fix other source of box deref

Author: beepster4096

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -441,11 +441,19 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     .find(|elem| matches!(elem.1, mir::ProjectionElem::Deref))
                 {
                     base = elem.0 + 1;
-                    self.codegen_consume(
+                    let cg_base = self.codegen_consume(
                         bx,
                         mir::PlaceRef { projection: &place_ref.projection[..elem.0], ..place_ref },
-                    )
-                    .deref(bx.cx())
+                    );
+
+                    // a box with a non-zst allocator should not be directly dereferenced
+                    if cg_base.layout.ty.is_box() && !cg_base.layout.field(cx, 1).is_zst() {
+                        let ptr = cg_base.extract_field(bx, 0).extract_field(bx, 0);
+
+                        ptr.deref(bx.cx())
+                    } else {
+                        cg_base.deref(bx.cx())
+                    }
                 } else {
                     bug!("using operand local {:?} as place", place_ref);
                 }

src/test/ui/box/issue-95036.rs

@@ -3,8 +3,20 @@
 
 #![feature(allocator_api, bench_black_box)]
 
+#[inline(never)]
+pub fn by_ref(node: &mut Box<[u8; 1], &std::alloc::Global>) {
+    node[0] = 9u8;
+}
+
 pub fn main() {
     let mut node = Box::new_in([5u8], &std::alloc::Global);
     node[0] = 7u8;
+
+    std::hint::black_box(node);
+
+    let mut node = Box::new_in([5u8], &std::alloc::Global);
+
+    by_ref(&mut node);
+
     std::hint::black_box(node);
 }