From 0de74158757dcb2db583c27b412f14391ae9ce0c Mon Sep 17 00:00:00 2001 From: Yuki Okushi Date: Sat, 7 Feb 2026 19:44:34 +0900 Subject: [PATCH] borrowck: suggest `&mut *x` for pattern reborrows --- .../src/diagnostics/mutability_errors.rs | 73 +++++++++++++++++-- tests/ui/borrowck/mut-borrow-of-mut-ref.fixed | 37 ++++++++++ tests/ui/borrowck/mut-borrow-of-mut-ref.rs | 3 +- .../ui/borrowck/mut-borrow-of-mut-ref.stderr | 16 +--- .../reborrow-in-match-suggest-deref.fixed | 23 ++++++ .../reborrow-in-match-suggest-deref.rs | 23 ++++++ .../reborrow-in-match-suggest-deref.stderr | 23 ++++++ 7 files changed, 175 insertions(+), 23 deletions(-) create mode 100644 tests/ui/borrowck/mut-borrow-of-mut-ref.fixed create mode 100644 tests/ui/borrowck/reborrow-in-match-suggest-deref.fixed create mode 100644 tests/ui/borrowck/reborrow-in-match-suggest-deref.rs create mode 100644 tests/ui/borrowck/reborrow-in-match-suggest-deref.stderr diff --git a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs index 4fb739403ced..07d2313cc7fd 100644 --- a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs +++ b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs @@ -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"); diff --git a/tests/ui/borrowck/mut-borrow-of-mut-ref.fixed b/tests/ui/borrowck/mut-borrow-of-mut-ref.fixed new file mode 100644 index 000000000000..f70983c65223 --- /dev/null +++ b/tests/ui/borrowck/mut-borrow-of-mut-ref.fixed @@ -0,0 +1,37 @@ +// Suggest not mutably borrowing a mutable reference +//@ run-rustfix +#![crate_type = "rlib"] + +pub fn f(mut b: &mut i32) { + //~^ ERROR: cannot borrow + //~| NOTE: not mutable + //~| NOTE: the binding is already a mutable borrow + //~| HELP: consider making the binding mutable if you need to reborrow multiple times + h(b); + //~^ NOTE: cannot borrow as mutable + //~| HELP: if there is only one mutable reborrow, remove the `&mut` + g(&mut &mut b); + //~^ NOTE: cannot borrow as mutable +} + +pub fn g(mut b: &mut i32) { //~ NOTE: the binding is already a mutable borrow + //~^ HELP: consider making the binding mutable if you need to reborrow multiple times + h(&mut &mut b); + //~^ ERROR: cannot borrow + //~| NOTE: cannot borrow as mutable +} + +pub fn h(_: &mut i32) {} + +trait Foo { + fn bar(&mut self); +} + +impl Foo for &mut String { + fn bar(&mut self) {} +} + +pub fn baz(mut f: &mut String) { //~ HELP consider making the binding mutable + f.bar(); //~ ERROR cannot borrow `f` as mutable, as it is not declared as mutable + //~^ NOTE cannot borrow as mutable +} diff --git a/tests/ui/borrowck/mut-borrow-of-mut-ref.rs b/tests/ui/borrowck/mut-borrow-of-mut-ref.rs index 33954592bf65..d150b249bf2d 100644 --- a/tests/ui/borrowck/mut-borrow-of-mut-ref.rs +++ b/tests/ui/borrowck/mut-borrow-of-mut-ref.rs @@ -1,4 +1,5 @@ // Suggest not mutably borrowing a mutable reference +//@ run-rustfix #![crate_type = "rlib"] pub fn f(b: &mut i32) { @@ -11,7 +12,6 @@ pub fn f(b: &mut i32) { //~| HELP: if there is only one mutable reborrow, remove the `&mut` g(&mut &mut b); //~^ NOTE: cannot borrow as mutable - //~| HELP: if there is only one mutable reborrow, remove the `&mut` } pub fn g(b: &mut i32) { //~ NOTE: the binding is already a mutable borrow @@ -19,7 +19,6 @@ pub fn g(b: &mut i32) { //~ NOTE: the binding is already a mutable borrow h(&mut &mut b); //~^ ERROR: cannot borrow //~| NOTE: cannot borrow as mutable - //~| HELP: if there is only one mutable reborrow, remove the `&mut` } pub fn h(_: &mut i32) {} diff --git a/tests/ui/borrowck/mut-borrow-of-mut-ref.stderr b/tests/ui/borrowck/mut-borrow-of-mut-ref.stderr index 4c4a5e718393..01ceba90c943 100644 --- a/tests/ui/borrowck/mut-borrow-of-mut-ref.stderr +++ b/tests/ui/borrowck/mut-borrow-of-mut-ref.stderr @@ -1,5 +1,5 @@ error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable - --> $DIR/mut-borrow-of-mut-ref.rs:4:10 + --> $DIR/mut-borrow-of-mut-ref.rs:5:10 | LL | pub fn f(b: &mut i32) { | ^ not mutable @@ -11,7 +11,7 @@ LL | g(&mut &mut b); | ------ cannot borrow as mutable | note: the binding is already a mutable borrow - --> $DIR/mut-borrow-of-mut-ref.rs:4:13 + --> $DIR/mut-borrow-of-mut-ref.rs:5:13 | LL | pub fn f(b: &mut i32) { | ^^^^^^^^ @@ -24,11 +24,6 @@ help: if there is only one mutable reborrow, remove the `&mut` LL - h(&mut b); LL + h(b); | -help: if there is only one mutable reborrow, remove the `&mut` - | -LL - g(&mut &mut b); -LL + g(&mut b); - | error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable --> $DIR/mut-borrow-of-mut-ref.rs:19:12 @@ -45,14 +40,9 @@ help: consider making the binding mutable if you need to reborrow multiple times | LL | pub fn g(mut b: &mut i32) { | +++ -help: if there is only one mutable reborrow, remove the `&mut` - | -LL - h(&mut &mut b); -LL + h(&mut b); - | error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable - --> $DIR/mut-borrow-of-mut-ref.rs:36:5 + --> $DIR/mut-borrow-of-mut-ref.rs:35:5 | LL | f.bar(); | ^ cannot borrow as mutable diff --git a/tests/ui/borrowck/reborrow-in-match-suggest-deref.fixed b/tests/ui/borrowck/reborrow-in-match-suggest-deref.fixed new file mode 100644 index 000000000000..3f52e57a804c --- /dev/null +++ b/tests/ui/borrowck/reborrow-in-match-suggest-deref.fixed @@ -0,0 +1,23 @@ +// Regression test for #81059. +// edition:2024 +//@ run-rustfix + +#![allow(unused)] + +fn test(mut outer: &mut Option) { + //~^ NOTE: the binding is already a mutable borrow + //~| HELP: consider making the binding mutable if you need to reborrow multiple times + match (&mut *outer, 23) { + //~^ ERROR: cannot borrow `outer` as mutable, as it is not declared as mutable + //~| NOTE: cannot borrow as mutable + //~| HELP: to reborrow the mutable reference, add `*` + (Some(inner), _) => { + *inner = 17; + } + _ => { + *outer = Some(2); + } + } +} + +fn main() {} diff --git a/tests/ui/borrowck/reborrow-in-match-suggest-deref.rs b/tests/ui/borrowck/reborrow-in-match-suggest-deref.rs new file mode 100644 index 000000000000..34c299f4f7b1 --- /dev/null +++ b/tests/ui/borrowck/reborrow-in-match-suggest-deref.rs @@ -0,0 +1,23 @@ +// Regression test for #81059. +// edition:2024 +//@ run-rustfix + +#![allow(unused)] + +fn test(outer: &mut Option) { + //~^ NOTE: the binding is already a mutable borrow + //~| HELP: consider making the binding mutable if you need to reborrow multiple times + match (&mut outer, 23) { + //~^ ERROR: cannot borrow `outer` as mutable, as it is not declared as mutable + //~| NOTE: cannot borrow as mutable + //~| HELP: to reborrow the mutable reference, add `*` + (Some(inner), _) => { + *inner = 17; + } + _ => { + *outer = Some(2); + } + } +} + +fn main() {} diff --git a/tests/ui/borrowck/reborrow-in-match-suggest-deref.stderr b/tests/ui/borrowck/reborrow-in-match-suggest-deref.stderr new file mode 100644 index 000000000000..db3129461518 --- /dev/null +++ b/tests/ui/borrowck/reborrow-in-match-suggest-deref.stderr @@ -0,0 +1,23 @@ +error[E0596]: cannot borrow `outer` as mutable, as it is not declared as mutable + --> $DIR/reborrow-in-match-suggest-deref.rs:10:12 + | +LL | match (&mut outer, 23) { + | ^^^^^^^^^^ cannot borrow as mutable + | +note: the binding is already a mutable borrow + --> $DIR/reborrow-in-match-suggest-deref.rs:7:16 + | +LL | fn test(outer: &mut Option) { + | ^^^^^^^^^^^^^^^^ +help: consider making the binding mutable if you need to reborrow multiple times + | +LL | fn test(mut outer: &mut Option) { + | +++ +help: to reborrow the mutable reference, add `*` + | +LL | match (&mut *outer, 23) { + | + + +error: aborting due to 1 previous error + +For more information about this error, try `rustc --explain E0596`.