IntoIterator for arrays
Summary
Details
Until Rust 1.53, only references to arrays implement IntoIterator
.
This means you can iterate over &[1, 2, 3]
and &mut [1, 2, 3]
,
but not over [1, 2, 3]
directly.
for &e in &[1, 2, 3] {} // Ok :)
for e in [1, 2, 3] {} // Error :(
This has been a long-standing issue, but the solution is not as simple as it seems.
Just adding the trait implementation would break existing code.
array.into_iter()
already compiles today because that implicitly calls
(&array).into_iter()
due to how method call syntax works.
Adding the trait implementation would change the meaning.
Usually we categorize this type of breakage (adding a trait implementation) 'minor' and acceptable. But in this case there is too much code that would be broken by it.
It has been suggested many times to "only implement IntoIterator
for arrays in Rust 2021".
However, this is simply not possible.
You can't have a trait implementation exist in one edition and not in another,
since editions can be mixed.
Instead, we decided to add the trait implementation in all editions (starting in Rust 1.53.0),
but add a small hack to avoid breakage until Rust 2021.
In Rust 2015 and 2018 code, the compiler will still resolve array.into_iter()
to (&array).into_iter()
like before, as if the trait implementation does not exist.
This only applies to the .into_iter()
method call syntax.
It does not affect any other syntax such as for e in [1, 2, 3]
, iter.zip([1, 2, 3])
or
IntoIterator::into_iter([1, 2, 3])
.
Those will start to work in all editions.
While it's a shame that this required a small hack to avoid breakage, we're very happy with how this solution keeps the difference between the editions to an absolute minimum. Since the hack is only present in the older editions, there is no added complexity in the new edition.