Refer to the second borrow as the "second borrow".

src/librustc_borrowck/borrowck/check_loans.rs

@@ -615,7 +615,7 @@ pub fn report_error_if_loan_conflicts_with_restriction(&self,
                     let new_loan_str = &new_loan.kind.to_user_str();
                     self.bccx.cannot_reborrow_already_uniquely_borrowed(
                         new_loan.span, "closure", &nl, &new_loan_msg, new_loan_str,
-                        old_loan.span, &old_loan_msg, previous_end_span, Origin::Ast)
+                        old_loan.span, &old_loan_msg, previous_end_span, "", Origin::Ast)
                 }
                 (..) =>
                     self.bccx.cannot_reborrow_already_borrowed(

src/librustc_mir/borrow_check/error_reporting.rs

@@ -344,6 +344,13 @@ pub(super) fn report_conflicting_borrow(
 
         let first_borrow_desc;
 
+        let explanation = self.explain_why_borrow_contains_point(context, issued_borrow, None);
+        let second_borrow_desc = if explanation.is_explained() {
+            "second "
+        } else {
+            ""
+        };
+
         // FIXME: supply non-"" `opt_via` when appropriate
         let mut err = match (
             gen_borrow_kind,
@@ -454,6 +461,7 @@ pub(super) fn report_conflicting_borrow(
                     issued_span,
                     "",
                     None,
+                    second_borrow_desc,
                     Origin::Mir,
                 )
             }
@@ -469,6 +477,7 @@ pub(super) fn report_conflicting_borrow(
                     issued_span,
                     "",
                     None,
+                    second_borrow_desc,
                     Origin::Mir,
                 )
             }
@@ -513,7 +522,7 @@ pub(super) fn report_conflicting_borrow(
             );
         }
 
-        self.explain_why_borrow_contains_point(context, issued_borrow, None)
+        explanation
             .add_explanation_to_diagnostic(self.infcx.tcx, self.mir, &mut err, first_borrow_desc);
 
         err.buffer(&mut self.errors_buffer);

src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs

@@ -52,6 +52,12 @@ pub(in borrow_check) enum LaterUseKind {
 }
 
 impl BorrowExplanation {
+    pub(in borrow_check) fn is_explained(&self) -> bool {
+        match self {
+            BorrowExplanation::Unexplained => false,
+            _ => true,
+        }
+    }
     pub(in borrow_check) fn add_explanation_to_diagnostic<'cx, 'gcx, 'tcx>(
         &self,
         tcx: TyCtxt<'cx, 'gcx, 'tcx>,

src/librustc_mir/util/borrowck_errors.rs

@@ -261,6 +261,7 @@ fn cannot_reborrow_already_uniquely_borrowed(
         old_loan_span: Span,
         old_opt_via: &str,
         previous_end_span: Option<Span>,
+        second_borrow_desc: &str,
         o: Origin,
     ) -> DiagnosticBuilder<'cx> {
         let mut err = struct_span_err!(
@@ -274,7 +275,10 @@ fn cannot_reborrow_already_uniquely_borrowed(
             kind_new,
             OGN = o
         );
-        err.span_label(new_loan_span, format!("borrow occurs here{}", opt_via));
+        err.span_label(
+            new_loan_span,
+            format!("{}borrow occurs here{}", second_borrow_desc, opt_via),
+        );
         err.span_label(
             old_loan_span,
             format!("{} construction occurs here{}", container_name, old_opt_via),

src/test/ui/E0501.ast.nll.stderr

@@ -7,7 +7,7 @@ LL |         inside_closure(a)
    |                        - first borrow occurs due to use of `a` in closure
 LL |     };
 LL |     outside_closure_1(a); //[ast]~ ERROR cannot borrow `*a` as mutable because previous closure requires unique access
-   |                       ^ borrow occurs here
+   |                       ^ second borrow occurs here
 ...
 LL |     drop(bar);
    |          --- first borrow later used here
@@ -21,7 +21,7 @@ LL |         inside_closure(a)
    |                        - first borrow occurs due to use of `a` in closure
 ...
 LL |     outside_closure_2(a); //[ast]~ ERROR cannot borrow `*a` as immutable because previous closure requires unique access
-   |                       ^ borrow occurs here
+   |                       ^ second borrow occurs here
 ...
 LL |     drop(bar);
    |          --- first borrow later used here

src/test/ui/E0501.mir.stderr

@@ -7,7 +7,7 @@ LL |         inside_closure(a)
    |                        - first borrow occurs due to use of `a` in closure
 LL |     };
 LL |     outside_closure_1(a); //[ast]~ ERROR cannot borrow `*a` as mutable because previous closure requires unique access
-   |                       ^ borrow occurs here
+   |                       ^ second borrow occurs here
 ...
 LL |     drop(bar);
    |          --- first borrow later used here
@@ -21,7 +21,7 @@ LL |         inside_closure(a)
    |                        - first borrow occurs due to use of `a` in closure
 ...
 LL |     outside_closure_2(a); //[ast]~ ERROR cannot borrow `*a` as immutable because previous closure requires unique access
-   |                       ^ borrow occurs here
+   |                       ^ second borrow occurs here
 ...
 LL |     drop(bar);
    |          --- first borrow later used here

src/test/ui/borrowck/borrowck-insert-during-each.nll.stderr

@@ -10,7 +10,7 @@ LL | |         |a| { //~ ERROR closure requires unique access to `f`
 LL | |             f.n.insert(*a);
    | |             - first borrow occurs due to use of `f` in closure
 LL | |         })
-   | |__________^ borrow occurs here
+   | |__________^ second borrow occurs here
 
 error[E0500]: closure requires unique access to `f` but it is already borrowed
   --> $DIR/borrowck-insert-during-each.rs:27:9

src/test/ui/generator/yield-while-ref-reborrowed.nll.stderr

@@ -7,7 +7,7 @@ LL |         let a = &mut *x;
    |                       - first borrow occurs due to use of `x` in generator
 ...
 LL |     println!("{}", x); //~ ERROR
-   |                    ^ borrow occurs here
+   |                    ^ second borrow occurs here
 LL |     b.resume();
    |     - first borrow later used here
 

src/test/ui/nll/closure-borrow-spans.stderr

@@ -126,7 +126,7 @@ LL |     let f = || *x = 0;
    |             |
    |             closure construction occurs here
 LL |     let y = &x; //~ ERROR
-   |             ^^ borrow occurs here
+   |             ^^ second borrow occurs here
 LL |     f.use_ref();
    |     - first borrow later used here
 
@@ -138,7 +138,7 @@ LL |     let f = || *x = 0;
    |             |
    |             closure construction occurs here
 LL |     let y = &mut x; //~ ERROR
-   |             ^^^^^^ borrow occurs here
+   |             ^^^^^^ second borrow occurs here
 LL |     f.use_ref();
    |     - first borrow later used here