Why does using a turbofish with into give "wrong number of type arguments"?

Question

This works:

use std::error::Error;

fn main() {
    let _: Box<dyn Error> = "test".into();
}

But this gives an error:

use std::error::Error;

fn main() {
    let _ = "test".into::<Box<dyn Error>>();
}

use std::error::Error;

fn main() {
    let _: Box<dyn Error> = "test".into();
}

Why?

Answer 1

这来自Into特征定义：

pub trait Into<T> {
    fn into(self) -> T;
}

如您所见，into没有泛型参数，但它来自特征定义本身。正确的完全限定语法是：

use std::error::Error;

fn main() {
    let _ = Into::<Box<dyn Error>>::into("asd");
}