• Feature Name: dyn_type
• Start Date: 2018-01-04
• RFC PR: (leave this empty)
• Rust Issue: (leave this empty)

Summary

Generalize the concept of Dynamic Sized Types (DSTs) and enable custom DSTs.

Motivation

This RFC is a replacement of RFC #1524. The main motivations of custom DSTs are:

• DST allows us to get rid of combinatorial explosion when interacting with smart pointers. Instead of defining BoxMatrix, RcMatrix, GcMatrix etc, we get Box<Matrix>, Rc<Matrix>, Gc<Matrix> from a single type.

• The Index and Deref trait requires us to return a reference without allocation, i.e. indexing operation can’t return a MatrixRef<'a> even if this is the only solution without custom DST.

• Allow more possibilities for modifying existing APIs involving DST types without breakage.

We expect many types will benefit from custom DSTs, here are some examples:

• Multi-dimensional arrays (matrix and tensor libraries).
• Length-prefixed structures e.g. Pascal strings or C structures with a flexible array member.
• CStr.
• Multi-encoded string.
• Bit array.

Guide-level explanation

• Tutorial
• Examples

Reference-level explanation

• Changes
• Custom DST from scratch

Drawbacks

• Drawbacks

Rationale and alternatives

• Rationales
• Alternatives
• Extensions
• Bikeshed

Unresolved questions

• Currently all super-traits of Sized (i.e. Aligned, DynSized, and Object) are marked #[fundamental], but it is unclear whether we really need this attribute on them.