Document allowed vector lengths

Author: calebzulawski

crates/core_simd/src/masks.rs

@@ -118,6 +118,8 @@ impl_element! { isize, usize }
 /// The layout of this type is unspecified, and may change between platforms
 /// and/or Rust versions, and code should not assume that it is equivalent to
 /// `[T; N]`.
+///
+/// `N` cannot be 0 and may be at most 64.
 #[repr(transparent)]
 pub struct Mask<T, const N: usize>(Simd<T, N>)
 where

crates/core_simd/src/vector.rs

@@ -51,6 +51,8 @@ use crate::simd::{
 /// Thus it is sound to [`transmute`] `Simd<T, N>` to `[T; N]` and should optimize to "zero cost",
 /// but the reverse transmutation may require a copy the compiler cannot simply elide.
 ///
+/// `N` cannot be 0 and may be at most 64.
+///
 /// # ABI "Features"
 /// Due to Rust's safety guarantees, `Simd<T, N>` is currently passed and returned via memory,
 /// not SIMD registers, except as an optimization. Using `#[inline]` on functions that accept