Debuginfo does not describe enums with uninhabited variants correctly

[wesleywiser]

I tried this code:

enum Uninhabited { }

fn main() {
    let x = Option::<Uninhabited>::None;

    foo(x);
}

#[inline(never)]
fn foo<T>(_: T) { }

I expected to see this happen: gdb reports that x is None.

Instead, this happened: gdb reports that x is Some:

$ gdb ./uninhabited_enum_test
GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1
...
(gdb) break uninhabited_enum_test.rs:4
Breakpoint 1 at 0x8491: file uninhabited_enum_test.rs, line 6.
(gdb) r
Starting program: /tmp/uninhabited_enum_test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, uninhabited_enum_test::main () at uninhabited_enum_test.rs:6
6           foo(x);
(gdb) info locals
x = core::option::Option<uninhabited_enum_test::Uninhabited>::Some(uninhabited_enum_test::Uninhabited)

Meta

rustc --version --verbose:

rustc 1.68.0 (2c8cc3432 2023-03-06)
binary: rustc
commit-hash: 2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74
commit-date: 2023-03-06
host: x86_64-unknown-linux-gnu
release: 1.68.0
LLVM version: 15.0.6

Also affects beta and nightly.

[wesleywiser]

DWARF is this which looks wrong as there is no way for a debugger to determine which variant is active:

< 3><0x000000a6>        DW_TAG_structure_type
                          DW_AT_name                  Option<uninhabited_enum_test::Uninhabited>
                          DW_AT_byte_size             0x00000000
                          DW_AT_alignment             0x00000001
< 4><0x000000ad>          DW_TAG_variant_part
< 5><0x000000ae>            DW_TAG_variant
< 6><0x000000af>              DW_TAG_member
                                DW_AT_name                  None
                                DW_AT_type                  <0x000000c9>
                                DW_AT_alignment             0x00000001
                                DW_AT_data_member_location  0
< 5><0x000000bb>            DW_TAG_variant
< 6><0x000000bc>              DW_TAG_member
                                DW_AT_name                  Some
                                DW_AT_type                  <0x000000da>
                                DW_AT_alignment             0x00000001
                                DW_AT_data_member_location  0
< 4><0x000000c9>          DW_TAG_structure_type
                            DW_AT_name                  None
                            DW_AT_byte_size             0x00000000
                            DW_AT_alignment             0x00000001
< 5><0x000000d0>            DW_TAG_template_type_parameter
                              DW_AT_type                  <0x00000092>
                              DW_AT_name                  T
< 4><0x000000da>          DW_TAG_structure_type
                            DW_AT_name                  Some
                            DW_AT_byte_size             0x00000000
                            DW_AT_alignment             0x00000001
< 5><0x000000e1>            DW_TAG_template_type_parameter
                              DW_AT_type                  <0x00000092>
                              DW_AT_name                  T
< 5><0x000000ea>            DW_TAG_member
                              DW_AT_name                  __0
                              DW_AT_type                  <0x00000092>
                              DW_AT_alignment             0x00000001
                              DW_AT_data_member_location  0