not a function pointer, it is an anonymous ) (of size 0) indicating exactly the function it was created from. This type coerces to a function pointer, but cannot know what type to coerce it to. Printing function pointers is easy (well, if they are not polymorphic via ), but printing these function singleton ZSTs is hard.
It might also be worth linting against .
helps with the docs. But to close this issue, it would probably make sense to write a lint against where is the name of a function. That should be a matter of adding a new in which checks all expressions for being . That path we can pass to to get a . Now we just need to check if that refers to a function: pass the to . At least I hope that makes sense, someone stop me if this is all wrong. ;)
I'd like to help write this lint. The default level for the lint should be right? claim
Yeah, feels like a reasonable start.
Nominated for lang team discussion. It seems we usually ask for lints to be approved before shipping them, so let's get that process started. sorry for this, I wasn't aware of this part of the process. You can still go ahead and implement this if you want, but depending on what the lang team decides, we should submit this lint to clippy instead of the compiler itself. That might require minor adjustments, but the main work you are putting in and the testcases will be useful either way.