Test foreign CoerceShared private tuple field

Author: P8L1

tests/ui/reborrow/auxiliary/reborrow_foreign_private.rs

@@ -1,7 +1,22 @@
-#![feature(reborrow)]
 #![allow(dead_code)]
 
+use std::marker::PhantomData;
+
 #[derive(Clone, Copy)]
 pub struct ForeignRef<'a> {
     value: &'a i32,
 }
+
+// SAFETY invariant: the pointer is valid as `&'a i32`.
+#[derive(Clone, Copy)]
+pub struct ForeignPtrRef<'a>((*const i32, PhantomData<&'a ()>));
+
+impl<'a> ForeignPtrRef<'a> {
+    pub fn new(r: &'a i32) -> Self {
+        ForeignPtrRef((r, PhantomData))
+    }
+
+    pub fn to_ref(self) -> &'a i32 {
+        unsafe { &*self.0.0 }
+    }
+}

tests/ui/reborrow/coerce-shared-foreign-private-tuple-field.rs

@@ -0,0 +1,24 @@
+//@ normalize-stderr: "\n\n\z" -> "\n"
+
+//@ aux-build: reborrow_foreign_private.rs
+
+#![feature(reborrow)]
+
+extern crate reborrow_foreign_private;
+
+use reborrow_foreign_private::ForeignPtrRef;
+use std::marker::{CoerceShared, PhantomData, Reborrow};
+use std::ptr;
+
+struct LocalPtrMut<'a>((*const i32, PhantomData<&'a ()>));
+
+impl<'a> Reborrow for LocalPtrMut<'a> {}
+
+impl<'a> CoerceShared<ForeignPtrRef<'a>> for LocalPtrMut<'a> {}
+//~^ ERROR
+
+fn main() {
+    let local = LocalPtrMut((ptr::null(), PhantomData));
+    let foreign: ForeignPtrRef<'_> = local;
+    let _ = foreign.to_ref();
+}

tests/ui/reborrow/coerce-shared-foreign-private-tuple-field.stderr

@@ -0,0 +1,7 @@
+error: implementing `CoerceShared` requires all source and target fields to be accessible from the impl
+  --> $DIR/coerce-shared-foreign-private-tuple-field.rs:17:1
+   |
+LL | impl<'a> CoerceShared<ForeignPtrRef<'a>> for LocalPtrMut<'a> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error