MERGEME Add fn mark_borrowed method to rustc_mir::build::Builder.

Of course this should only be kept if I actually find `mark_borrowed` called elsewhere.

Author: pnkfelix

src/librustc_mir/build/scope.rs

@@ -666,6 +666,23 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
         success_block
     }
+
+    pub(crate) fn mark_borrowed(&mut self, extent: CodeExtent, span: Span) {
+        let opt_scope = self.scopes
+            .iter_mut()
+            .find(|s| s.extent == extent);
+        match opt_scope {
+            None => {
+                self.hir.tcx().sess.span_note_without_error(
+                    span,
+                    &format!("extent {:?} not found for borrow", extent));
+            }
+            Some(scope) => {
+                // borrowed scopes will need to emit an EndRegion.
+                scope.needs_cleanup = true;
+            }
+        }
+    }
 }
 
 /// Builds drops for pop_scope and exit_scope.