mir-borrowck: Add span labels to report_illegal_mutation_of_borrowed()

Author: zilbuz

src/librustc_mir/borrow_check.rs

@@ -979,9 +979,15 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
                                            _: Context,
                                            (lvalue, span): (&Lvalue, Span),
                                            loan: &BorrowData) {
+        let describe_lvalue = self.describe_lvalue(lvalue);
+        let borrow_span = self.retrieve_borrow_span(loan);
+
         let mut err = self.tcx.cannot_assign_to_borrowed(
             span, &self.describe_lvalue(lvalue), Origin::Mir);
-        // FIXME: add span labels for borrow and assignment points
+
+        err.span_label(borrow_span, format!("borrow of `{}` occurs here", describe_lvalue));
+        err.span_label(span, format!("assignment to borrowed `{}` occurs here", describe_lvalue));
+
         err.emit();
     }