Support all types that CXX supports for C++ generation side

[ahayzen-kdab]

In the C++ generation we have to map Rust types to C++ types, here not all types have been mapped.

Some of the missing types

• Pointers (eg * const T) needs to handle TypePtr (related Nested objects #367 Defining nested objects with the new API #299 )
• Slice to rust::Slice cxx-qt-gen: add support for more CXX types #389
• Function pointer to Fn cxx-qt-gen: add support for more CXX types #389
• Array type cxx-qt-gen: add support for more CXX types #389
• Result ? Split into Support Result<T, E> as a return type from Rust -> C++ #404 for later as it's more complicated
• Do we handle &str correctly? cxx-qt-gen: add support for more CXX types #389
• i64,u64 ( These may help as then we can use std types cxx-qt-gen: use std types for integers instead of qint #317 Support u64 and i64 #36 )
• Pin cxx-qt-gen: generate C++ types for Pin<T> correctly #353
• Any others :-)

https://github.com/KDAB/cxx-qt/blob/main/crates/cxx-qt-gen/src/generator/cpp/types.rs

[LeonMatthesKDAB]

Could we get this exposed by upstream?
E.g. could CXX somehow tell us how to transform this stuff...

[ahayzen-kdab]

Maybe, but they have already parsed the types into their own enum via syn. We would have to expose some API to get from our syn types to the theirs etc.

[ahayzen-kdab]

Could be worth a discussion upstream though if something like the cxx-gen crate could expose something like this. It would need to be able to go from a Rust syn::Type to a C++ String. We would also need to know if the type means that we need to mark the CXX bridge method as unsafe (eg fn is_unsafe(ty: &syn::Type) -> bool).

[ahayzen-kdab]

Note that CXX has it's own Type can calls syn's Type as RustType https://github.com/dtolnay/cxx/blob/a6e1cd1e8d9d6df20e88e7443963dc4c5c8c4875/syntax/mod.rs#L226

Effectively we need fn parse_type(ty: &RustType) -> Result<Type> to be public or part of a public API to then generate the C++ string https://github.com/dtolnay/cxx/blob/a6e1cd1e8d9d6df20e88e7443963dc4c5c8c4875/syntax/parse.rs#L1134

[ahayzen-kdab]

autocxx also has similar things in their code

• Registering of the types https://github.com/google/autocxx/blob/6cca9b29c1b2a64a6ee7d442f35d6fbfbc76cc42/engine/src/known_types.rs#L363
• Type -> C++ conversion https://github.com/google/autocxx/blob/6cca9b29c1b2a64a6ee7d442f35d6fbfbc76cc42/engine/src/conversion/codegen_cpp/type_to_cpp.rs#L67 (note this goes from a syn::Type to a String so is the same as what we want.

[ahayzen-kdab]

Split out Result as that is more complicated #404 .