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Rollup merge of rust-lang#124313 - estebank:split-at-mut, r=fee1-dead
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Detect borrow error involving sub-slices and suggest `split_at_mut`

```
error[E0499]: cannot borrow `foo` as mutable more than once at a time
  --> $DIR/suggest-split-at-mut.rs:13:18
   |
LL |     let a = &mut foo[..2];
   |                  --- first mutable borrow occurs here
LL |     let b = &mut foo[2..];
   |                  ^^^ second mutable borrow occurs here
LL |     a[0] = 5;
   |     ---- first borrow later used here
   |
   = help: use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
```

Address most of rust-lang#58792.

For follow up work, we should emit a structured suggestion for cases where we can identify the exact `let (a, b) = foo.split_at_mut(2);` call that is needed.
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matthiaskrgr committed Apr 25, 2024
2 parents 9e6c4fd + 64a4cdc commit 60c825f
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84 changes: 66 additions & 18 deletions compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1527,7 +1527,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow },
) => {
first_borrow_desc = "mutable ";
self.cannot_reborrow_already_borrowed(
let mut err = self.cannot_reborrow_already_borrowed(
span,
&desc_place,
&msg_place,
Expand All @@ -1537,7 +1537,15 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
"mutable",
&msg_borrow,
None,
)
);
self.suggest_slice_method_if_applicable(
&mut err,
place,
issued_borrow.borrowed_place,
span,
issued_span,
);
err
}
(
BorrowKind::Mut { kind: MutBorrowKind::Default | MutBorrowKind::TwoPhaseBorrow },
Expand All @@ -1555,6 +1563,13 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
&msg_borrow,
None,
);
self.suggest_slice_method_if_applicable(
&mut err,
place,
issued_borrow.borrowed_place,
span,
issued_span,
);
self.suggest_binding_for_closure_capture_self(&mut err, &issued_spans);
self.suggest_using_closure_argument_instead_of_capture(
&mut err,
Expand All @@ -1581,6 +1596,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
&mut err,
place,
issued_borrow.borrowed_place,
span,
issued_span,
);
self.suggest_using_closure_argument_instead_of_capture(
&mut err,
Expand Down Expand Up @@ -2011,40 +2028,47 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
err: &mut Diag<'_>,
place: Place<'tcx>,
borrowed_place: Place<'tcx>,
span: Span,
issued_span: Span,
) {
let tcx = self.infcx.tcx;
let hir = tcx.hir();

let has_split_at_mut = |ty: Ty<'tcx>| {
let ty = ty.peel_refs();
match ty.kind() {
ty::Array(..) | ty::Slice(..) => true,
ty::Adt(def, _) if tcx.get_diagnostic_item(sym::Vec) == Some(def.did()) => true,
_ if ty == tcx.types.str_ => true,
_ => false,
}
};
if let ([ProjectionElem::Index(index1)], [ProjectionElem::Index(index2)])
| (
[ProjectionElem::Deref, ProjectionElem::Index(index1)],
[ProjectionElem::Deref, ProjectionElem::Index(index2)],
) = (&place.projection[..], &borrowed_place.projection[..])
{
let decl1 = &self.body.local_decls[*index1];
let decl2 = &self.body.local_decls[*index2];

let mut note_default_suggestion = || {
err.help(
"consider using `.split_at_mut(position)` or similar method to obtain \
two mutable non-overlapping sub-slices",
"consider using `.split_at_mut(position)` or similar method to obtain two \
mutable non-overlapping sub-slices",
)
.help("consider using `.swap(index_1, index_2)` to swap elements at the specified indices");
};

let Some(body_id) = tcx.hir_node(self.mir_hir_id()).body_id() else {
note_default_suggestion();
return;
.help(
"consider using `.swap(index_1, index_2)` to swap elements at the specified \
indices",
);
};

let mut expr_finder =
FindExprBySpan::new(self.body.local_decls[*index1].source_info.span, tcx);
expr_finder.visit_expr(hir.body(body_id).value);
let Some(index1) = expr_finder.result else {
let Some(index1) = self.find_expr(decl1.source_info.span) else {
note_default_suggestion();
return;
};

expr_finder = FindExprBySpan::new(self.body.local_decls[*index2].source_info.span, tcx);
expr_finder.visit_expr(hir.body(body_id).value);
let Some(index2) = expr_finder.result else {
let Some(index2) = self.find_expr(decl2.source_info.span) else {
note_default_suggestion();
return;
};
Expand Down Expand Up @@ -2092,7 +2116,13 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
None
}
}) else {
note_default_suggestion();
let hir::Node::Expr(parent) = tcx.parent_hir_node(index1.hir_id) else { return };
let hir::ExprKind::Index(_, idx1, _) = parent.kind else { return };
let hir::Node::Expr(parent) = tcx.parent_hir_node(index2.hir_id) else { return };
let hir::ExprKind::Index(_, idx2, _) = parent.kind else { return };
if !idx1.equivalent_for_indexing(idx2) {
err.help("use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices");
}
return;
};

Expand All @@ -2102,7 +2132,25 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
format!("{obj_str}.swap({index1_str}, {index2_str})"),
Applicability::MachineApplicable,
);
return;
}
let place_ty = PlaceRef::ty(&place.as_ref(), self.body, tcx).ty;
let borrowed_place_ty = PlaceRef::ty(&borrowed_place.as_ref(), self.body, tcx).ty;
if !has_split_at_mut(place_ty) && !has_split_at_mut(borrowed_place_ty) {
// Only mention `split_at_mut` on `Vec`, array and slices.
return;
}
let Some(index1) = self.find_expr(span) else { return };
let hir::Node::Expr(parent) = tcx.parent_hir_node(index1.hir_id) else { return };
let hir::ExprKind::Index(_, idx1, _) = parent.kind else { return };
let Some(index2) = self.find_expr(issued_span) else { return };
let hir::Node::Expr(parent) = tcx.parent_hir_node(index2.hir_id) else { return };
let hir::ExprKind::Index(_, idx2, _) = parent.kind else { return };
if idx1.equivalent_for_indexing(idx2) {
// `let a = &mut foo[0]` and `let b = &mut foo[0]`? Don't mention `split_at_mut`
return;
}
err.help("use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices");
}

/// Suggest using `while let` for call `next` on an iterator in a for loop.
Expand Down
38 changes: 38 additions & 0 deletions compiler/rustc_hir/src/hir.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1811,6 +1811,44 @@ impl Expr<'_> {
}
}

/// Whether this and the `other` expression are the same for purposes of an indexing operation.
///
/// This is only used for diagnostics to see if we have things like `foo[i]` where `foo` is
/// borrowed multiple times with `i`.
pub fn equivalent_for_indexing(&self, other: &Expr<'_>) -> bool {
match (self.kind, other.kind) {
(ExprKind::Lit(lit1), ExprKind::Lit(lit2)) => lit1.node == lit2.node,
(
ExprKind::Path(QPath::LangItem(item1, _)),
ExprKind::Path(QPath::LangItem(item2, _)),
) => item1 == item2,
(
ExprKind::Path(QPath::Resolved(None, path1)),
ExprKind::Path(QPath::Resolved(None, path2)),
) => path1.res == path2.res,
(
ExprKind::Struct(QPath::LangItem(LangItem::RangeTo, _), [val1], None),
ExprKind::Struct(QPath::LangItem(LangItem::RangeTo, _), [val2], None),
)
| (
ExprKind::Struct(QPath::LangItem(LangItem::RangeToInclusive, _), [val1], None),
ExprKind::Struct(QPath::LangItem(LangItem::RangeToInclusive, _), [val2], None),
)
| (
ExprKind::Struct(QPath::LangItem(LangItem::RangeFrom, _), [val1], None),
ExprKind::Struct(QPath::LangItem(LangItem::RangeFrom, _), [val2], None),
) => val1.expr.equivalent_for_indexing(val2.expr),
(
ExprKind::Struct(QPath::LangItem(LangItem::Range, _), [val1, val3], None),
ExprKind::Struct(QPath::LangItem(LangItem::Range, _), [val2, val4], None),
) => {
val1.expr.equivalent_for_indexing(val2.expr)
&& val3.expr.equivalent_for_indexing(val4.expr)
}
_ => false,
}
}

pub fn method_ident(&self) -> Option<Ident> {
match self.kind {
ExprKind::MethodCall(receiver_method, ..) => Some(receiver_method.ident),
Expand Down
56 changes: 55 additions & 1 deletion tests/ui/suggestions/suggest-split-at-mut.rs
Original file line number Diff line number Diff line change
@@ -1,8 +1,62 @@
fn main() {
fn foo() {
let mut foo = [1, 2, 3, 4];
let a = &mut foo[2];
let b = &mut foo[3]; //~ ERROR cannot borrow `foo[_]` as mutable more than once at a time
*a = 5;
*b = 6;
println!("{:?} {:?}", a, b);
}

fn bar() {
let mut foo = [1,2,3,4];
let a = &mut foo[..2];
let b = &mut foo[2..]; //~ ERROR cannot borrow `foo` as mutable more than once at a time
a[0] = 5;
b[0] = 6;
println!("{:?} {:?}", a, b);
}

fn baz() {
let mut foo = [1,2,3,4];
let a = &foo[..2];
let b = &mut foo[2..]; //~ ERROR cannot borrow `foo` as mutable because it is also borrowed as immutable
b[0] = 6;
println!("{:?} {:?}", a, b);
}

fn qux() {
let mut foo = [1,2,3,4];
let a = &mut foo[..2];
let b = &foo[2..]; //~ ERROR cannot borrow `foo` as immutable because it is also borrowed as mutable
a[0] = 5;
println!("{:?} {:?}", a, b);
}

fn bad() {
let mut foo = [1,2,3,4];
let a = &foo[1];
let b = &mut foo[2]; //~ ERROR cannot borrow `foo[_]` as mutable because it is also borrowed as immutable
*b = 6;
println!("{:?} {:?}", a, b);
}

fn bat() {
let mut foo = [1,2,3,4];
let a = &mut foo[1];
let b = &foo[2]; //~ ERROR cannot borrow `foo[_]` as immutable because it is also borrowed as mutable
*a = 5;
println!("{:?} {:?}", a, b);
}

fn ang() {
let mut foo = [1,2,3,4];
let a = &mut foo[0..];
let b = &foo[0..]; //~ ERROR cannot borrow `foo` as immutable because it is also borrowed as mutable
a[0] = 5;
println!("{:?} {:?}", a, b);
}

fn main() {
foo();
bar();
}
80 changes: 76 additions & 4 deletions tests/ui/suggestions/suggest-split-at-mut.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -8,9 +8,81 @@ LL | let b = &mut foo[3];
LL | *a = 5;
| ------ first borrow later used here
|
= help: consider using `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
= help: consider using `.swap(index_1, index_2)` to swap elements at the specified indices
= help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

error: aborting due to 1 previous error
error[E0499]: cannot borrow `foo` as mutable more than once at a time
--> $DIR/suggest-split-at-mut.rs:13:18
|
LL | let a = &mut foo[..2];
| --- first mutable borrow occurs here
LL | let b = &mut foo[2..];
| ^^^ second mutable borrow occurs here
LL | a[0] = 5;
| ---- first borrow later used here
|
= help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

error[E0502]: cannot borrow `foo` as mutable because it is also borrowed as immutable
--> $DIR/suggest-split-at-mut.rs:22:18
|
LL | let a = &foo[..2];
| --- immutable borrow occurs here
LL | let b = &mut foo[2..];
| ^^^ mutable borrow occurs here
LL | b[0] = 6;
LL | println!("{:?} {:?}", a, b);
| - immutable borrow later used here
|
= help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

error[E0502]: cannot borrow `foo` as immutable because it is also borrowed as mutable
--> $DIR/suggest-split-at-mut.rs:30:14
|
LL | let a = &mut foo[..2];
| --- mutable borrow occurs here
LL | let b = &foo[2..];
| ^^^ immutable borrow occurs here
LL | a[0] = 5;
| ---- mutable borrow later used here
|
= help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

error[E0502]: cannot borrow `foo[_]` as mutable because it is also borrowed as immutable
--> $DIR/suggest-split-at-mut.rs:38:13
|
LL | let a = &foo[1];
| ------- immutable borrow occurs here
LL | let b = &mut foo[2];
| ^^^^^^^^^^^ mutable borrow occurs here
LL | *b = 6;
LL | println!("{:?} {:?}", a, b);
| - immutable borrow later used here
|
= help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

error[E0502]: cannot borrow `foo[_]` as immutable because it is also borrowed as mutable
--> $DIR/suggest-split-at-mut.rs:46:13
|
LL | let a = &mut foo[1];
| ----------- mutable borrow occurs here
LL | let b = &foo[2];
| ^^^^^^^ immutable borrow occurs here
LL | *a = 5;
| ------ mutable borrow later used here
|
= help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

error[E0502]: cannot borrow `foo` as immutable because it is also borrowed as mutable
--> $DIR/suggest-split-at-mut.rs:54:14
|
LL | let a = &mut foo[0..];
| --- mutable borrow occurs here
LL | let b = &foo[0..];
| ^^^ immutable borrow occurs here
LL | a[0] = 5;
| ---- mutable borrow later used here

error: aborting due to 7 previous errors

For more information about this error, try `rustc --explain E0499`.
Some errors have detailed explanations: E0499, E0502.
For more information about an error, try `rustc --explain E0499`.

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