Rust TryFrom: slice pattern let-else replaces manual length check

When implementing TryFrom<&[T]> for a fixed-arity type, let [a, b, c] = *slice else { return Err(BadLen) }; checks length and destructures in one step (works for Copy element types). Delegate array and slice impls to a single tuple impl to avoid duplicated range logic. For i16->u8 range checks, u8::try_from(x).map_err(|_| MyErr::IntConversion)? replaces hand-written 0..=255 comparisons. For AsMut<u32> on Box<u32>, let n = arg.as_mut(); *n *= *n; compiles fine since both reads happen through the same &mut.