From bc84e211075e4c53e6fd122353f3132939e45ff9 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 3 Feb 2021 12:33:27 -0500
Subject: [PATCH] Fix panic when emitting diagnostic for closure mutable
 binding error

Fixes #81700

The upvar borrow kind may be `ty::BorrowKind::UniqueImmBorrow`, which is
still a mutable borrow for the purposes of this diagnostic code.
---
 .../borrow_check/diagnostics/mutability_errors.rs   |  4 ++--
 src/test/ui/closures/issue-81700-mut-borrow.rs      |  5 +++++
 src/test/ui/closures/issue-81700-mut-borrow.stderr  | 13 +++++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)
 create mode 100644 src/test/ui/closures/issue-81700-mut-borrow.rs
 create mode 100644 src/test/ui/closures/issue-81700-mut-borrow.stderr

diff --git a/compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs b/compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs
index 74abe2d35ee74..333ac0738d45b 100644
--- a/compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs
+++ b/compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs
@@ -514,7 +514,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             let upvar = ty::place_to_string_for_capture(tcx, place);
             match tables.upvar_capture(upvar_id) {
                 ty::UpvarCapture::ByRef(ty::UpvarBorrow {
-                    kind: ty::BorrowKind::MutBorrow,
+                    kind: ty::BorrowKind::MutBorrow | ty::BorrowKind::UniqueImmBorrow,
                     ..
                 }) => {
                     format!("mutable borrow of `{}`", upvar)
@@ -522,7 +522,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 ty::UpvarCapture::ByValue(_) => {
                     format!("possible mutation of `{}`", upvar)
                 }
-                _ => bug!("upvar `{}` borrowed, but not mutably", upvar),
+                val => bug!("upvar `{}` borrowed, but not mutably: {:?}", upvar, val),
             }
         } else {
             bug!("not an upvar")
diff --git a/src/test/ui/closures/issue-81700-mut-borrow.rs b/src/test/ui/closures/issue-81700-mut-borrow.rs
new file mode 100644
index 0000000000000..a27a6160142ad
--- /dev/null
+++ b/src/test/ui/closures/issue-81700-mut-borrow.rs
@@ -0,0 +1,5 @@
+fn foo(x: &mut u32) {
+    let bar = || { foo(x); };
+    bar(); //~ ERROR cannot borrow
+}
+fn main() {}
diff --git a/src/test/ui/closures/issue-81700-mut-borrow.stderr b/src/test/ui/closures/issue-81700-mut-borrow.stderr
new file mode 100644
index 0000000000000..3f564afff58e2
--- /dev/null
+++ b/src/test/ui/closures/issue-81700-mut-borrow.stderr
@@ -0,0 +1,13 @@
+error[E0596]: cannot borrow `bar` as mutable, as it is not declared as mutable
+  --> $DIR/issue-81700-mut-borrow.rs:3:5
+   |
+LL |     let bar = || { foo(x); };
+   |         ---            - calling `bar` requires mutable binding due to mutable borrow of `x`
+   |         |
+   |         help: consider changing this to be mutable: `mut bar`
+LL |     bar();
+   |     ^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.