test that we also find bad uses of mem::uninitialized

Author: RalfJung

tests/fail/validity/uninit_float.rs

@@ -1,6 +1,8 @@
+#![allow(deprecated)]
 // This test is adapted from https://github.com/rust-lang/miri/issues/1340#issue-600900312.
 
 fn main() {
-    let _val = unsafe { std::mem::MaybeUninit::<f32>::uninit().assume_init() };
+    // Deliberately using `mem::uninitialized` to make sure that despite all the mitigations, we consider this UB.
+    let _val: f32 = unsafe { std::mem::uninitialized() };
     //~^ ERROR: constructing invalid value at .value: encountered uninitialized bytes, but expected initialized bytes
 }

tests/fail/validity/uninit_float.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: constructing invalid value at .value: encountered uninitialized bytes, but expected initialized bytes
   --> $DIR/uninit_float.rs:LL:CC
    |
-LL |     let _val = unsafe { std::mem::MaybeUninit::<f32>::uninit().assume_init() };
-   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .value: encountered uninitialized bytes, but expected initialized bytes
+LL |     let _val: f32 = unsafe { std::mem::uninitialized() };
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .value: encountered uninitialized bytes, but expected initialized bytes
    |
    = 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