test that partially uninit MaybeUninit works correctly

rust-version

@@ -1 +1 @@
-634770c0a7f8598164ab825cfe419cc8b03c36e5
+f262ca12aac76152c4b46cefcf8300f0249a5eb2

tests/run-pass/partially-uninit.rs

@@ -0,0 +1,15 @@
+// compile-flags: -Zmiri-check-number-validity
+
+use std::mem::{self, MaybeUninit};
+
+#[repr(C)]
+#[derive(Copy, Clone, Debug, PartialEq)]
+struct Demo(bool, u16);
+
+fn main() { unsafe {
+    // Transmute-round-trip through a type with Scalar layout is lossless.
+    // This is tricky because that 'scalar' is *partially* uninitialized.
+    let x = Demo(true, 3);
+    let y: MaybeUninit<u32> = mem::transmute(x);
+    assert_eq!(x, mem::transmute(y));
+} }