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Fix needless_collect suggests a suggestion that cannot be typed #17383

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Fix needless_collect suggests a suggestion that cannot be typed #17383

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355b42a
Fix needless_collect suggests a suggestion that cannot be typed
AnHoang-alex Jul 9, 2026
0c209c1
Remove expect now that needless_collect is fixed
AnHoang-alex Jul 9, 2026
5f14555
Code simplification (collect has only one generic parameter)
AnHoang-alex Jul 15, 2026
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39 changes: 39 additions & 0 deletions clippy_lints/src/methods/needless_collect.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,10 @@ pub(super) fn check<'tcx>(
return; // don't lint if the iterator has side effects
}

if collect_turbofish_is_fully_concrete(collect_expr) {
return; // don't lint if turbofish on collect maybe the only thing anchoring the type
}

match cx.tcx.parent_hir_node(collect_expr.hir_id) {
Node::Expr(parent) => {
check_collect_into_intoiterator(cx, parent, collect_expr, call_span, iter_expr);
Expand Down Expand Up @@ -230,6 +234,41 @@ fn check_collect_into_intoiterator<'tcx>(
}
}

/// Returns `true` if `collect_expr`'s turbofish is fully concrete (has
/// generic arguments and none of them are inference placeholders)
fn collect_turbofish_is_fully_concrete(collect_expr: &Expr<'_>) -> bool {
if let ExprKind::MethodCall(segment, ..) = collect_expr.kind
&& let Some(args) = segment.args
&& let [a] = args.args
{
generic_arg_is_fully_concrete(a)
} else {
false
}
}
Comment on lines +237 to +248

@hkBst hkBst Jul 11, 2026 •
edited by rustbot
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collect has only a single generic parameter, so either it is given and concrete, or it isn't. The code makes it seem like the number of generic parameters is unknown, and thus has to manually fix the empty case, because all would give true.

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@llogiq llogiq Jul 15, 2026

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I guess matching args.args with [a] if generic_arg_is_fully_concrete(a) should do the trick?


fn generic_arg_is_fully_concrete(arg: &rustc_hir::GenericArg<'_>) -> bool {
match arg {
rustc_hir::GenericArg::Infer(_) => false,
rustc_hir::GenericArg::Type(ty) => ty_is_fully_concrete(ty.as_unambig_ty()),
rustc_hir::GenericArg::Const(ct) => !matches!(ct.as_unambig_ct().kind, rustc_hir::ConstArgKind::Infer(..)),
rustc_hir::GenericArg::Lifetime(_) => true,
}
}

fn ty_is_fully_concrete(ty: &rustc_hir::Ty<'_>) -> bool {
match &ty.kind {
rustc_hir::TyKind::Infer(..) => false,
rustc_hir::TyKind::Path(rustc_hir::QPath::Resolved(_, path)) => path.segments.iter().all(|seg| {
seg.args
.is_none_or(|a| a.args.iter().all(generic_arg_is_fully_concrete))
}),
rustc_hir::TyKind::Ref(_, mut_ty) => ty_is_fully_concrete(mut_ty.ty),
rustc_hir::TyKind::Slice(ty) | rustc_hir::TyKind::Array(ty, _) => ty_is_fully_concrete(ty),
rustc_hir::TyKind::Tup(tys) => tys.iter().all(ty_is_fully_concrete),
_ => true,
}
}
/// Checks if the given method call matches the expected signature of `([&[mut]] self) -> bool`
fn is_is_empty_sig(cx: &LateContext<'_>, call_id: HirId) -> bool {
cx.typeck_results().type_dependent_def_id(call_id).is_some_and(|id| {
Expand Down
4 changes: 0 additions & 4 deletions tests/ui/needless_collect.fixed
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Original file line number Diff line number Diff line change
Expand Up @@ -198,10 +198,6 @@ mod issue8055_regression {
}
}

#[expect(
clippy::needless_collect,
reason = "FIXME: proposed fix cannot determine type, see issue #17315"
)]
fn foo() {
Foo {
inner: [].iter(),
Expand Down
4 changes: 0 additions & 4 deletions tests/ui/needless_collect.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -198,10 +198,6 @@ mod issue8055_regression {
}
}

#[expect(
clippy::needless_collect,
reason = "FIXME: proposed fix cannot determine type, see issue #17315"
)]
fn foo() {
Foo {
inner: [].iter(),
Expand Down
12 changes: 6 additions & 6 deletions tests/ui/needless_collect.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -122,7 +122,7 @@ LL | baz((0..10), (), ('a'..='z').collect::<Vec<_>>())
| ^^^^^^^^^^^^^^^^^^^^ help: remove this call

error: avoid using `collect()` when not needed
--> tests/ui/needless_collect.rs:224:26
--> tests/ui/needless_collect.rs:220:26
|
LL | let mut v = iter.collect::<Vec<_>>();
| ^^^^^^^
Expand All @@ -139,7 +139,7 @@ LL ~ iter.chain([1]).map(|x| x + 1).collect()
|

error: avoid using `collect()` when not needed
--> tests/ui/needless_collect.rs:236:26
--> tests/ui/needless_collect.rs:232:26
|
LL | let mut v = iter.collect::<Vec<_>>();
| ^^^^^^^
Expand All @@ -157,7 +157,7 @@ LL ~ iter.chain([1, 2]).map(|x| x + 1).collect()
|

error: avoid using `collect()` when not needed
--> tests/ui/needless_collect.rs:244:26
--> tests/ui/needless_collect.rs:240:26
|
LL | let mut v = iter.collect::<LinkedList<_>>();
| ^^^^^^^
Expand All @@ -174,7 +174,7 @@ LL ~ iter.chain([1]).map(|x| x + 1).collect()
|

error: avoid using `collect()` when not needed
--> tests/ui/needless_collect.rs:264:27
--> tests/ui/needless_collect.rs:260:27
|
LL | let mut ll = iter.collect::<LinkedList<_>>();
| ^^^^^^^
Expand All @@ -191,7 +191,7 @@ LL ~ iter.chain(s).map(|x| x + 1).collect()
|

error: avoid using `collect()` when not needed
--> tests/ui/needless_collect.rs:271:26
--> tests/ui/needless_collect.rs:267:26
|
LL | let mut v = iter.collect::<VecDeque<_>>();
| ^^^^^^^
Expand All @@ -209,7 +209,7 @@ LL ~ [1, 2].into_iter().chain(iter).map(|x| x + 1).collect()
|

error: avoid using `collect()` when not needed
--> tests/ui/needless_collect.rs:282:26
--> tests/ui/needless_collect.rs:278:26
|
LL | let mut v = iter.collect::<LinkedList<_>>();
| ^^^^^^^
Expand Down