Auto merge of rust-lang#2268 - RalfJung:not-unpin-protected, r=RalfJung

test that &mut !Unpin references are protected

Author: bors

tests/fail/stacked_borrows/deallocate_against_barrier2.rs

@@ -0,0 +1,16 @@
+// error-pattern: deallocating while item is protected
+use std::marker::PhantomPinned;
+
+pub struct NotUnpin(i32, PhantomPinned);
+
+fn inner(x: &mut NotUnpin, f: fn(&mut NotUnpin)) {
+    // `f` may mutate, but it may not deallocate!
+    f(x)
+}
+
+fn main() {
+    inner(Box::leak(Box::new(NotUnpin(0, PhantomPinned))), |x| {
+        let raw = x as *mut _;
+        drop(unsafe { Box::from_raw(raw) });
+    });
+}

tests/fail/stacked_borrows/deallocate_against_barrier2.stderr

@@ -0,0 +1,38 @@
+error: Undefined Behavior: deallocating while item is protected: [SharedReadWrite for <TAG> (call ID)]
+  --> RUSTLIB/alloc/src/alloc.rs:LL:CC
+   |
+LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ deallocating while item is protected: [SharedReadWrite for <TAG> (call ID)]
+   |
+   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
+   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
+           
+   = note: inside `std::alloc::dealloc` at RUSTLIB/alloc/src/alloc.rs:LL:CC
+   = note: inside `<std::alloc::Global as std::alloc::Allocator>::deallocate` at RUSTLIB/alloc/src/alloc.rs:LL:CC
+   = note: inside `alloc::alloc::box_free::<NotUnpin, std::alloc::Global>` at RUSTLIB/alloc/src/alloc.rs:LL:CC
+   = note: inside `std::ptr::drop_in_place::<std::boxed::Box<NotUnpin>> - shim(Some(std::boxed::Box<NotUnpin>))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
+   = note: inside `std::mem::drop::<std::boxed::Box<NotUnpin>>` at RUSTLIB/core/src/mem/mod.rs:LL:CC
+note: inside closure at $DIR/deallocate_against_barrier2.rs:LL:CC
+  --> $DIR/deallocate_against_barrier2.rs:LL:CC
+   |
+LL |         drop(unsafe { Box::from_raw(raw) });
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: inside `<[closure@$DIR/deallocate_against_barrier2.rs:LL:CC] as std::ops::FnOnce<(&mut NotUnpin,)>>::call_once - shim` at RUSTLIB/core/src/ops/function.rs:LL:CC
+note: inside `inner` at $DIR/deallocate_against_barrier2.rs:LL:CC
+  --> $DIR/deallocate_against_barrier2.rs:LL:CC
+   |
+LL |     f(x)
+   |     ^^^^
+note: inside `main` at $DIR/deallocate_against_barrier2.rs:LL:CC
+  --> $DIR/deallocate_against_barrier2.rs:LL:CC
+   |
+LL | /     inner(Box::leak(Box::new(NotUnpin(0, PhantomPinned))), |x| {
+LL | |         let raw = x as *mut _;
+LL | |         drop(unsafe { Box::from_raw(raw) });
+LL | |     });
+   | |______^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+