Fix debuginfo for unsized struct members

[philipc]

The member was given the size of a fat pointer, which caused
llvm to emit DWARF attributes for a 128-bit bitfield.

[rust-highfive]

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[michaelwoerister]

Thanks for the PR!

The member was given the size of a fat pointer, which caused llvm to emit DWARF attributes for a 128-bit bitfield.

What does it emit after this change?

[philipc]

For the test case I added, it emitted this before:

< 2><0x000000e9>      DW_TAG_structure_type
                        DW_AT_name                  "Foo<unsized::Foo<[u8]>>"
                        DW_AT_byte_size             0x00000001
< 3><0x000000ef>        DW_TAG_member
                          DW_AT_name                  "value"
                          DW_AT_type                  <0x000000d8>
                          DW_AT_byte_size             0x00000001
                          DW_AT_bit_size              0x00000080
                          DW_AT_bit_offset            0xffffffffffffff80
                          DW_AT_data_member_location  2305843009213693951

and this after:

< 2><0x000000eb>      DW_TAG_structure_type
                        DW_AT_name                  "Foo<unsized::Foo<[u8]>>"
                        DW_AT_byte_size             0x00000001
< 3><0x000000f1>        DW_TAG_member
                          DW_AT_name                  "value"
                          DW_AT_type                  <0x000000da>
                          DW_AT_data_member_location  0

In both case, the DW_AT_byte_size of the member type is 1. For the before case, the problem is that we tell LLVM that the type size is 1 byte, but the member size is 128 bits. For the after case, we tell LLVM that both type and member size are 1 byte, so it doesn't need to add any size attributes.

I'm not sure that this is completely correct yet, but at least it is consistent. The reason I'm not sure it is correct is that in C the equivalent type is a flexible array, and I think these are given a size of 0 in DWARF, whereas rust unsized arrays are given the size of the first element.

Also note that this only caused a problem for gdb without rust support, because the rust support in gdb ignores bitfield attributes.

[michaelwoerister]

Thanks for the additional info. It's still not really correct but it's definitely an improvement over what we have now.

[michaelwoerister]

@bors r+

[bors]

📌 Commit af34f91 has been approved by michaelwoerister

[bors]

⌛ Testing commit af34f91 with merge d3a2efa...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: michaelwoerister
Pushing d3a2efa to master...

[michaelwoerister]

Thanks again for the PR!