Add a regression test for #109141

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

tests/ui/const-generics/generic_const_exprs/issue-109141.rs

@@ -0,0 +1,13 @@
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+impl EntriesBuffer {
+    fn a(&self) -> impl Iterator {
+        self.0.iter_mut() //~ ERROR: cannot borrow `*self.0` as mutable, as it is behind a `&` reference
+    }
+}
+
+struct EntriesBuffer(Box<[[u8; HashesEntryLEN]; 5]>);
+//~^ ERROR: cannot find value `HashesEntryLEN` in this scope
+
+fn main() {}

tests/ui/const-generics/generic_const_exprs/issue-109141.stderr

@@ -0,0 +1,26 @@
+error[E0425]: cannot find value `HashesEntryLEN` in this scope
+  --> $DIR/issue-109141.rs:10:32
+   |
+LL | struct EntriesBuffer(Box<[[u8; HashesEntryLEN]; 5]>);
+   |                                ^^^^^^^^^^^^^^ not found in this scope
+   |
+help: you might be missing a const parameter
+   |
+LL | struct EntriesBuffer<const HashesEntryLEN: /* Type */>(Box<[[u8; HashesEntryLEN]; 5]>);
+   |                     ++++++++++++++++++++++++++++++++++
+
+error[E0596]: cannot borrow `*self.0` as mutable, as it is behind a `&` reference
+  --> $DIR/issue-109141.rs:6:9
+   |
+LL |         self.0.iter_mut()
+   |         ^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider changing this to be a mutable reference
+   |
+LL |     fn a(&mut self) -> impl Iterator {
+   |          ~~~~~~~~~
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0425, E0596.
+For more information about an error, try `rustc --explain E0425`.