add test for intermediate reference in '&(*x).0 as *const i32'

Author: RalfJung

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_to_raw_pointer.rs

@@ -0,0 +1,20 @@
+#![feature(raw_ref_op)]
+#![feature(strict_provenance)]
+use std::ptr;
+
+fn direct_raw(x: *const (i32, i32)) -> *const i32 {
+    unsafe { &raw const (*x).0 }
+}
+
+// Ensure that if a raw pointer is created via an intermediate
+// reference, we catch that. (Just in case someone decides to
+// desugar this differenly or so.)
+fn via_ref(x: *const (i32, i32)) -> *const i32 {
+    unsafe { &(*x).0 as *const i32 } //~ERROR: dangling pointer
+}
+
+fn main() {
+    let ptr = ptr::without_provenance(0x10);
+    direct_raw(ptr); // this is fine
+    via_ref(ptr); // this is not
+}

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_to_raw_pointer.stderr

@@ -0,0 +1,20 @@
+error: Undefined Behavior: out-of-bounds pointer use: 0x10[noalloc] is a dangling pointer (it has no provenance)
+  --> $DIR/dangling_pointer_to_raw_pointer.rs:LL:CC
+   |
+LL |     unsafe { &(*x).0 as *const i32 }
+   |              ^^^^^^^ out-of-bounds pointer use: 0x10[noalloc] is a dangling pointer (it has no provenance)
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `via_ref` at $DIR/dangling_pointer_to_raw_pointer.rs:LL:CC
+note: inside `main`
+  --> $DIR/dangling_pointer_to_raw_pointer.rs:LL:CC
+   |
+LL |     via_ref(ptr); // this is not
+   |     ^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+