rust-by-practice/solutions/generics-traits/const-generics.md

1.

```rust
struct Array<T, const N: usize> {
    data : [T; N]
}

fn main() {
    let arrays = [
        Array{
            data: [1, 2, 3],
        },
        Array {
            data: [1, 2, 3],
        },
        Array {
            data: [1, 2,4]
        }
    ];
}
```

2.

```rust
fn print_array<T: std::fmt::Debug, const N: usize>(arr: [T; N]) {
    println!("{:?}", arr);
}
fn main() {
    let arr = [1, 2, 3];
    print_array(arr);

    let arr = ["hello", "world"];
    print_array(arr);
}
```

3.

```rust
#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

fn check_size<T>(val: T)
where
    Assert<{ core::mem::size_of::<T>() < 768 }>: IsTrue,
{
    //...
}

// fix the errors in main
fn main() {
    check_size([0u8; 767]); 
    check_size([0i32; 191]);
    check_size(["hello你好"; 47]); // &str is a string reference, containing a pointer and string length in it, so it takes two word long, in x86-64, 1 word = 8 bytes
    check_size(["hello你好".to_string(); 31]);  // String is a smart pointer struct, it has three fields: pointer, length and capacity, each takes 8 bytes
    check_size(['中'; 191]); // A char takes 4 bytes in Rust
}



pub enum Assert<const CHECK: bool> {}

pub trait IsTrue {}

impl IsTrue for Assert<true> {}
```