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.last() to .next_back() requires a mutable receiver #14140

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.last() to .next_back() requires a mutable receiver #14140

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samueltardieu
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@samueltardieu samueltardieu commented Feb 2, 2025 •
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In the case where iter is a DoubleEndedIterator, replacing a call to iter.last() (which consumes iter) by iter.next_back() (which requires a mutable reference to iter) cannot be done when iter is a non-mutable binding which is not a mutable reference. When possible, a local immutable binding is made into a mutable one.

Also, the applicability is switched to MaybeIncorrect and a note is added to the output when the element types have a significant drop, because the drop order will potentially be modified because .next_back() does not consume the iterator nor the elements before the last one.

Fix #14139

changelog: [double_ended_iterator_last]: do not trigger on non-reference immutable receiver, and warn about possible drop order change

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rustbot commented Feb 2, 2025

r? @Centri3

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@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Feb 2, 2025
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Commits logically separated for easier review.

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Rebased

Centri3
Centri3 approved these changes Feb 14, 2025
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Just one thing: I suppose we shouldn't just change it to be mutable in the suggestion? I'm not sure if there'd be any case that doesn't work/causes an error considering .last() will already consume it.

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Just one thing: I suppose we shouldn't just change it to be mutable in the suggestion? I'm not sure if there'd be any case that doesn't work/causes an error considering .last() will already consume it.

Good question, let me try that, I'll switch to @rustbot author in the meantime.

On an unrelated topic, I also realized that this lint should probably not be MachineApplicable if the elements have a significant drop, as they will be dropped in a different order with .last() and .next_back().

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Feb 14, 2025
@samueltardieu
Add clippy_utils::is_mutable()
9c5ad66
@samueltardieu
Use clippy_utils::is_mutable() in unnecessary_struct_initialization
560bd36
@samueltardieu
Add error markers to doubleended_iterator_last ui test
30813c3
@samueltardieu
.last() to .next_back() requires a mutable receiver
8c58c18 
In the case where `iter` is a `DoubleEndedIterator`, replacing a call to
`iter.last()` (which consumes `iter`) by `iter.next_back()` (which
requires a mutable reference to `iter`) cannot be done when `iter`
Is not a mutable binding or a mutable reference.

When `iter` is a local binding, it can be made mutable by fixing its
definition site.
@samueltardieu
double_ended_iterator_last: note when drop order is changed
dd54d79 
`iter.last()` will drop all elements of `iter` in order, while
`iter.next_back()` will drop the non-last elements of `iter` when
`iter` goes out of scope since `.next_back()` does not consume its
argument.

When the transformation proposed by `double_ended_iterator_last` would
concern an iterator whose element type has a significant drop, a note is
added to warn about the possible drop order change, and the suggestion
is switched from `MachineApplicable` to `MaybeIncorrect`.
@samueltardieu
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Changes done, mutability is turned on whenever possible (good idea @Centri3). Also, a note is added and applicability is adjusted when the drop order may change.

@rustbot ready

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties and removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Feb 14, 2025
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double_ended_iterator_last should not trigger on immutable non-ref receiver
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