Why can't rust automatically infer the type of closure put in Vec?

Question

let c1 = || { "c1"; };
let c2 = || { "c2"; };
let v = [c1, c2];
let v: Vec<fn()> = vec![c1, c2];
let v = vec![1, 2, 3];

Why the compiler cannot automatically infer the type of v in the fourth line of the code, but needs to be explicitly specified. But the fifth line can automatically infer the type of v