Fix unwrapping usize issue in HasMutInterior

src/librustc_mir/transform/qualify_consts.rs

@@ -316,8 +316,9 @@ impl Qualif for HasMutInterior {
                     } else if let ty::Array(_, len) = ty.sty {
                         // FIXME(eddyb) the `cx.mode == Mode::Fn` condition
                         // seems unnecessary, given that this is merely a ZST.
-                        if !(len.unwrap_usize(cx.tcx) == 0 && cx.mode == Mode::Fn) {
-                            return true;
+                        match len.assert_usize(cx.tcx) {
+                            Some(0) if cx.mode == Mode::Fn => {},
+                            _ => return true,
                         }
                     } else {
                         return true;

src/test/ui/const-generics/mut-ref-const-param-array.rs

@@ -0,0 +1,19 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+use std::ops::AddAssign;
+
+fn inc<T: AddAssign + Clone, const N: usize>(v: &mut [T; N]) -> &mut [T; N] {
+    for x in v.iter_mut() {
+        *x += x.clone();
+    }
+    v
+}
+
+fn main() {
+    let mut v = [1, 2, 3];
+    inc(&mut v);
+    assert_eq!(v, [2, 4, 6]);
+}

src/test/ui/const-generics/mut-ref-const-param-array.stderr

@@ -0,0 +1,6 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/mut-ref-const-param-array.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+