Named functions do not implement traits implemented on fn()

[yvie-k]

I tried this code:

trait Foo {}

impl Foo for fn() {}

fn main() {
    let x: &dyn Foo = &main;
}

playground link

I expected this code to compile, as main is a fn() pointer, which implements the trait Foo

However, rustc seems to need additiontal casting to convert a named fn() to a unnamed one:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `fn() {main}: Foo` is not satisfied
 --> src/lib.rs:6:23
  |
6 |     let x: &dyn Foo = &main;
  |                       ^^^^^ the trait `Foo` is not implemented for fn item `fn() {main}`
  |
  = note: required for the cast from `&fn() {main}` to `&dyn Foo`
help: the trait `Foo` is implemented for fn pointer `fn()`, try casting using `as`
  |
6 |     let x: &dyn Foo = &main as fn();
  |                             +++++++

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to 1 previous error

On a side note, the solution suggested by rustc is missing parentheses. The correct way would be &(main as fn()). When following the suggestions given by rustc, the code goes &main -> &main as fn() -> &(&main as fn()). The last code suggestion is an invalid cast, because you cannot cast &fn() to fn().

Meta

rustc --version --verbose:

rustc 1.76.0 (07dca489a 2024-02-04)
binary: rustc
commit-hash: 07dca489ac2d933c78d3c5158e3f43beefeb02ce
commit-date: 2024-02-04
host: x86_64-unknown-linux-gnu
release: 1.76.0
LLVM version: 17.0.6

(also happens on beta and nightly)

[asquared31415]

main is a fn() pointer

This is not true, and that's what the diagnostic is trying to explain.

fn item fn() {main}

main (and all other functions) are what's called a "function item" which is a unique type per function that can coerce to a fn pointer, but only at certain coercion sites. This is not one of those places, so the compiler is trying to tell you to force the coercion.

The suggestion is incorrect though, it needs parentheses.

An alternative is to do impl<T: Fn()> Foo for T {}, since function items and function pointers both implement the Fn trait family where possible. (note that closures also implement this trait, which may be helpful, but also may not).

@rustbot label -needs-triage +A-diagnostics +D-papercut +D-incorrect

[rustbot]

Error: Parsing relabel command in comment failed: ...'papercut +' | error: empty label at >| ' D-incorre'...

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[asquared31415]

@rustbot claim

I'll work on fixing the suggestion at least, and possibly see about clarifying the diagnostic

[rustbot]

Error: Parsing assign command in comment failed: ...' claim' | error: expected end of command at >| ' I'll work'...

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.