Use unions for uninhabitedness checking rather than mem::transmute

3 files changed, 18 insertions, 26 deletions

diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index c210372c4e7..3ebdd44aba9 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -557,7 +557,7 @@ impl<'a, 'tcx> LayoutCx<'tcx, TyCtxt<'a, 'tcx, 'tcx>> {
                 let size = element.size.checked_mul(count, dl)
                     .ok_or(LayoutError::SizeOverflow(ty))?;
 
-                let abi = if size != Size::ZERO && ty.conservative_is_uninhabited(tcx) {
+                let abi = if count != 0 && ty.conservative_is_uninhabited(tcx) {
                     Abi::Uninhabited
                 } else {
                     Abi::Aggregate { sized: true }
diff --git a/src/test/ui/consts/const-eval/ub-uninhabit.rs b/src/test/ui/consts/const-eval/ub-uninhabit.rs
index 74713af2ea0..ba083df4bee 100644
--- a/src/test/ui/consts/const-eval/ub-uninhabit.rs
+++ b/src/test/ui/consts/const-eval/ub-uninhabit.rs
@@ -1,13 +1,3 @@
-// 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.
-
 #![feature(const_transmute)]
 #![allow(const_err)] // make sure we cannot allow away the errors tested here
 
@@ -16,14 +6,18 @@ use std::mem;
 #[derive(Copy, Clone)]
 enum Bar {}
 
-const BAD_BAD_BAD: Bar = unsafe { mem::transmute(()) };
-//~^ ERROR it is undefined behavior to use this value
+union TransmuteUnion<A: Clone + Copy, B: Clone + Copy> {
+    a: A,
+    b: B,
+}
+
+const BAD_BAD_BAD: Bar = unsafe { (TransmuteUnion::<(), Bar> { a: () }).b };
+//~^ ERROR this constant likely exhibits undefined behavior
 
 const BAD_BAD_REF: &Bar = unsafe { mem::transmute(1usize) };
 //~^ ERROR it is undefined behavior to use this value
 
-const BAD_BAD_ARRAY: [Bar; 1] = unsafe { mem::transmute(()) };
-//~^ ERROR it is undefined behavior to use this value
+const BAD_BAD_ARRAY: [Bar; 1] = unsafe { (TransmuteUnion::<(), [Bar; 1]> { a: () }).b };
+//~^ ERROR this constant likely exhibits undefined behavior
 
-fn main() {
-}
+fn main() {}
diff --git a/src/test/ui/consts/const-eval/ub-uninhabit.stderr b/src/test/ui/consts/const-eval/ub-uninhabit.stderr
index cadcedd6239..c4ed503635a 100644
--- a/src/test/ui/consts/const-eval/ub-uninhabit.stderr
+++ b/src/test/ui/consts/const-eval/ub-uninhabit.stderr
@@ -1,12 +1,10 @@
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-uninhabit.rs:19:1
    |
-LL | const BAD_BAD_BAD: Bar = unsafe { mem::transmute(()) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^------------------^^^
-   |                                   |
-   |                                   entered unreachable code
+LL | const BAD_BAD_BAD: Bar = unsafe { (TransmuteUnion::<(), Bar> { a: () }).b };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a value of an uninhabited type
    |
-   = note: #[deny(const_err)] on by default
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-uninhabit.rs:22:1
@@ -19,10 +17,10 @@ LL | const BAD_BAD_REF: &Bar = unsafe { mem::transmute(1usize) };
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-uninhabit.rs:25:1
    |
-LL | const BAD_BAD_ARRAY: [Bar; 1] = unsafe { mem::transmute(()) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^------------------^^^
-   |                                          |
-   |                                          entered unreachable code
+LL | const BAD_BAD_ARRAY: [Bar; 1] = unsafe { (TransmuteUnion::<(), [Bar; 1]> { a: () }).b };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a value of an uninhabited type
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error: aborting due to 3 previous errors