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Enhance never loop #17145

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Merged
samueltardieu merged 1 commit into from
Jun 10, 2026
+321 −3
Merged

Enhance never loop #17145

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82 changes: 80 additions & 2 deletions clippy_lints/src/loops/never_loop.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,7 @@ pub(super) fn check<'tcx>(
NeverLoopResult::Diverging {
ref break_spans,
ref never_spans,
ref non_obvious_exprs,
} => {
span_lint_and_then(cx, NEVER_LOOP, span, "this loop never actually loops", |diag| {
if let Some(ForLoop {
Expand All @@ -54,7 +55,7 @@ pub(super) fn check<'tcx>(
for_span.with_hi(iterator.span.hi()),
for_to_if_let_sugg(cx, iterator, pat),
)];
// Make sure to clear up the diverging sites when we remove a loopp.
// Make sure to clear up the diverging sites when we remove a loop.
suggestions.extend(break_spans.iter().map(|span| (*span, String::new())));
diag.multipart_suggestion(
"if you need the first element of the iterator, try writing",
Expand All @@ -69,6 +70,15 @@ pub(super) fn check<'tcx>(
);
}
}

let non_obvious_spans = non_obvious_exprs
.iter()
.map(|hir_id| cx.tcx.hir_expect_expr(*hir_id))
.flat_map(|expr| find_non_obvious_spans(cx, expr));

for span in non_obvious_spans {
diag.span_note(span, "this expression never returns");
}
});
},
NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Normal => (),
Expand Down Expand Up @@ -135,6 +145,7 @@ enum NeverLoopResult {
Diverging {
break_spans: Vec<Span>,
never_spans: Vec<Span>,
non_obvious_exprs: Vec<HirId>,
},
/// We have not encountered any main loop continue,
/// and subsequent control flow is (possibly) reachable
Expand Down Expand Up @@ -181,17 +192,21 @@ fn combine_branches(b1: NeverLoopResult, b2: NeverLoopResult) -> NeverLoopResult
NeverLoopResult::Diverging {
break_spans: mut break_spans1,
never_spans: mut never_spans1,
non_obvious_exprs: mut non_obvious_exprs1,
},
NeverLoopResult::Diverging {
break_spans: mut break_spans2,
never_spans: mut never_spans2,
non_obvious_exprs: mut non_obvious_exprs2,
},
) => {
break_spans1.append(&mut break_spans2);
never_spans1.append(&mut never_spans2);
non_obvious_exprs1.append(&mut non_obvious_exprs2);
NeverLoopResult::Diverging {
break_spans: break_spans1,
never_spans: never_spans1,
non_obvious_exprs: non_obvious_exprs1,
}
},
}
Expand Down Expand Up @@ -278,7 +293,7 @@ fn is_label_for_block(cx: &LateContext<'_>, dest: &Destination) -> bool {
#[expect(clippy::too_many_lines)]
fn never_loop_expr<'tcx>(
cx: &LateContext<'tcx>,
expr: &Expr<'tcx>,
expr: &'tcx Expr<'tcx>,
local_labels: &mut Vec<(HirId, bool)>,
main_loop_id: HirId,
) -> NeverLoopResult {
Expand Down Expand Up @@ -335,6 +350,7 @@ fn never_loop_expr<'tcx>(
NeverLoopResult::Diverging {
break_spans: vec![],
never_spans: vec![],
non_obvious_exprs: vec![],
},
|a, b| combine_branches(a, never_loop_expr(cx, b.body, local_labels, main_loop_id)),
)
Expand All @@ -361,6 +377,7 @@ fn never_loop_expr<'tcx>(
NeverLoopResult::Diverging {
break_spans: all_spans_after_expr(cx, expr),
never_spans: vec![],
non_obvious_exprs: vec![],
}
}
},
Expand All @@ -374,6 +391,7 @@ fn never_loop_expr<'tcx>(
NeverLoopResult::Diverging {
break_spans: vec![],
never_spans: vec![],
non_obvious_exprs: vec![],
}
})
},
Expand All @@ -391,13 +409,15 @@ fn never_loop_expr<'tcx>(
all_spans_after_expr(cx, expr)
},
never_spans: vec![],
non_obvious_exprs: vec![],
}
})
},
ExprKind::Become(e) => combine_seq(never_loop_expr(cx, e, local_labels, main_loop_id), || {
NeverLoopResult::Diverging {
break_spans: vec![],
never_spans: vec![],
non_obvious_exprs: vec![],
}
}),
ExprKind::InlineAsm(asm) => combine_seq_many(asm.operands.iter().map(|(o, _)| match o {
Expand Down Expand Up @@ -434,11 +454,13 @@ fn never_loop_expr<'tcx>(
| ExprKind::Lit(_)
| ExprKind::Err(_) => NeverLoopResult::Normal,
};

let result = combine_seq(result, || {
if cx.typeck_results().expr_ty(expr).is_never() {
NeverLoopResult::Diverging {
break_spans: vec![],
never_spans: all_spans_after_expr(cx, expr),
non_obvious_exprs: vec![expr.hir_id],
}
} else {
NeverLoopResult::Normal
Expand Down Expand Up @@ -480,3 +502,59 @@ fn mark_block_as_reachable(expr: &Expr<'_>, local_labels: &mut [(HirId, bool)])
*reachable = true;
}
}

fn find_non_obvious_spans<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'tcx>) -> Vec<Span> {
let mut spans = vec![];

for_each_expr_without_closures(e, |expr: &'tcx Expr<'tcx>| -> ControlFlow<(), Descend> {
if cx.typeck_results().expr_ty(expr).is_never() && !expr.span.from_expansion() {
match expr.kind {

@samueltardieu samueltardieu Jun 7, 2026

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How did you choose the list here? Can you add a comment so that future kinds can be added if needed?

I think I would have preferred if you listed the 27 missing variants instead of _ so that if a new ExprKind is added it has to be handled properly here.

// The first arm handles both directly divergent expressions and expressions
// that contain divergence indirectly. The latter are inspected to identify
// possible inner non-trivial divergent expressions.
ExprKind::Break(..)
| ExprKind::Continue(..)
| ExprKind::Ret(..)
| ExprKind::Become(..)
| ExprKind::Loop(..)
| ExprKind::Block(..)
| ExprKind::Match(..)
| ExprKind::If(..) => {
return ControlFlow::Continue(Descend::Yes);
},
ExprKind::ConstBlock(..)
| ExprKind::Array(..)
| ExprKind::Call(..)
| ExprKind::MethodCall(..)
| ExprKind::Use(..)
| ExprKind::Tup(..)
| ExprKind::Binary(..)
| ExprKind::Unary(..)
| ExprKind::Lit(..)
| ExprKind::Cast(..)
| ExprKind::Type(..)
| ExprKind::DropTemps(..)
| ExprKind::Let(..)
| ExprKind::Closure(..)
| ExprKind::Assign(..)
| ExprKind::AssignOp(..)
| ExprKind::Field(..)
| ExprKind::Index(..)
| ExprKind::Path(..)
| ExprKind::AddrOf(..)
| ExprKind::InlineAsm(..)
| ExprKind::OffsetOf(..)
| ExprKind::Struct(..)
| ExprKind::Repeat(..)
| ExprKind::Yield(..)
| ExprKind::UnsafeBinderCast(..)
| ExprKind::Err(..) => {
spans.push(expr.span);
return ControlFlow::Continue(Descend::No);
},
}
}
ControlFlow::Continue(Descend::Yes)
});
spans
}
103 changes: 103 additions & 0 deletions tests/ui/never_loop.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -541,3 +541,106 @@ fn issue16462() {
n >= 0 || break;
}
}

fn issue16056_basic() {
let x = || Some(panic!());

//~v never_loop
loop {
x().unwrap();
}
}

fn issue16056_nested_loops() {
let x = || Some(panic!());

//~v never_loop
loop {
//~v never_loop
loop {
x().unwrap();
}
}
}

fn issue16056_nested_block() {
let x = || Some(panic!());

//~v never_loop
loop {
{
x().unwrap();
}
}
}

fn issue16056_nested_blocks() {
let x = || Some(panic!());

//~v never_loop
loop {
{
{
x().unwrap();
}
}
}
}

fn issue16056_match() {
let x = || Some(panic!());

let mut y = 1;
//~v never_loop
loop {
y += 1;
match y {
5 => return,
_ => {
x().unwrap();
},
}
}
}

fn issue16056_if_no_trigger() {
let x = || Some(panic!());
let mut y = 0;
//~v never_loop
loop {
y += 1;
if y == 1 {
// No extra note for this
x().unwrap();
}
break;
}
}

fn issue16056_if_trigger() {
let x = || Some(panic!());

//~v never_loop
loop {
if true {
x().unwrap();
} else {
break;
}
}
}

macro_rules! test_macro {
($e:expr) => {
$e
};
}

fn issue16056_macro() {
let x = || Some(panic!());

//~v never_loop
loop {
test_macro!(x().unwrap());
}
}
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