Don't suggest split_at_mut when the multiple borrows have the same index

Author: estebank

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2136,10 +2136,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         }
         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 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(..) = parent.kind else { return };
+        let hir::ExprKind::Index(_, idx2, _) = parent.kind else { return };
+        if idx1.equals(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");
     }
 

compiler/rustc_hir/src/hir.rs

@@ -1811,6 +1811,41 @@ 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 equals(&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.equals(val2.expr),
+            (
+                ExprKind::Struct(QPath::LangItem(LangItem::Range, _), [val1, val3], None),
+                ExprKind::Struct(QPath::LangItem(LangItem::Range, _), [val2, val4], None),
+            ) => val1.expr.equals(val2.expr) && val3.expr.equals(val4.expr),
+            _ => false,
+        }
+    }
+
     pub fn method_ident(&self) -> Option<Ident> {
         match self.kind {
             ExprKind::MethodCall(receiver_method, ..) => Some(receiver_method.ident),

tests/ui/borrowck/borrowck-assign-comp-idx.stderr

@@ -9,8 +9,6 @@ LL |     p[0] = 5;
 LL |
 LL |     println!("{}", *q);
    |                    -- immutable borrow later used here
-   |
-   = help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices
 
 error[E0502]: cannot borrow `p` as mutable because it is also borrowed as immutable
   --> $DIR/borrowck-assign-comp-idx.rs:27:9

tests/ui/suggestions/suggest-split-at-mut.rs

@@ -48,6 +48,14 @@ fn bat() {
     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();

tests/ui/suggestions/suggest-split-at-mut.stderr

@@ -75,7 +75,17 @@ 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 6 previous errors
+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
 
 Some errors have detailed explanations: E0499, E0502.
 For more information about an error, try `rustc --explain E0499`.