generics - What is Vec<_>?

Question

I have seen Vec<_> a couple of times already. For example:

let a = "line1
line2
line3";
println!("{:?}", a.lines().collect::<Vec<_>>());

But what does that 'uncertain face' <_> mean?

I'm used to a typename in angle brackets, but what type can that be? The only meaning of underscore that I'm aware of is from Python as a name for an unused variable.

Answer 1

It means "Rust compiler, infer what type goes into the Vec". And it is indeed analogous to the unused variable in Python (and in Rust itself), in that it represents a placeholder for a type, like it can represent a placeholder for a variable name.

You can find an explanation in The Rust Programming Language chapter about iterator consumers:

Using a _ will let you provide a partial hint:

let one_to_one_hundred = (1..101).collect::<Vec<_>>(); This says "Collect into a Vec<T>, please, but infer what the T is for me." _ is sometimes called a "type placeholder" for this reason.