Tracking issue for future-incompatibility lint `array_into_iter`

[LukasKalbertodt]

What is this lint about?

Method resolution is responsible for finding a fitting method for a method call expression receiver.name(args). The expression array.into_iter() (where array has an array type [T; N]) currently resolves to either <&[T; N] as IntoIterator>::into_iter (for arrays smaller than 33) or <&[T] as IntoIterator>::into_iter (for larger arrays). In either way, an iterator over references to the array's elements is returned.

In the future, we might want to add impl IntoIterator for [T; N] (for arrays by value). In that case, method resolution would prioritize <[T;N] as IntoIterator>::into_iter as that method call would not require an autoref-coercion. In other words: the receiver expression (left of the dot in the method call) fits the receiver type of the method perfectly, so that method is preferred. In the &[T; N] or &[T] case, coercions are necessary to make the method call work.

Since the new method is prioritized over the old ones, some code can break. Usually that code looks somewhat like this:

[1, 2, 3].into_iter().for_each(|n| { *n; });

Currently this works, as into_iter returns an iterator over references to the array's values, meaning that n is indeed &{integer} and can be dereferenced. With the new impl, that code would stop compiling. The lint has been put in place to warn of this potentially upcoming breaking change.

How to fix this warning/error

Replace .into_iter() with .iter(). The latter is guaranteed to always resolve to an iterator over references to the elements.

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Current status

• Lint merged as warn by default in 1.41 (stable on 2020-01-31): Add future incompatibility lint for array.into_iter() #66017
• Lint also lints boxed arrays starting from 1.42 (stable on 2020-03-13): Generalize array_into_iter lint to also lint for boxed arrays #67524

[est31]

Was hitting this in a private (currently not open source) project of mine. Thanks for introducing this slowly so that it can be caught in time instead of just breaking it!