don't activate borrows that are already activated

Author: nikomatsakis

src/librustc_mir/borrow_check/mod.rs

@@ -317,7 +317,7 @@ impl<'cx, 'gcx, 'tcx> DataflowResultsConsumer<'cx, 'tcx> for MirBorrowckCtxt<'cx
     }
 
     fn visit_block_entry(&mut self, bb: BasicBlock, flow_state: &Self::FlowState) {
-        debug!("MirBorrowckCtxt::process_block({:?}): {}", bb, flow_state);
+        debug!("MirBorrowckCtxt::visit_block_entry({:?}): {}", bb, flow_state);
     }
 
     fn visit_statement_entry(
@@ -327,7 +327,7 @@ impl<'cx, 'gcx, 'tcx> DataflowResultsConsumer<'cx, 'tcx> for MirBorrowckCtxt<'cx
         flow_state: &Self::FlowState,
     ) {
         debug!(
-            "MirBorrowckCtxt::process_statement({:?}, {:?}): {}",
+            "MirBorrowckCtxt::visit_statement_entry({:?}, {:?}): {}",
             location,
             stmt,
             flow_state
@@ -1127,6 +1127,11 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                     BorrowKind::Unique | BorrowKind::Mut => {}
                 }
 
+                // If this is already activated, no need to do further checks.
+                if flow_state.borrows.contains(&gen) {
+                    return;
+                }
+
                 self.access_place(
                     ContextKind::Activation.new(location),
                     (&borrow.borrowed_place, span),

src/test/ui/issue-45157.stderr

@@ -7,14 +7,5 @@ error[E0502]: cannot borrow `u.z.c` as immutable because it is also borrowed as
 37 |         let nref = &u.z.c;
    |                    ^^^^^^ immutable borrow occurs here
 
-error[E0502]: cannot borrow `u.s.a` as mutable because it is also borrowed as immutable
-  --> $DIR/issue-45157.rs:39:27
-   |
-37 |         let nref = &u.z.c;
-   |                    ------ immutable borrow occurs here
-38 |         //~^ ERROR cannot borrow `u.z.c` as immutable because it is also borrowed as mutable [E0502]
-39 |         println!("{} {}", mref, nref)
-   |                           ^^^^ mutable borrow occurs here
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error