error[E0015]: calls in constants are limited to tuple structs and tuple variants
> src\main.rs:3:20

3  const A: f64 = 1.0/4.0_f64.sqrt();
 ^^^^^^^^^^^^^^

note: a limited form of compiletime function evaluation is available on a nightly compiler via `const fn`
> src\main.rs:3:20
Your example is obviously trivial, but if you can express your number in terms of sqrt(2) or sqrt(π), you may be able to build it with the standard consts: std::f64::consts  Rust
The following constants are required globally in my code:
const A: f64 = 1.0/(2.0_f64.sqrt());
const B: f64 = 1.0/10.0_f64.sqrt();
const C: f64 = 1.0/42.0_f64.sqrt();
Rust should support this by default. It is not efficient to hard code the sqrt of a number before it can be assigned to a const variable.