Really stuck out to me too. Does that mean that somewhere in there there are 32 different "overloads" (or whatever the right term is) of this functionality for 1 to 32 elements? It feels so out of place...
Here's what's left (this was greatly reduced by const generics already, as another poster has explained):
https://doc.rust-lang.org/src/core/array/mod.rs.html#383
That macro implements Default, but only for arrays of size 1 to 32. It doesn't make any sense to talk about a Default array of size 0, the array is empty, there's no "default" content at all.
You can see the same verbosity for places where in some other languages you'd be able to just say all these things just work with any number of elements in a tuple, and Rust has chosen "up to 12" instead because it cannot express that.
https://doc.rust-lang.org/src/core/tuple.rs.html#100
The top half of that file is explaining to Rust's macro language how to do various things for an N-tuple, and then the bottom half call that macro for the 1 through 12 sizes.
If you really need a 16-tuple (unlikely, but...) you could copy paste the code to make one with the same features. This isn't compiler support, so you could just do it yourself.
There's one wrinkle in that strategy in that you might run afoul of the orphan rule[1][2] where you can't write:
impl ForeignTrait for ForeignType {}
struct MyArray<T>([T; 33]);
impl ForeignTrait for ForeignType {}
[2]: https://play.rust-lang.org/?version=stable&mode=debug&editio...
impl ForeignTrait for (MyType,u8)
The compiler will conclude that ForeignTrait is foreign, of course which is what we expected, but (MyType,u8) is also foreign even though MyType is local and so the orphan rule forbids this definition. Shame.
It doesn't make any sense to sidestep with new type because if you didn't want a tuple you could already define whatever type you wanted and this isn't an issue anyway, whereas if you do really want a tuple then a new type isn't a substitute.
32 was the limit selected.
- &[Elem]: A "slice" of elements, represented as a pointer and a length.
- [Elem; N]. A fixed-size array where the number of elements is known at compile time. This might be used for 2D or 3D coordinates, for example.
Array slices have always supported arbitrary lengths. But arrays with fixed compile-time sizes are represented as entirely different types: it's an error to pass a [i32; 2] to a function that wants an [i32; 3]. The size of the fixed-size array is part of the type.
And until very recently, Rust's type system didn't support defining methods on [Elem; N] that were generic over N. This is a feature that tends to be added later in many languages that support generics, not just Rust.