为什么rustc说推断的类型不匹配？

如果我尝试创建一个函数指针向量，编译器总是会抱怨错误的类型（尽管我没有明确声明任何类型）：

fn abc() {}

fn def() {}

fn main()
{
    let a = vec![abc, def];
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=ef52fe778db1112433d20dd02b3d0b54

error[E0308]: mismatched types
 --> src/main.rs:5:13
  |
5 |     let a = vec![abc, def];
  |             ^^^^^^^^^^^^^^ expected slice, found array of 2 elements
  |
  = note: expected struct `Box<[fn() {abc}], _>`
             found struct `Box<[fn(); 2], std::alloc::Global>`
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `tes`

To learn more, run the command again with --verbose.

回答

fn abc() {}
fn def() {}

fn main() {
    let a: Vec<fn()> = vec![abc, def];
}

fn abc() {}
fn def() {}

fn main() {
    let a = vec![abc as fn(), def];
}