How to infer the return type of a function? [duplicate]

问题

This question already has an answer here:

Differences in Type inference for closures and functions in rust (1 answer)

Closed 2 years ago.

The return type of a block is inferred.

fn main() {
    let x = { 5 };
    println!("{}", x);
}

But when I give the block a name, I have to specify a type.

fn five() -> i32 {
    5
}

fn main() {
    let x = five();
    println!("{}", x);
}

How can I avoid selecting a type?

回答1:

You cannot. Rust explicitly prohibits this by design.

However, for large and complex return types, you have the following options:

• Use a closure instead - As it is local, it is allowed to infer its type
• Return a boxed type
• Return an abstract type

You can see a practical example of these in the answer for What is the correct way to return an Iterator (or any other trait)?

来源：https://stackoverflow.com/questions/47347962/how-to-infer-the-return-type-of-a-function