Fix unwrapping usize issue with transparent MaybeUnit array wrapper

Author: varkor

src/librustc/ty/layout.rs

@@ -549,8 +549,8 @@ impl<'a, 'tcx> LayoutCx<'tcx, TyCtxt<'a, 'tcx, 'tcx>> {
                     }
                 }
 
+                let count = count.assert_usize(tcx).ok_or(LayoutError::Unknown(ty))?;
                 let element = self.layout_of(element)?;
-                let count = count.unwrap_usize(tcx);
                 let size = element.size.checked_mul(count, dl)
                     .ok_or(LayoutError::SizeOverflow(ty))?;
 

src/test/ui/const-generics/transparent-maybeunit-array-wrapper.rs

@@ -0,0 +1,11 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+use std::mem::MaybeUninit;
+
+#[repr(transparent)]
+pub struct MaybeUninitWrapper<const N: usize>(MaybeUninit<[u64; N]>);
+
+fn main() {}

src/test/ui/const-generics/transparent-maybeunit-array-wrapper.stderr

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