Rollup merge of rust-lang#94414 - DrMeepster:box_alloc_ice2, r=tmiasko

Fix ICE when using Box<T, A> with large A

A sequel to rust-lang#94043 that fixes rust-lang#81270 and rust-lang#92054 (duplicate).

Author: matthiaskrgr

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -453,7 +453,18 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         };
         for elem in place_ref.projection[base..].iter() {
             cg_base = match elem.clone() {
-                mir::ProjectionElem::Deref => bx.load_operand(cg_base).deref(bx.cx()),
+                mir::ProjectionElem::Deref => {
+                    // custom allocators can change box's abi, making it unable to be derefed directly
+                    if cg_base.layout.ty.is_box()
+                        && matches!(cg_base.layout.abi, Abi::Aggregate { .. } | Abi::Uninhabited)
+                    {
+                        let ptr = cg_base.project_field(bx, 0).project_field(bx, 0);
+
+                        bx.load_operand(ptr).deref(bx.cx())
+                    } else {
+                        bx.load_operand(cg_base).deref(bx.cx())
+                    }
+                }
                 mir::ProjectionElem::Field(ref field, _) => {
                     cg_base.project_field(bx, field.index())
                 }

src/test/ui/box/issue-78459-ice.rs

@@ -0,0 +1,23 @@
+// build-pass
+#![feature(allocator_api)]
+
+use std::alloc::Allocator;
+
+struct BigAllocator([usize; 2]);
+
+unsafe impl Allocator for BigAllocator {
+    fn allocate(
+        &self,
+        _: std::alloc::Layout,
+    ) -> Result<std::ptr::NonNull<[u8]>, std::alloc::AllocError> {
+        todo!()
+    }
+    unsafe fn deallocate(&self, _: std::ptr::NonNull<u8>, _: std::alloc::Layout) {
+        todo!()
+    }
+}
+
+fn main() {
+    Box::new_in((), &std::alloc::Global);
+    Box::new_in((), BigAllocator([0; 2]));
+}