Auto merge of #100043 - RalfJung:scalar-always-init, r=RalfJung

interpret: remove support for uninitialized scalars

With Miri no longer supporting `-Zmiri-allow-uninit-numbers`, we no longer need to support storing uninit data in a `Scalar`. We anyway already only use this representation for types with *initialized* `Scalar` layout (and we have to, due to partial initialization), so let's get rid of the `ScalarMaybeUninit` type entirely.

I tried to stage this into meaningful commits, but the one that changes `read_immediate` to always trigger UB on uninit is the largest chunk of the PR and I don't see how it could be subdivided.

Fixes https://github.com/rust-lang/miri/issues/2187
r? `@oli-obk`

1 files changed, 4 insertions, 2 deletions

diff --git a/src/test/ui/consts/const-eval/ub-ref-ptr.rs b/src/test/ui/consts/const-eval/ub-ref-ptr.rs
index d0216f74668..c62848f70db 100644
--- a/src/test/ui/consts/const-eval/ub-ref-ptr.rs
+++ b/src/test/ui/consts/const-eval/ub-ref-ptr.rs
@@ -51,12 +51,14 @@ const USIZE_AS_BOX: Box<u8> = unsafe { mem::transmute(1337usize) };
 //~^ ERROR it is undefined behavior to use this value
 
 const UNINIT_PTR: *const i32 = unsafe { MaybeUninit { uninit: () }.init };
-//~^ ERROR it is undefined behavior to use this value
+//~^ ERROR evaluation of constant value failed
+//~| uninitialized
 
 const NULL_FN_PTR: fn() = unsafe { mem::transmute(0usize) };
 //~^ ERROR it is undefined behavior to use this value
 const UNINIT_FN_PTR: fn() = unsafe { MaybeUninit { uninit: () }.init };
-//~^ ERROR it is undefined behavior to use this value
+//~^ ERROR evaluation of constant value failed
+//~| uninitialized
 const DANGLING_FN_PTR: fn() = unsafe { mem::transmute(13usize) };
 //~^ ERROR it is undefined behavior to use this value
 const DATA_FN_PTR: fn() = unsafe { mem::transmute(&13) };