`autoclone` variable marker

language design
25

The original problem was the bad ergonomics when cloning things into a closure, because one also needs to introduce a binding holding a variable often named very similar to the original.

The suggestion by @scottmcm of extending move with a list of how variables are captured solves this issue perfectly (at least in my eyes).

Other situations (when needing to clone something a lot) do not have this ergonomic problem of requiring you to create a binding for each clone. You might need to write .clone() a couple of times, but I think the explicit call has multiple benefits:

  • easily scan for uses of clone using text search
  • no implicitly executed code
  • clone is not special, it is just another function (drop on the other hand is special)

While a binding marker like autoclone would improve this situation I think it is unnecessary in the face of better ergonomics for specifying closure capturing.

I am firmly against introducing an AutoClone Trait (because it executes non-special code implicitly), when I come across cloning data, it is one of these situations:

  • I want to turn a reference into something that i own (should be explicit)
  • I need to pass the same data to multiple functions (should be explicit)
  • I need to pass the same data to multiple threads (should be explicit, but without the additional let bindings)

I have not yet seen an example where autoclone/AutoClone would be useful (prevent a lot of clone calls) that is not situation 3 (because there it would also get rid of the additional bindings).

When you really have such extensive usage of clone (situation 1 & 2), why not use a closure to clone your data?

let data = ...;
let data = || data.clone();
consume(data());
consume(data());
consume(data());
consume(data());
consume(data());
consume(data());
consume(data());
consume(data());
consume(data());
consume(data());

Aside from the additional () and creation of the closure, this is identical to the autoclone/AutoClone syntax. But you still have the clone call to find with text search and the explicit closure call indicating that some code is run there.

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