also add a Miri test

Author: RalfJung

src/tools/miri/tests/fail/extern-type-field-offset.rs

@@ -0,0 +1,29 @@
+#![feature(extern_types)]
+
+extern"C"{
+typeOpaque;
+}
+
+structNewtype(Opaque);
+
+structS{
+i:i32,
+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 _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 _field = &x.a;//~ERROR: does not have a known offset
+}

src/tools/miri/tests/fail/extern-type-field-offset.stderr

@@ -0,0 +1,14 @@
+error: unsupported operation: `extern type` does not have a known offset
+ --> $DIR/extern-type-field-offset.rs:LL:CC
+ |
+LL | let _field = &x.a;
+ | ^^^^ `extern type` does not have a known offset
+ |
+ = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
+ = note: BACKTRACE:
+ = note: inside `main` at $DIR/extern-type-field-offset.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+