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.

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);
 

src/test/debuginfo/unsized.rs

@@ -16,13 +16,17 @@
 // gdbg-check:$3 = {pointer = [...], vtable = [...]}
 // gdbr-check:$3 = &unsized::Foo<dyn core::fmt::Debug> {pointer: [...], vtable: [...]}
 
+// gdb-command:print _box
+// gdbg-check:$4 = {pointer = [...], vtable = [...]}
+// gdbr-check:$4 = alloc::boxed::Box<unsized::Foo<dyn core::fmt::Debug>, alloc::alloc::Global> {pointer: [...], vtable: [...]}
+
 // gdb-command:print tuple_slice
-// gdbg-check:$4 = {data_ptr = [...], length = 2}
-// gdbr-check:$4 = &(i32, i32, [i32]) {data_ptr: [...], length: 2}
+// gdbg-check:$5 = {data_ptr = [...], length = 2}
+// gdbr-check:$5 = &(i32, i32, [i32]) {data_ptr: [...], length: 2}
 
 // gdb-command:print tuple_dyn
-// gdbg-check:$5 = {pointer = [...], vtable = [...]}
-// gdbr-check:$5 = &(i32, i32, dyn core::fmt::Debug) {pointer: [...], vtable: [...]}
+// gdbg-check:$6 = {pointer = [...], vtable = [...]}
+// gdbr-check:$6 = &(i32, i32, dyn core::fmt::Debug) {pointer: [...], vtable: [...]}
 
 // === CDB TESTS ===================================================================================
 
@@ -42,6 +46,12 @@
 // cdb-check:    [+0x000] pointer          : 0x[...] [Type: unsized::Foo<dyn$<core::fmt::Debug> > *]
 // cdb-check:    [...] vtable           : 0x[...] [Type: unsigned [...]int[...] (*)[3]]
 
+// cdb-command:dx _box
+// cdb-check:
+// cdb-check:_box             [Type: alloc::boxed::Box<unsized::Foo<dyn$<core::fmt::Debug> >,alloc::alloc::Global>]
+// cdb-check:[+0x000] pointer          : 0x[...] [Type: unsized::Foo<dyn$<core::fmt::Debug> > *]
+// cdb-check:[...] vtable           : 0x[...] [Type: unsigned [...]int[...] (*)[3]]
+
 // cdb-command:dx tuple_slice
 // cdb-check:tuple_slice      [Type: ref$<tuple$<i32,i32,slice$<i32> > >]
 // cdb-check:    [+0x000] data_ptr         : 0x[...] [Type: tuple$<i32,i32,slice$<i32> > *]
@@ -69,6 +79,7 @@ fn main() {
     let a: &Foo<[u8]> = &foo.value;
     let b: &Foo<Foo<[u8]>> = &foo;
     let c: &Foo<dyn std::fmt::Debug> = &Foo { value: 7i32 };
+    let _box: Box<Foo<dyn std::fmt::Debug>> = Box::new(Foo { value: 8i32 });
 
     // Also check unsized tuples
     let tuple_slice: &(i32, i32, [i32]) = &(0, 1, [2, 3]);