const validation: add test for uninit bool

Author: RalfJung

src/test/ui/consts/const-eval/union-ub.rs

@@ -2,6 +2,7 @@
 
 #[repr(C)]
 union DummyUnion {
+    unit: (),
     u8: u8,
     bool: bool,
 }
@@ -30,6 +31,8 @@ union Bar {
 // the value is not valid for bools
 const BAD_BOOL: bool = unsafe { DummyUnion { u8: 42 }.bool};
 //~^ ERROR it is undefined behavior to use this value
+const UNINIT_BOOL: bool = unsafe { DummyUnion { unit: () }.bool};
+//~^ ERROR it is undefined behavior to use this value
 
 // The value is not valid for any union variant, but that's fine
 // unions are just a convenient way to transmute bits around

src/test/ui/consts/const-eval/union-ub.stderr

@@ -1,11 +1,19 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub.rs:31:1
+  --> $DIR/union-ub.rs:32:1
    |
 LL | const BAD_BOOL: bool = unsafe { DummyUnion { u8: 42 }.bool};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0x2a, but expected a boolean
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
 
-error: aborting due to previous error
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/union-ub.rs:34:1
+   |
+LL | const UNINIT_BOOL: bool = unsafe { DummyUnion { unit: () }.bool};
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized bytes, but expected a boolean
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.