如何根据 Rust 中的泛型选择常量？

Question

I'm trying to work on a math library that has constants and functions. What I need to do to bind a value to a trait that uses generics?

The functions in my Rust 1.30.0 project that employs generics as parameters and I need the constants to match the same type of T so I can operate with them in a function (here I am also using the num crate):

use num_traits as ntraits; // 0.2.6
use std::{f32, f64};

pub trait consts<T> {
    const INGA: T;
}

impl<T> consts<f32> for T {
    const INGA: f32 = f32::consts::FRAC_2_SQRT_PI;
}

impl<T> consts<f64> for T {
    const INGA: f64 = f64::consts::FRAC_2_SQRT_PI;
}

pub struct CMPS<T> {
    pub a: T,
    pub b: T,
}

pub type CMPS32 = CMPS<f32>;
pub type CMPS64 = CMPS<f64>;

impl<T: Clone + ntraits::Float + ntraits::FromPrimitive> CMPS<T> {

    pub fn cerf(a: T, b: T) -> CMPS<T> {
        let pr: T = consts::INGA;
        let rtr = a;
        let rti = b;
        CMPS { a: rtr, b: rti }
    }
}

But when I try to perform this binding, expecting pr to be the same type as T and match the value of INGA as this:

let pr: T = consts::INGA;

It only outputs errors like:

error[E0283]: type annotations required: cannot resolve `_: consts<T>`
  --> src/lib.rs:27:21
   |
27 |         let pr: T = consts::INGA;
   |                     ^^^^^^^^^^^^
   |
note: required by `consts::INGA`
  --> src/lib.rs:5:5
   |
5  |     const INGA: T;
   |     ^^^^^^^^^^^^^^

Answer 1

I got it by adding + consts<T> in the CMPS implementation and changing let pr: T = consts::INGA; to let pr = T::INGA resulting in the following:

impl<T: Clone + ntraits::Float + ntraits::FromPrimitive + consts<T>> CMPS<T> {
    pub fn cerf(a: T, b: T) -> CMPS<T> {
        let pr = T::INGA;
        CMPS { a: rtr, b: rti }
    }
}