Add a section for const value repetition

Author: cuviper

posts/2021-02-11-Rust-1.50.0.md

@@ -52,6 +52,36 @@ fn main() {
 }
 ```
 
+### `const` value repetition for arrays
+
+Arrays in Rust can be written either as a list `[a, b, c]` or a repetition `[x; N]`.
+For lengths `N` greater than one, repetition has only been allowed for `Copy`
+types, and [RFC 2203] sought to allow any `const` expression there. However,
+while that feature was unstable for arbitrary expressions, its implementation
+since Rust 1.38 accidentally allowed stable use of `const` _values_ in array
+repetition.
+
+```rust
+fn main() {
+    // This is not allowed, because `Option<Vec<i32>>` does not implement `Copy`.
+    let array: [Option<Vec<i32>>; 10] = [None; 10];
+
+    const NONE: Option<Vec<i32>> = None;
+    const EMPTY: Option<Vec<i32>> = Some(Vec::new());
+
+    // However, repeating a `const` value is allowed!
+    let nones = [NONE; 10];
+    let empties = [EMPTY; 10];
+}
+```
+
+In Rust 1.50, that stabilization is formally acknowledged, so you may use this
+construct without fear. In the future, to avoid such "temporary" named
+constants, you can look forward to inline `const` expressions per [RFC 2920].
+
+[RFC 2203]: https://rust-lang.github.io/rfcs/2203-const-repeat-expr.html
+[RFC 2920]: https://rust-lang.github.io/rfcs/2920-inline-const.html
+
 ### Safe assignments to `ManuallyDrop<T>` union fields
 
 Rust 1.49 made it possible to add `ManuallyDrop<T>` fields to a `union` as part