Use DW_LANG_Rust in DWARF

[tromey]

This changes the GCC DWARF emitterto use DW_LANG_Rust or
DW_LANG_Rust_old as appropriate.

[dkm]

Maybe the PR template content (Thank you for making Rust GCC better!....) could be removed from the first comment? It will end-up in the git log :)

[tromey]

I edited the comment, no idea if that really works.

[philberty]

bors r+

[bors]

Build succeeded:

• build-and-check
• clang-format

[dkm]

I edited the comment, no idea if that really works.

Yes, editing the first comment works ! Thanks !

[tschwinge]

My understanding of DWARF is limited, so please bear with me. ;-)

@tromey, I suppose that DW_LANG_Rust/DW_LANG_Rust_old have originally been created for rustc/LLVM proper. Meaning that GDB (or any other consumer) will (I suppose) use this to "switch to expecting to see rustc/LLVM idioms" in the debug information (whatever that exactly means in practice). Are there now any caveats that GCC/Rust has to make sure to follow (produce) the same idioms, so that GDB etc. understand GCC/Rust debug information the same way as what rustc/LLVM produces?

Or, are DW_LANG_Rust/DW_LANG_Rust_old agnostic of the exact producer (rustc/LLVM vs. GCC/Rust), meaning that as long as the producer follows the specification, GDB etc. don't care -- in theory as well as in practice? (I further suppose that if in theory but not in practice, that then points to GDB etc. bugs to be fixed.)

And no, I don't have a specific concern, I was just curious. (Hope you understood my concern.)

[tschwinge]

Aha -- what I mean are things like #514. ;-) But I suppose that's fine -- we just have to pay attention to such conventions.

[tromey]

I understand your concern, but there's nothing too much to be worried about. DWARF varies between compilers but also between different versions of compilers. Debuggers are naturally resilient to this to some degree; and GDB at least will add compiler-specific workarounds when needed (either by pattern matching or by examining the producer string). Mentioning the language of the CU is just a good practice and it lets GDB know that it should start up in "rust mode".