Reintroduce zst-slice reading read_bytes method on Memory

Author: oli-obk

src/librustc_mir/interpret/memory.rs

@@ -567,6 +567,22 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
     }
 }
 
+/// Byte Accessors
+impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
+    pub fn read_bytes(
+        &self,
+        ptr: Scalar<M::PointerTag>,
+        size: Size,
+    ) -> EvalResult<'tcx, &[u8]> {
+        if size.bytes() == 0 {
+            Ok(&[])
+        } else {
+            let ptr = ptr.to_ptr()?;
+            self.get(ptr.alloc_id)?.read_bytes(self, ptr, size)
+        }
+    }
+}
+
 /// Interning (for CTFE)
 impl<'a, 'mir, 'tcx, M> Memory<'a, 'mir, 'tcx, M>
 where

src/librustc_mir/interpret/operand.rs

@@ -351,10 +351,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
         mplace: MPlaceTy<'tcx, M::PointerTag>,
     ) -> EvalResult<'tcx, &str> {
         let len = mplace.len(self)?;
-        let ptr = mplace.ptr.to_ptr()?;
-        let bytes = self.memory
-            .get(ptr.alloc_id)?
-            .read_bytes(self, ptr, Size::from_bytes(len as u64))?;
+        let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len as u64))?;
         let str = ::std::str::from_utf8(bytes)
             .map_err(|err| EvalErrorKind::ValidationFailure(err.to_string()))?;
         Ok(str)

src/test/ui/consts/int_ptr_for_zst_slices.rs

@@ -1,6 +1,7 @@
+// compile-pass
+
 #![feature(const_raw_ptr_deref)]
 
 const FOO: &str = unsafe { &*(1_usize as *const [u8; 0] as *const [u8] as *const str) };
-//~^ ERROR it is undefined behaviour to use this value
 
 fn main() {}