Rust collections should support inline capacity specified as a compile-time integer

[zwarich]

Many C++ projects (including all web browser engines that I am aware of, LLVM, etc.) use their own C++ collections where collections have an inline capacity specified at compile time, e.g. Vec<T, 4>. This means that the collection reserves space for a fixed number of elements, and it until its capacity goes above the inline capacity it can avoid a heap allocation.

Obviously, this feature requires support for type-level integers in the first place. It also increases the pressure to deal with existing ergonomic issues regarding type constructor parameters, since e.g. HashMap would have key, value, hasher, inline capacity, and allocator parameters.

[zwarich]

CC: @larsbergstrom @pcwalton

[pcwalton]

Note that I've implemented this in Servo without the type-level integer support by just macro-expanding SmallVec1, SmallVec2, SmallVec4, etc. Obviously not ideal but works in a pinch.

[pcwalton]

Oops, hit wrong button.

[bluss]

Quite an esoteric feature as an addon to the more general issue of Supporting Integer parameters in generics

[Gankra]

CC me

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#319