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.

2 files changed, 18 insertions, 0 deletions

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 00000000000..a27a6160142
--- /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 00000000000..3f564afff58
--- /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`.