Multi fn items can be casted to fn pointers implicitly but it does not work with only one fn item

[mu001999]

Code

fn foo() {}

fn bar() {}

fn main() {
    let _x: Vec<fn()> = [foo].into_iter().collect();
    // let _x: Vec<fn()> = [foo, bar].into_iter().collect();
}

Current output

error[E0277]: a value of type `Vec<fn()>` cannot be built from an iterator over elements of type `fn() {foo}`
    --> src/main.rs:6:43
     |
6    |     let _x: Vec<fn()> = [foo].into_iter().collect();
     |                                           ^^^^^^^ value of type `Vec<fn()>` cannot be built from `std::iter::Iterator<Item=fn() {foo}>`
     |
     = help: the trait `FromIterator<fn() {foo}>` is not implemented for `Vec<fn()>`
             but trait `FromIterator<fn()>` is implemented for it
     = help: for that trait implementation, expected `fn()`, found `fn() {foo}`
note: the method call chain might not have had the expected associated types
    --> src/main.rs:6:31
     |
6    |     let _x: Vec<fn()> = [foo].into_iter().collect();
     |                         ----- ^^^^^^^^^^^ `Iterator::Item` is `fn() {foo}` here
     |                         |
     |                         this expression has type `[fn() {foo}; 1]`
note: required by a bound in `collect`
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:1967:19
     |
1967 |     fn collect<B: FromIterator<Self::Item>>(self) -> B
     |                   ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Iterator::collect`

For more information about this error, try `rustc --explain E0277`.

Desired output

Compiled successfully

Rationale and extra context

The following call with two fn items would be compiled successfully:

fn foo() {}

fn bar() {}

fn main() {
    // let _x: Vec<fn()> = [foo].into_iter().collect();
    let _x: Vec<fn()> = [foo, bar].into_iter().collect();
}

Other cases

Rust Version

Build using the Nightly version: 1.84.0-nightly

(2024-11-17 5ec7d6eee7e0f5236ec1)

Anything else?

No response

[gr1mpatr0n]

Don't think this is a bug.

In [foo], foo is a "function item type", hence the fn() {foo} type. This is intentional to provide superior safety and optimisations.
In [foo, bar], array construction allows for type decay to fn() in order to accommodate both functions.

.collect due to its generic nature is conservative about coercions like this.
So, if you really want to use the first form, you will need something like [foo as fn()].into_iter().collect() to be explicit that you want to strip that layer of information.