How to print the type of a variable?

help
1

Is there a way to print the type of a variable?

Having this code:

fn main() {
    let a = vec![2, 3];
    println!("{}", a);
}

I get a compile error:

`Vec<{integer}>` doesn't implement `std::fmt::Display`

So not surprisingly the compiler know what type is the variable a

Is there a macro or a formatting option I could use in println! that would print Vec<{integer}> when the a variable is supplied?

2

Vec<{integer}> is not quote the type here, actual type is Vec<i32>

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3

Thanks, but I don't seem to be able to use this. Where do I put the a variable?

4

Something like this. You can call it as print_type(&a).

fn print_type<T>(_: &T) { 
    println!("{:?}", std::any::type_name::<T>());
}
1 Like
5

That worked quite well. Thanks.

It seems such a neat tool for understand code, why is this function, the way you wrote it, not part of the standard library?

6

It is, it's just not stabilized yet, due to some corner cases.

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7

Are you using rust-analyzer? It integrates into your editor and prints types of all the variables inside the editor itself.

8

@Cerber-Ursi thanks for the link

@manpacket When I use VS Code then I do have rust-analyzer, but I write a lot in vim and I have not set up the integration yet.

9

You can add it to vim as well with this: GitHub - simrat39/rust-tools.nvim: Tools for better development in rust using neovim's builtin lsp

1 Like
10

If you just need to know in development, a trick is to cause a type error:

let a: () = vec![2,3];

will print something like expected (), but found Vec<i32>.

5 Likes
11

A pedantic tangent:

You will never get a program to print Vec<{integer}> as a type name. This is because {integer} is a pseudo-type that only appears when a program is being compiled: it means “some integer type we haven't yet decided on” (that came from an integer literal). When the compiler finishes, it will always have either replaced {integer} with a specific integer type (i32 if nothing else constrains it), or have reported a fatal error. Any successfully compiled program contains no {integer}s.

2 Likes
12

Thanks for the explanation. That now also explains why it said "integer", something that was not clear to me in this and in other cases.