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0231-upvar-capture-inference.md

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Summary

The || unboxed closure form should be split into two forms—|| for nonescaping closures and move || for escaping closures—and the capture clauses and self type specifiers should be removed.

Motivation

Having to specify ref and the capture mode for each unboxed closure is inconvenient (see Rust PR rust-lang/rust#16610). It would be more convenient for the programmer if the type of the closure and the modes of the upvars could be inferred. This also eliminates the "line-noise" syntaxes like |&:|, which are arguably unsightly.

Not all knobs can be removed, however—the programmer must manually specify whether each closure is escaping or nonescaping. To see this, observe that no sensible default for the closure || (*x).clone() exists: if the function is nonescaping, it's a closure that returns a copy of x every time but does not move x into it; if the function is escaping, it's a closure that returns a copy of x and takes ownership of x.

Therefore, we need two forms: one for nonescaping closures and one for escaping closures. Nonescaping closures are the commonest, so they get the || syntax that we have today, and a new move || syntax will be introduced for escaping closures.

Detailed design

For unboxed closures specified with ||, the capture modes of the free variables are determined as follows:

  1. Any variable which is closed over and borrowed mutably is by-reference and mutably borrowed.

  2. Any variable of a type that does not implement Copy which is moved within the closure is captured by value.

  3. Any other variable which is closed over is by-reference and immutably borrowed.

The trait that the unboxed closure implements is FnOnce if any variables were moved out of the closure; otherwise FnMut if there are any variables that are closed over and mutably borrowed; otherwise Fn.

The ref prefix for unboxed closures is removed, since it is now essentially implied.

We introduce a new grammar production, move ||. The value returned by a move || implements FnOnce, FnMut, or Fn, as determined above; thus, for example, move |x: int, y| x + y produces an unboxed closure that implements the Fn(int, int) -> int trait (and thus the FnOnce(int, int) -> int trait by inheritance). Free variables referenced by a move || closure are always captured by value.

In the trait reference grammar, we will change the |&:| sugar to Fn(), the |&mut:| sugar to FnMut(), and the |:| sugar to FnOnce(). Thus what was before written fn foo<F:|&mut: int, int| -> int>() will be fn foo<F:FnMut(int, int) -> int>().

It is important to note that the trait reference syntax and closure construction syntax are purposefully distinct. This is because either the || form or the move || form can construct any of FnOnce, FnMut, or Fn closures.

Drawbacks

  1. Having two syntaxes for closures could be seen as unfortunate.

  2. move becomes a keyword.

Alternatives

  1. Keep the status quo: |:|/|&mut:/|&:| are the only ways to create unboxed closures, and ref must be used to get by-reference upvars.

  2. Use some syntax other than move || for escaping closures.

  3. Keep the |:|/|&mut:/|&:| syntax only for trait reference sugar.

  4. Use fn() syntax for trait reference sugar.

Unresolved questions

There may be unforeseen complications in doing the inference.