debug info emitted for trait objects has wrong size/alignment

[khuey]

Consider:

use std::sync::Arc;

trait Foo {
    fn bar(&self) -> bool;
}

struct Baz;

impl Foo for Baz {
    fn bar(&self) -> bool {
        return false;
    }
}

fn do_it(foo: Arc<dyn Foo>) {
    println!("{}", foo.bar());
}

fn main() {
    do_it(Arc::new(Baz));
}

The DWARF debug info for this will contain the following DIE

< 1><0x000004e8 GOFF=0x00000ad5>    DW_TAG_structure_type
                                      DW_AT_name                  dyn rust_scratch::Foo
                                      DW_AT_byte_size             0x00000000
                                      DW_AT_alignment             0x00000001
< 2><0x000004ef GOFF=0x00000adc>      DW_TAG_member
                                        DW_AT_name                  pointer
                                        DW_AT_type                  <0x00000506 GOFF=0x00000af3>
                                        DW_AT_alignment             0x00000008
                                        DW_AT_data_member_location  0
                                        DW_AT_artificial            yes(1)
< 2><0x000004fa GOFF=0x00000ae7>      DW_TAG_member
                                        DW_AT_name                  vtable
                                        DW_AT_type                  <0x0000051a GOFF=0x00000b07>
                                        DW_AT_alignment             0x00000008
                                        DW_AT_data_member_location  8
                                        DW_AT_artificial            yes(1)

The DW_TAG_members have been synthesized via trait_pointer_metadata and are correct. But the DW_TAG_structure_type's size and alignment have been computed from the Layout of the type and are wrong.

These incorrect values are computed by the ty::Dynamic branch of layout_of_uncached which does

            ty::Dynamic(..) | ty::Foreign(..) => {
                let mut unit = self.univariant_uninterned(
                    ty,
                    &[],
                    &ReprOptions::default(),
                    StructKind::AlwaysSized,
		)?;
                match unit.abi {
                    Abi::Aggregate { ref mut sized } => *sized = false,
                    _ => bug!(),
                }
                tcx.intern_layout(unit)
            }

Unsurprisingly, passing in an empty set of members produces a zero sized layout.

For comparison, ty::Ref is handled differently and produces the correct output

< 1><0x00000555 GOFF=0x00000b42>    DW_TAG_structure_type
                                      DW_AT_name                  &mut dyn rust_scratch::Foo
                                      DW_AT_byte_size             0x00000010
                                      DW_AT_alignment             0x00000008
< 2><0x0000055c GOFF=0x00000b49>      DW_TAG_member
                                        DW_AT_name                  pointer
                                        DW_AT_type                  <0x00000506 GOFF=0x00000af3>
                                        DW_AT_alignment             0x00000008
                                        DW_AT_data_member_location  0
                                        DW_AT_artificial            yes(1)
< 2><0x00000567 GOFF=0x00000b54>      DW_TAG_member
                                        DW_AT_name                  vtable
                                        DW_AT_type                  <0x0000051a GOFF=0x00000b07>
                                        DW_AT_alignment             0x00000008
                                        DW_AT_data_member_location  8
                                        DW_AT_artificial            yes(1)

The incorrect values above result in our debugger being unable to do anything useful with do_it's foo parameter.

[Manishearth]

cc @michaelwoerister

[michaelwoerister]

Thanks for the bug report, @khuey! Yes, it looks like debuginfo for dyn pointers is broken in a number of ways. I'll try to improve it over the next few weeks.

[michaelwoerister]

#93006 (which was just merged and should be in a nightly soon) improves debuginfo for fat pointers. You now get the following debuginfo for the actual fat pointer type:

< 1><0x00000380>    DW_TAG_structure_type
                      DW_AT_name                  *mut alloc::sync::ArcInner<dyn issue88873::Foo>
                      DW_AT_byte_size             0x00000010
                      DW_AT_alignment             0x00000008
< 2><0x00000387>      DW_TAG_member
                        DW_AT_name                  pointer
                        DW_AT_type                  <0x000002d6>
                        DW_AT_alignment             0x00000008
                        DW_AT_data_member_location  0
< 2><0x00000392>      DW_TAG_member
                        DW_AT_name                  vtable
                        DW_AT_type                  <0x000002df>
                        DW_AT_alignment             0x00000008
                        DW_AT_data_member_location  8

Does that help?

[khuey]

I expect it will. I'll test next week.

[khuey]

I actually can't really test this in my debugger because rustc's vtable debug info changed and deleted the DW_AT_containing_type entries. It looks like that's getting fixed in #93503 though.

[khuey]

Alright I did get around to testing this and what you landed is sufficient. Thanks!

[michaelwoerister]

Great!