Just nitpicking, but a better explanation of the expansion of `vec![1,2]` would use `Vec::with_capacity(2)` instead of `Vec::new()`, that is, it already allocat

Just nitpicking, but a better explanation of the expansion of `vec![1,2]` would use `Vec::with_capacity(2)` instead of `Vec::new()`, that is, it already allocates the Vec with the correct capacity (the real expansion uses in-place construction of a boxed slice, but the effect is similar).