Auto merge of #54667 - RalfJung:maybe-uninit, r=pnkfelix

Panic when using mem::uninitialized or mem::zeroed on an uninhabited type

All code by @japaric. This re-submits one half of https://github.com/rust-lang/rust/pull/53508. This is likely not the one that introduced the perf regression, but just to be sure I'll do a perf run anyway.

1 files changed, 7 insertions, 2 deletions

diff --git a/src/test/debuginfo/nil-enum.rs b/src/test/debuginfo/nil-enum.rs
index 94377421c0b..ab9c7e2dd27 100644
--- a/src/test/debuginfo/nil-enum.rs
+++ b/src/test/debuginfo/nil-enum.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// NOTE Instantiating an empty enum is UB. This test may break in the future.
+
 // LLDB can't handle zero-sized values
 // ignore-lldb
 
@@ -25,8 +27,11 @@
 
 #![allow(unused_variables)]
 #![feature(omit_gdb_pretty_printer_section)]
+#![feature(maybe_uninit)]
 #![omit_gdb_pretty_printer_section]
 
+use std::mem::MaybeUninit;
+
 enum ANilEnum {}
 enum AnotherNilEnum {}
 
@@ -35,8 +40,8 @@ enum AnotherNilEnum {}
 // The error from gdbr is expected since nil enums are not supposed to exist.
 fn main() {
     unsafe {
-        let first: ANilEnum = ::std::mem::zeroed();
-        let second: AnotherNilEnum = ::std::mem::zeroed();
+        let first: ANilEnum = MaybeUninit::uninitialized().into_inner();
+        let second: AnotherNilEnum = MaybeUninit::uninitialized().into_inner();
 
         zzz(); // #break
     }