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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 #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.
This commit is contained in:
Esteban Küber 2024-04-24 01:26:32 +00:00
parent 6a9758d4f3
commit 386236f289
4 changed files with 57 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

41
compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
View File

@ -1581,6 +1581,8 @@ 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,
@ -2011,10 +2013,11 @@ 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();
if let ([ProjectionElem::Index(index1)], [ProjectionElem::Index(index2)])
| (
[ProjectionElem::Deref, ProjectionElem::Index(index1)],
@ -2023,28 +2026,23 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
{
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");
.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 {
let Some(index1) = self.find_expr(self.body.local_decls[*index1].source_info.span)
else {
note_default_suggestion();
return;
};
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 {
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(self.body.local_decls[*index2].source_info.span)
else {
note_default_suggestion();
return;
};
@ -2102,7 +2100,18 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
format!("{obj_str}.swap({index1_str}, {index2_str})"),
Applicability::MachineApplicable,
);
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(..) = 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(..) = parent.kind else { return };
err.help(
"use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping \
sub-slices",
);
}
/// Suggest using `while let` for call `next` on an iterator in a for loop.

4
tests/ui/borrowck/borrowck-overloaded-index-autoderef.stderr
View File

@ -17,6 +17,8 @@ LL | let q = &mut f[&s];
| ^ second mutable borrow occurs here
LL | p.use_mut();
| - first borrow later used here
|
= help: use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
error[E0499]: cannot borrow `f.foo` as mutable more than once at a time
--> $DIR/borrowck-overloaded-index-autoderef.rs:53:18
@ -27,6 +29,8 @@ LL | let q = &mut f.foo[&s];
| ^^^^^ second mutable borrow occurs here
LL | p.use_mut();
| - first borrow later used here
|
= help: use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
error[E0502]: cannot borrow `f.foo` as mutable because it is also borrowed as immutable
--> $DIR/borrowck-overloaded-index-autoderef.rs:65:18

16
tests/ui/suggestions/suggest-split-at-mut.rs
View File

@ -1,4 +1,4 @@
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
@ -6,3 +6,17 @@ fn main() {
*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 main() {
foo();
bar();
}

14
tests/ui/suggestions/suggest-split-at-mut.stderr
View File

@ -11,6 +11,18 @@ LL | *a = 5;
= 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
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)` or similar method to obtain two mutable non-overlapping sub-slices
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0499`.