No method named `inv` found for struct `rug::Rational` in the current scope

[SpinTensor]

Hi,
here is a code snippet utilizing the rug 1.19.2 crate:

use rug::Rational;
fn main() {
    let a = Rational::from((137,-42));
    let b = Rational::from((2,3));
    println!("{:?}*{:?} = {:?}", a, b, a.clone()*b.clone());
    println!("({:?})^(-1) = {:?}", a, a.clone().inv());
}

I create two Rational-structs. In the first println I multiply them, utilizing the Mul-tait, and in the second line I try to invert one of the Rationals, utilizing the Inv-trait. Both traits are implemented for the rational struct:
impl-Inv-for-Rational
impl-Mul-for-Rational
When trying to build the code I get the following error

error[E0599]: no method named `inv` found for struct `Rational` in the current scope
 --> src/main.rs:7:49
  |
7 |     println!("({:?})^(-1) = {:?}", a, a.clone().inv());
  |                                                 ^^^ method not found in `Rational`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `testing` (bin "testing") due to previous error

For some reason I cannot use the inv on the Rational struct, although it should be implemented.
The error message is not helpful at all. I know that the method is not in scope, but how do I get it?
Can someone explain to me what I did wrong and how to fix it?

Thanks

[inejge]

1. You need to include the num-traits feature in the rug dependency.
2. You need to bring the Inv trait from the num-traits crate into scope, and for that...
3. You need to add num-traits as a dependency.

So, in Cargo.toml:

rug = { version = "1.19.2", features = ["num-traits"] }
num-traits = "0.2.15"

...and in your program:

use num_traits::Inv;

(This is for Inv; similarly for Mul.)

[SkiFire13]

Alternatively rug::Rational provides a method recip that does the same thing as inv, but since it's defined on the struct itself it doesn't need other traits to be in scope.

[SpinTensor]

Thank you. That solves it.
Why is it designed that way? I would have expected the rug crate to have num-traits as a dependency and automatically load all the features.

[erelde]

Short FYI: You can write let product = dbg!(a * b); (or dbg!(a, b, a * c)) instead of the complex println! here.

[DanielKeep]

As previously noted, the operation you want is implemented as recip by rug. Compatibility with num-traits is an optional extra.

In general, there is an expectation that crates don't just auto-enable everything by default, since turning features off is more difficult and inconvenient than turning them on. So crates will typically default to a conservative set of functionality, with useful but potentially unnecessary code disabled until explicitly enabled. For example: if you aren't already using the num-traits crate, or don't need to interoperate with code that uses it, then rug supporting num-traits probably isn't particularly useful.

It's also worth noting that rug does have num-traits as a dependency (specifically an optional dependency). Adding the dependency to your code is so that you have access to its traits.