Add iterator reduction coverage to never_loop

[cuongleqq]

Fixes #16061.

Extend never_loop to also lint iterator reduction methods (e.g. for_each, try_for_each, fold, try_fold, reduce, all, any) when the closure’s body diverges (return type !). Add a UI test never_loop_iterator_reduction to cover these cases.

Testing:

• TESTNAME=never_loop_iterator_reduction cargo uitest

changelog: [never_loop]: lint diverging iterator reduction closures like for_each and fold

[rustbot]

r? @Jarcho

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[Jarcho]

This should be combined with the previous method check as something like

if let ExprKind::MethodCall(path, recv, args, _) = expr.kind {
  match (path.ident.name, args) {
    // methods here
  }
}

[cuongleqq]

I used a single MethodCall destructure as suggested, but I kept 2 if branches instead of a single match because some methods like for_each, all, any are handled by both lints. Match will short-circuit to only the first matching arm.

[Jarcho]

Still make it a single match. A match arm can do multiple things.

[cuongleqq]

Ok! I switched it to a single match as suggested. For the overlapping methods, the match arm now calls both unused_enumerate_index and never_loop. Other arms handle the single-lint cases.

[Jarcho]

Should be WithPlaceholders

[Jarcho]

This should just be snippet_with_context. We already know the receiver is an iterator.

[Jarcho]

This should be recv.ty_based_def(cx).assoc_fn_parent(cx).is_diag_item(cx, sym::Iterator).

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[cuongleqq]

I pushed an update to address all 4 comments

• Combined the method check for both unused_enumerate_index and never_loop
• Use a more direct check for Iterator trait
• Use HasPlaceholders for the applicability
• Use snippet_with_context instead of make_iterator_snippet

@rustbot ready

[cuongleqq]

Addressed the requested changes:

• Switched to a single match for the MethodCall handling

@rustbot ready