Module index_ext::tag

Not quite dependent typing for eliding bounds checks.

Lifetime generativity

The main idea is to use lifetimes as a compile time tag to identify a particular exact slice without keeping a direct reference of it. This means that you can not choose any of the lifetime parameters that you see in this module. Instead, you must be prepared to handle arbitrary lifetime which will make it opaque to you and the compiler. Each lifetime guarantees that all Ref and Mut with that exact lifetime are at least as long as all the sizes in Len structs of the same lifetime and each Idx is bounded by some Len. While this may seem very restrictive at first, it still allows you to pass information on a slice's length across function boundaries by explicitly mentioning the same lifetime twice. Additionally you're allowed some mutable operations on indices that can not exceed the original bounds.

Use with_ref and with_mut as the main entry constructors.

Checked constant bounds

Alternatively we can choose other unique type instances. By that we mean that for any particular type exactly one value must be used to construct ExactSize. One possible way is if this is simply a constant which is implemented by the Constant wrapper and its ConstantSource trait. For example one may define:

use index_ext::tag::{Constant, ConstantSource, ExactSize};

const BUFFER_SIZE: usize = 4096;
struct BufferSize4K;

impl ConstantSource for BufferSize4K {
    const LEN: usize = BUFFER_SIZE;
}

const LEN: ExactSize<Constant<BufferSize4K>> = Constant::EXACT_SIZE;

Named bounds

And finally one might come up with an internal naming scheme where types are used to express unique bounds. This requires some unsafe code and the programmers guarantee of uniqueness of values but permits the combination of runtime values with 'static lifetime.

Structs

Boxed	An owned, allocated slice with a checked length.
Constant	A tag using a ConstantSource.
ExactSize	The exact length separating slices and indices for a tag.
Generative	A generative lifetime.
Idx	A valid index for all slices of the same length.
Len	The length of a particular slice (or a number of slices).
Mut	A mutable slice with a unique lifetime.
Named	A named unique tag.
NonZeroLen	The length of a non-empty slice.
Ref	A slice with a unique lifetime.

Traits

ConstantSource	A type that names a constant buffer size.
Tag	A type suitable for tagging length, indices, and containers.

Functions

with_mut	Enter a region for soundly indexing a mutable slice without bounds checks.
with_ref	Enter a region for soundly indexing a slice without bounds checks.