also test projecting to some sized fields at non-zero offset in structs with an extern type tail

Author: RalfJung

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

@@ -13,7 +13,8 @@ struct Newtype(Opaque);
 
 structS{
 i:i32,
-a:Opaque,
+j:i32,
+a:Newtype,
 }
 
 constNEWTYPE:() = unsafe{
@@ -28,6 +29,10 @@ const OFFSET: () = unsafe {
 let buf = [0i32;4];
 let x:&S = &*(&buf as*const_as*constS);
 
+// Accessing sized fields is perfectly fine, even at non-zero offsets.
+let field = &x.i;
+let field = &x.j;
+
 // This needs to compute the field offset, but we don't know the type's alignment, so this
 // fails.
 let field = &x.a;

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:33:17
+ --> $DIR/issue-91827-extern-types-field-offset.rs:38:17
  |
 LL | let field = &x.a;
  | ^^^^ `extern type` does not have a known offset

tests/ui/extern/extern-types-field-offset.rs

@@ -12,16 +12,22 @@ struct Newtype(Opaque);
 
 structS{
 i:i32,
-a:Opaque,
+j:i32,
+a:Newtype,
 }
 
 fnmain(){
+let buf = [0i32;4];
+
+let x:&Newtype = unsafe{&*(&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;
 
+let x:&S = unsafe{&*(&buf as*const_as*constS)};
+// Accessing sized fields is perfectly fine, even at non-zero offsets.
+let field = &x.i;
+let field = &x.j;
 // This needs to compute the field offset, but we don't know the type's alignment,
 // so this panics.
-let x:&S = unsafe{&*(1usizeas*constS)};
 let field = &x.a;
 }