fix UB in a test

also add an explicit test for the fact that a Option<WidePtr> has padding when it is None

library/core/tests/mem.rs

@@ -773,15 +773,20 @@ struct Foo {
 #[test]
 fn const_maybe_uninit_zeroed() {
     // Sanity check for `MaybeUninit::zeroed` in a realistic const situation (plugin array term)
+
+    // It is crucial that this type has no padding!
     #[repr(C)]
     struct Foo {
-        a: Option<&'static str>,
+        a: Option<&'static u8>,
         b: Bar,
         c: f32,
+        _pad: u32,
         d: *const u8,
     }
+
     #[repr(C)]
     struct Bar(usize);
+
     struct FooPtr(*const Foo);
     unsafe impl Sync for FooPtr {}
 

src/tools/miri/tests/fail/uninit/padding-enum.rs

@@ -18,6 +18,6 @@ fn main() { unsafe {
     // Turns out the discriminant is (currently) stored
     // in the 2nd pointer, so the first half is padding.
     let c = &p as *const _ as *const u8;
-    let _val = *c.add(0); // Get the padding byte.
+    let _val = *c.add(0); // Get a padding byte.
     //~^ERROR: uninitialized
 } }

src/tools/miri/tests/fail/uninit/padding-enum.stderr

@@ -1,7 +1,7 @@
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> $DIR/padding-enum.rs:LL:CC
    |
-LL |     let _val = *c.add(0); // Get the padding byte.
+LL |     let _val = *c.add(0); // Get a padding byte.
    |                ^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior

src/tools/miri/tests/fail/uninit/padding-wide-ptr.rs

@@ -0,0 +1,18 @@
+use std::mem;
+
+// If this is `None`, the metadata becomes padding.
+type T = Option<&'static str>;
+
+fn main() { unsafe {
+    let mut p: mem::MaybeUninit<T> = mem::MaybeUninit::zeroed();
+    // The copy when `T` is returned from `transmute` should destroy padding
+    // (even when we use `write_unaligned`, which under the hood uses an untyped copy).
+    p.as_mut_ptr().write_unaligned(mem::transmute((0usize, 0usize)));
+    // Null epresents `None`.
+    assert!(matches!(*p.as_ptr(), None));
+
+    // The second part, with the length, becomes padding.
+    let c = &p as *const _ as *const u8;
+    let _val = *c.add(mem::size_of::<*const u8>()); // Get a padding byte.
+    //~^ERROR: uninitialized
+} }

src/tools/miri/tests/fail/uninit/padding-wide-ptr.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
+  --> $DIR/padding-wide-ptr.rs:LL:CC
+   |
+LL |     let _val = *c.add(mem::size_of::<*const u8>()); // Get a padding byte.
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/padding-wide-ptr.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+