Support vec![const { ... }; n] syntax for creating a Vec of non-Clone values (original) (raw)

Proposal

Problem statement

For array types, I can use [const { ... }; N] syntax to create an array of const { ... }, even if it’s does not implement Clone. But I can’t do the same thing with Vec: vec![const { ... }; n] currently does not compile.

Motivating examples or use cases

Imagining implementing a DFS algorithm with an array to save the traversal states, I can write something like:

enum State { NotVisited, Visiting, Visited, }

let states = std::iter::repeat_with(|| State::NotVisited).take(n).collect::<Vec<_>>();

But it could be more simple if I could just write:

let states = vec![const { State::NotVisited }; n];

Deriving Clone might not always be possible because the value type could be from a third party crate.

Solution sketch

Reimplement vec! macro so it supports the const { ... } syntax above.

Alternatives

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

Second, if there's a concrete solution: