Fix Box deref for non-ZST allocators

Author: bjorn3

example/mini_core.rs

@@ -483,8 +483,17 @@ pub trait Deref {
     fn deref(&self) -> &Self::Target;
 }
 
+pub struct Unique<T: ?Sized> {
+    pub pointer: *const T,
+    pub _marker: PhantomData<T>,
+}
+
+impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
+
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}
+
 #[lang = "owned_box"]
-pub struct Box<T: ?Sized>(*mut T);
+pub struct Box<T: ?Sized>(Unique<T>, ());
 
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 
@@ -508,8 +517,8 @@ unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
 }
 
 #[lang = "box_free"]
-unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
-    libc::free(ptr as *mut u8);
+unsafe fn box_free<T: ?Sized>(ptr: Unique<T>, alloc: ()) {
+    libc::free(ptr.pointer as *mut u8);
 }
 
 #[lang = "drop"]

example/mini_core_hello_world.rs

@@ -105,12 +105,6 @@ fn start<T: Termination + 'static>(
 static mut NUM: u8 = 6 * 7;
 static NUM_REF: &'static u8 = unsafe { &NUM };
 
-struct Unique<T: ?Sized> {
-    pointer: *const T,
-    _marker: PhantomData<T>,
-}
-
-impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
 
 unsafe fn zeroed<T>() -> T {
     let mut uninit = MaybeUninit { uninit: () };

src/base.rs

@@ -25,7 +25,7 @@ pub(crate) fn codegen_fn<'tcx>(
     let mir = tcx.instance_mir(instance.def);
     let _mir_guard = crate::PrintOnPanic(|| {
         let mut buf = Vec::new();
-        rustc_middle::mir::write_mir_pretty(tcx, Some(instance.def_id()), &mut buf).unwrap();
+        rustc_middle::mir::pretty::write_mir_fn(tcx, mir, &mut |_, _| Ok(()), &mut buf).unwrap();
         String::from_utf8_lossy(&buf).into_owned()
     });
 
@@ -813,7 +813,14 @@ pub(crate) fn codegen_place<'tcx>(
     for elem in place.projection {
         match elem {
             PlaceElem::Deref => {
-                cplace = cplace.place_deref(fx);
+                if cplace.layout().ty.is_box() {
+                    cplace = cplace
+                        .place_field(fx, Field::new(0)) // Box<T> -> Unique<T>
+                        .place_field(fx, Field::new(0)) // Unique<T> -> *const T
+                        .place_deref(fx);
+                } else {
+                    cplace = cplace.place_deref(fx);
+                }
             }
             PlaceElem::Field(field, _ty) => {
                 cplace = cplace.place_field(fx, field);