Min_const_generics
- Rust course
- Advanced
- Lesson 69/81
- 4 min
Learn the limits and valid uses of min_const_generics.
Stable const generics limits
In stable Rust (min_const_generics), const parameters can only be integers, bool and char. Also, expressions involving const parameters can't be used in types:
fn f<const N: usize>() -> [u8; N] { [0; N] } // fine
// fn g<const N: usize>() -> [u8; N + 1] { .. } // error: generic expression
// struct S<const AD: &'static str>; // error: type not allowedExpressions like N + 1 are only possible on nightly with the generic_const_exprs feature. In stable code the solution is usually going back to slices or converting at runtime with try_into.
From slice to array
Taking a slice's first N elements as a fixed-size array, if the length allows:
fn ilk_n<const N: usize>(v: &[i32]) -> Option<[i32; N]> {
v.get(..N)?.try_into().ok()
}
println!("{:?}", ilk_n::<2>(&[5, 6, 7])); // Some([5, 6])
println!("{:?}", ilk_n::<5>(&[5, 6, 7])); // Noneget(..N) gives None out of bounds; try_into converts the slice of length N into an array.
Practice First N elements
Write the ilk_n function above. Print the results of ilk_n::<2>(&[5, 6, 7]) and ilk_n::<5>(&[5, 6, 7]) with {:?}, separated by a space.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn ilk_n<const N: usize>(v: &[i32]) -> Option<[i32; N]> {
v.get(..N)?.try_into().ok()
}
fn main() {
println!("{:?} {:?}", ilk_n::<2>(&[5, 6, 7]), ilk_n::<5>(&[5, 6, 7]));
}Expected output
Some([5, 6]) None
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How do you go from a slice to an array of a specific length?
- as [T; N]
- You can't
- It needs unsafe
- <[T; N]>::try_from(slice) or slice.try_into() Correct answer
Explanation: If the length doesn't match, Err is returned, so the runtime check stays safe.
2 Convert to an array
fn ilk_n<const N: usize>(v: &[i32]) -> Option<[i32; N]> {
v.get(..N)?.____().ok()
}fn ilk_n<const N: usize>(v: &[i32]) -> Option<[i32; N]> {
v.get(..N)?.try_into().ok()
}Explanation: try_into converts the slice into an array with a length check.
3 Syntax and stable status
| Term | Meaning |
|---|---|
<const N: usize> |
Stable |
<const B: bool> |
Stable (bool) |
[u8; N + 1] |
Nightly only |
<const S: &str> |
Not supported |