CODEWITHSUNDEEP
arraysstringrustslice
rencedm112
rencedm112

Reputation: 577

In Rust, why is '[T]' and 'str' slice type not a syntax error?

From what I understand, &[T] type is a 'fat' pointer (memory address and size) and not the slice itself and [T] is the actual slice that is being referenced. But why didn't they make [T] a syntax error in the context below?

let y: [i32; 6] = [1, 2, 3, 4, 5, 6];

// let z: [i32] = y[..]; // error: the size for values of type `[i32]` cannot be known at compilation time

let z: &[i32] = &y[..]; // OK

// let v: str = "Hello World"; // the size for values of type `str` cannot be known at compilation time

From the error: [i32] cannot be known at compilation, is this just an error to make me, the user, understand why this syntax isn't possible or is it because I'm not using this syntax right and it is valid in some context?

Upvotes: 0

Views: 165

Answers (2)

Colonel Thirty Two
Colonel Thirty Two

Reputation: 26549

Because it is possible to use unsized types like str, [T], and dyn MyTrait in some contexts.

The most common at the moment is in generics. You can, for example, have a Box<[T]> or Box<str>, Arc<[T]>, or any generic with a ?Sized bound.

There's also a few in-development features using such unsized types:

Upvotes: 5

Masklinn
Masklinn

Reputation: 42207

But why didn't they make [T] a syntax error in the context below?

Because there's nothing about the syntax that has any reason to be invalid?

While DSTs are difficult to work with and don't make sense everywhere, there are contexts in which they are perfectly legal e.g.

struct Foo {
    a: usize,
    b: [i32]
}

Upvotes: 2

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