util: Add tl::expected type

[CohenArthur]

Directly taken from https://github.com/TartanLlama/expected and adapted
slightly for our use case.

gcc/rust/ChangeLog:

* util/expected.h: New file.

This type offers a type similar to std::expected, which is only available since C++23. It will be useful for cleaning up our error handling, chaining errors together, and easier control flow in general.

This will be used for the upcoming experimental name resolution algorithm.

[philberty]

Added in other GCC folks, are we allowed to do this in GCC?

Another option might be to take your optional implementation and reuse it to bootstrap a Result<Value, Error> class helper.

[philberty]

I am not sure how i feel about it my initial thought is we might have issues using this with GCC but on the other hand it could be a useful interface.

@dkm and @tschwinge any opinion?

[tschwinge]

This has recently been discussed in GCC/Rust IRC, 2023-06-15: https://github.com/Rust-GCC/gccrs/wiki/IRC-2023-06-15.

[tschwinge]

This has recently been discussed in GCC/Rust IRC, 2023-06-15: https://github.com/Rust-GCC/gccrs/wiki/IRC-2023-06-15.

The outcome of that -- per my free interpretation -- was something like this:

Should talk to libstdc++ maintainers to determine whether doing this via existing (or future) libstdc++ code is feasible, whilst maintaining GCC 4.8 compatibility, and otherwise it's deemed OKish to import external code.

[tschwinge]

One thing that's going to be useful for future maintenance: first import the external file(s) verbatim, and then do local changes in separate commit(s). For some (?) external imports, LOCAL_PATCHES files are maintained -- not sure whether that'll be useful here, too?

$ find -name LOCAL_PATCHES
./libffi/LOCAL_PATCHES
./libsanitizer/LOCAL_PATCHES
./libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
./libstdc++-v3/src/c++17/ryu/LOCAL_PATCHES

[CohenArthur]

One thing that's going to be useful for future maintenance: first import the external file(s) verbatim, and then do local changes in separate commit(s). For some (?) external imports, LOCAL_PATCHES files are maintained -- not sure whether that'll be useful here, too?

$ find -name LOCAL_PATCHES
./libffi/LOCAL_PATCHES
./libsanitizer/LOCAL_PATCHES
./libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
./libstdc++-v3/src/c++17/ryu/LOCAL_PATCHES

@tschwinge this is going to create commits that do not compile, as the original implementation uses exceptions which we explicitly disable. So I'm not sure how needed this is?

[tschwinge]

this is going to create commits that do not compile, as the original implementation uses exceptions which we explicitly disable.

There's no such problem: the file will be unused initially. :-)

[CohenArthur]

this is going to create commits that do not compile, as the original implementation uses exceptions which we explicitly disable.

There's no such problem: the file will be unused initially. :-)

ah, that's a good point! :D

[CohenArthur]

I'll split up the commit then