make it more clear what comments refer to; avoid dangling unaligned references

Co-authored-by: Waffle Maybe <waffle.lapkin@gmail.com>

Author: RalfJung

tests/ui/consts/const-eval/issue-91827-extern-types-field-offset.rs

@@ -17,15 +17,21 @@ struct S {
 }
 
 constNEWTYPE:() = unsafe{
+let buf = [0i32;4];
+let x:&Newtype = &*(&buf as*const_as*constNewtype);
+
 // Projecting to the newtype works, because it is always at offset 0.
-let x:&Newtype = unsafe{&*(1usizeas*constNewtype)};
 let field = &x.0;
 };
 
 constOFFSET:() = unsafe{
-// This needs to compute the field offset, but we don't know the type's alignment, so this fail.
-let x:&S = unsafe{&*(1usizeas*constS)};
-let field = &x.a;//~ERROR: evaluation of constant value failed
+let buf = [0i32;4];
+let x:&S = &*(&buf as*const_as*constS);
+
+// This needs to compute the field offset, but we don't know the type's alignment, so this
+// fails.
+let field = &x.a;
+//~^ ERROR: evaluation of constant value failed
 //~| does not have a known offset
 };
 

tests/ui/consts/const-eval/issue-91827-extern-types-field-offset.stderr

@@ -1,5 +1,5 @@
 error[E0080]: evaluation of constant value failed
- --> $DIR/issue-91827-extern-types-field-offset.rs:28:17
+ --> $DIR/issue-91827-extern-types-field-offset.rs:33:17
  |
 LL | let field = &x.a;
  | ^^^^ `extern type` does not have a known offset