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);
 
 struct S {
     i: i32,
-    a: Opaque,
+    j: i32,
+    a: Newtype,
 }
 
 const NEWTYPE: () = unsafe {
@@ -28,6 +29,10 @@ const OFFSET: () = unsafe {
     let buf = [0i32; 4];
     let x: &S = &*(&buf as *const _ as *const S);
 
+    // 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);
 
 struct S {
     i: i32,
-    a: Opaque,
+    j: i32,
+    a: Newtype,
 }
 
 fn main() {
+    let buf = [0i32; 4];
+
+    let x: &Newtype = unsafe { &*(&buf as *const _ as *const Newtype) };
     // Projecting to the newtype works, because it is always at offset 0.
-    let x: &Newtype = unsafe { &*(1usize as *const Newtype) };
     let field = &x.0;
 
+    let x: &S = unsafe { &*(&buf as *const _ as *const S) };
+    // 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 { &*(1usize as *const S) };
     let field = &x.a;
 }