Print a Rust Value’s Type

This is a very short card about a tiny function I still find useful. After a few hours of coding, I sometimes want Rust to tell me the concrete type of a value without making me hover through an IDE or deliberately provoke the compiler.

Flashcard 06 Rust

The question

How can I print the concrete type of a Rust value?

The standard library already knows the type name. A generic helper can expose it.

Reveal the answer

Pass a reference into a generic function and print std::any::type_name::<T>().

The helper

use std::any::type_name;

fn what_type<T>(_: &T) {
    println!("type is: {}", type_name::<T>());
}

The argument gives Rust a concrete T. The function does not need the value itself, so the parameter is named _. type_name::<T>() returns the name of that type as a string slice.

Use it with a few values

fn main() {
    let number = 3232;
    let name = "Rahul".to_string();
    let list_of_nums = vec![1, 2];

    what_type(&number);
    what_type(&name);
    what_type(&list_of_nums);
}

type is: i32
type is: alloc::string::String
type is: alloc::vec::Vec<i32>

Before I knew this helper, I sometimes used IDE introspection or deliberately assigned a value to () so the compiler would complain and reveal the real type. This is a little less silly.

What I keep in my head

type_name::<T>() is useful for debugging and learning. The exact string is intended for diagnostics, so I would not build program logic around its format.

The helper

Use it with a few values

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