[[T]] is allowed in structs

[kornelski]

The following construct compiles:

struct Foo {
 foo: [[u8]],
}

Even though let foo: [[u8]] doesn't. From my very limited understanding this type doesn't make sense, so it'd be nice if the compiler rejected it at the point of definition.

[abonander]

Because of [[u8]], Foo is an unsized struct, or a dynamically sized type (DST), which means it can only be bound by let behind a pointer, e.g. &Foo or Box<Foo>. It's the same with a bare [[u8]].

[Diggsey]

No, pornel is right, [u8] is a DST, but [[u8]] is an invalid type - you can't have a slice of DSTs because arrays/slices require that all of their elements are the same size.

[abonander]

That makes sense now. The let foo: [[u8]] part threw me off, made it sound like the problem was simpler than that. You can't bind DST to the stack.

[Manishearth]

The same goes for [str], as was brought up in irc yesterday.

These aren't types that can ever be constructed, and the compiler doesn't know how to handle them, so they should be forbidden in all places.

[Manishearth]

See also: #21748

The generic issue is that [T] should be [T] where T: Sized internally.

[eddyb]

[[T]] makes perfect sense as exists N, M.[[T; N]; M], it's just not implemented - that's the main reason we should require T: Sized in [T] for now.

[brson]

Nominating because rustc accepts bogus syntax.

[nikomatsakis]

triage: P-medium

Assigning medium priority since it's hard to construct useful programs that rely on this, as far as I know.

[TimNN]

This seems to have been fixed on stable: http://is.gd/P0QUd6

[alexcrichton]

Yay!