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Rollup merge of #152281 - JohnTitor:sugg-mut-deref-borrows, r=estebank

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borrowck: suggest `&mut *x` for pattern reborrows

Fixes rust-lang/rust#81059
r? @estebank as you should have some context here, but feel free to re-assign if you don't have time to review right now.
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Urgau 2026-02-12 00:04:14 +01:00 • committed by GitHub
commit 050b48a693
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7 changed files with 175 additions and 23 deletions
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73
compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
View file

@ -354,14 +354,71 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
self.infcx.tcx.sess.source_map().span_to_snippet(source_info.span)
{
if snippet.starts_with("&mut ") {
// We don't have access to the HIR to get accurate spans, but we can
// give a best effort structured suggestion.
err.span_suggestion_verbose(
source_info.span.with_hi(source_info.span.lo() + BytePos(5)),
"if there is only one mutable reborrow, remove the `&mut`",
"",
Applicability::MaybeIncorrect,
);
// In calls, `&mut &mut T` may be deref-coerced to `&mut T`, and
// removing the extra `&mut` is the most direct suggestion. But for
// pattern-matching expressions (`match`, `if let`, `while let`), that
// can easily turn into a move, so prefer suggesting an explicit
// reborrow via `&mut *x` instead.
let mut in_pat_scrutinee = false;
let mut is_deref_coerced = false;
if let Some(expr) = self.find_expr(source_info.span) {
let tcx = self.infcx.tcx;
let span = expr.span.source_callsite();
for (_, node) in tcx.hir_parent_iter(expr.hir_id) {
if let Node::Expr(parent_expr) = node {
match parent_expr.kind {
ExprKind::Match(scrutinee, ..)
if scrutinee
.span
.source_callsite()
.contains(span) =>
{
in_pat_scrutinee = true;
break;
}
ExprKind::Let(let_expr)
if let_expr
.init
.span
.source_callsite()
.contains(span) =>
{
in_pat_scrutinee = true;
break;
}
_ => {}
}
}
}
let typeck = tcx.typeck(expr.hir_id.owner.def_id);
is_deref_coerced =
typeck.expr_adjustments(expr).iter().any(|adj| {
matches!(adj.kind, ty::adjustment::Adjust::Deref(_))
});
}
if in_pat_scrutinee {
// Best-effort structured suggestion: insert `*` after `&mut `.
err.span_suggestion_verbose(
source_info
.span
.with_lo(source_info.span.lo() + BytePos(5))
.shrink_to_lo(),
"to reborrow the mutable reference, add `*`",
"*",
Applicability::MaybeIncorrect,
);
} else if is_deref_coerced {
// We don't have access to the HIR to get accurate spans, but we
// can give a best effort structured suggestion.
err.span_suggestion_verbose(
source_info.span.with_hi(source_info.span.lo() + BytePos(5)),
"if there is only one mutable reborrow, remove the `&mut`",
"",
Applicability::MaybeIncorrect,
);
}
} else {
// This can occur with things like `(&mut self).foo()`.
err.span_help(source_info.span, "try removing `&mut` here");