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.

src/test/codegen/box-maybe-uninit.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -O
+#![crate_type="lib"]
+#![feature(maybe_uninit)]
+
+use std::mem::MaybeUninit;
+
+// Boxing a `MaybeUninit` value should not copy junk from the stack
+#[no_mangle]
+pub fn box_uninitialized() -> Box<MaybeUninit<usize>> {
+    // CHECK-LABEL: @box_uninitialized
+    // CHECK-NOT: store
+    Box::new(MaybeUninit::uninitialized())
+}

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
     }

src/test/run-pass/panic-uninitialized-zeroed.rs

@@ -0,0 +1,83 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-wasm32-bare always compiled as panic=abort right now and this requires unwinding
+// This test checks that instantiating an uninhabited type via `mem::{uninitialized,zeroed}` results
+// in a runtime panic.
+
+#![feature(never_type)]
+
+use std::{mem, panic};
+
+#[allow(dead_code)]
+struct Foo {
+    x: u8,
+    y: !,
+}
+
+enum Bar {}
+
+fn main() {
+    unsafe {
+        assert_eq!(
+            panic::catch_unwind(|| {
+                mem::uninitialized::<!>()
+            }).err().and_then(|a| a.downcast_ref::<String>().map(|s| {
+                s == "Attempted to instantiate uninhabited type ! using mem::uninitialized"
+            })),
+            Some(true)
+        );
+
+        assert_eq!(
+            panic::catch_unwind(|| {
+                mem::zeroed::<!>()
+            }).err().and_then(|a| a.downcast_ref::<String>().map(|s| {
+                s == "Attempted to instantiate uninhabited type ! using mem::zeroed"
+            })),
+            Some(true)
+        );
+
+        assert_eq!(
+            panic::catch_unwind(|| {
+                mem::uninitialized::<Foo>()
+            }).err().and_then(|a| a.downcast_ref::<String>().map(|s| {
+                s == "Attempted to instantiate uninhabited type Foo using mem::uninitialized"
+            })),
+            Some(true)
+        );
+
+        assert_eq!(
+            panic::catch_unwind(|| {
+                mem::zeroed::<Foo>()
+            }).err().and_then(|a| a.downcast_ref::<String>().map(|s| {
+                s == "Attempted to instantiate uninhabited type Foo using mem::zeroed"
+            })),
+            Some(true)
+        );
+
+        assert_eq!(
+            panic::catch_unwind(|| {
+                mem::uninitialized::<Bar>()
+            }).err().and_then(|a| a.downcast_ref::<String>().map(|s| {
+                s == "Attempted to instantiate uninhabited type Bar using mem::uninitialized"
+            })),
+            Some(true)
+        );
+
+        assert_eq!(
+            panic::catch_unwind(|| {
+                mem::zeroed::<Bar>()
+            }).err().and_then(|a| a.downcast_ref::<String>().map(|s| {
+                s == "Attempted to instantiate uninhabited type Bar using mem::zeroed"
+            })),
+            Some(true)
+        );
+    }
+}