add const generics

solutions/generics-traits/const-generics.md

@@ -0,0 +1,64 @@
+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> {}
+```