Rollup merge of #95270 - michaelwoerister:fix-box-unsized-debuginfo, r=wesleywiser

debuginfo: Fix debuginfo for Box<T> where T is unsized.

Before this fix, the debuginfo for the fields was generated from the struct defintion of Box<T>, but (at least at the moment) the compiler pretends that Box<T> is just a (fat) pointer, so the fields need to be `pointer` and `vtable` instead of `__0: Unique<T>` and `__1: Allocator`.

This is meant as a temporary mitigation until we can make sure that simply treating Box as a regular struct in debuginfo does not cause too much breakage in the ecosystem.

r? ````@wesleywiser````

1 files changed, 18 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
index f02c7b2d2e1..74e194750fa 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
@@ -166,6 +166,13 @@ fn build_pointer_or_reference_di_node<'ll, 'tcx>(
     pointee_type: Ty<'tcx>,
     unique_type_id: UniqueTypeId<'tcx>,
 ) -> DINodeCreationResult<'ll> {
+    // The debuginfo generated by this function is only valid if `ptr_type` is really just
+    // a (fat) pointer. Make sure it is not called for e.g. `Box<T, NonZSTAllocator>`.
+    debug_assert_eq!(
+        cx.size_and_align_of(ptr_type),
+        cx.size_and_align_of(cx.tcx.mk_mut_ptr(pointee_type))
+    );
+
     let pointee_type_di_node = type_di_node(cx, pointee_type);
 
     return_if_di_node_created_in_meantime!(cx, unique_type_id);
@@ -212,7 +219,17 @@ fn build_pointer_or_reference_di_node<'ll, 'tcx>(
                     DIFlags::FlagZero,
                 ),
                 |cx, owner| {
-                    let layout = cx.layout_of(ptr_type);
+                    // FIXME: If this fat pointer is a `Box` then we don't want to use its
+                    //        type layout and instead use the layout of the raw pointer inside
+                    //        of it.
+                    //        The proper way to handle this is to not treat Box as a pointer
+                    //        at all and instead emit regular struct debuginfo for it. We just
+                    //        need to make sure that we don't break existing debuginfo consumers
+                    //        by doing that (at least not without a warning period).
+                    let layout_type =
+                        if ptr_type.is_box() { cx.tcx.mk_mut_ptr(pointee_type) } else { ptr_type };
+
+                    let layout = cx.layout_of(layout_type);
                     let addr_field = layout.field(cx, abi::FAT_PTR_ADDR);
                     let extra_field = layout.field(cx, abi::FAT_PTR_EXTRA);