How do I print the type of a variable in Rust?

Question

I have the following:

let mut my_number = 32.90;

How do I print the type of my_number?

Using type and

Answer 1

If you merely wish to find out the type of a variable and are willing to do it at compile time, you can cause an error and get the compiler to pick it up.

For example, set the variable to a type which doesn't work:

let mut my_number: () = 32.90;
// let () = x; would work too

error[E0308]: mismatched types
 --> src/main.rs:2:29
  |
2 |     let mut my_number: () = 32.90;
  |                             ^^^^^ expected (), found floating-point number
  |
  = note: expected type `()`
             found type `{float}`

Or call an invalid method:

let mut my_number = 32.90;
my_number.what_is_this();

error[E0599]: no method named `what_is_this` found for type `{float}` in the current scope
 --> src/main.rs:3:15
  |
3 |     my_number.what_is_this();
  |               ^^^^^^^^^^^^

Or access an invalid field:

let mut my_number = 32.90;
my_number.what_is_this

error[E0610]: `{float}` is a primitive type and therefore doesn't have fields
 --> src/main.rs:3:15
  |
3 |     my_number.what_is_this
  |               ^^^^^^^^^^^^

These reveal the type, which in this case is actually not fully resolved. It’s called “floating-point variable” in the first example, and “{float}” in all three examples; this is a partially resolved type which could end up f32 or f64, depending on how you use it. “{float}” is not a legal type name, it’s a placeholder meaning “I’m not completely sure what this is”, but it is a floating-point number. In the case of floating-point variables, if you don't constrain it, it will default to f64¹. (An unqualified integer literal will default to i32.)

See also:

• What is the {integer} or {float} in a compiler error message?

---

¹ There may still be ways of baffling the compiler so that it can’t decide between f32 and f64; I’m not sure. It used to be as simple as 32.90.eq(&32.90), but that treats both as f64 now and chugs along happily, so I don’t know.