It would be awesome if the std::ffi module contained utilities which serves as common conversions from Rust types to C types in FFI functions. For example in git2-rs I have a macro which will call a function but convert each argument to it's "C equivalent" before doing so.
The conversion trait is pretty ad-hoc at the moment, but it encompasses a number of useful conversions such as:
&T => *const T&mut T => *mut TCString => *const c_charOption<CString> => *const c_char (nullable)Option<&T> => *const T (nullable)bool => c_int
These sorts of conversions may not be appropriate for Rust at large, but the general idea is that types like Option<CString> are pretty painful to convert to a nullable pointer today:
let foo: Option<CString> = ...;
let ptr = foo.as_ref().map(|s| s.as_ptr()).unwrap_or(0 as *const _);
This issue is also motivated by rust-lang/rust#16035 because it would serve two purposes:
- We could remove/deprecate
as_ptr in favor of as_ref. This would break code such as the line above (because as_ptr would return a reference that would not be coerced to a raw pointer) but the old behavior could be opted into with this conversion trait.
- The need to call
as_ptr in the first place actually largely goes away as you can in theory pass the &CString to the C call directly (when using a macro like shown above).
More broadly this sort of helper or trait plays into the story of writing unsafe code in Rust (and improving the ergonomics thereof) and may have other considerations. I'm sure there are also quite a few variants of the trait I've got already floating around in other crates as well :). Regardless, it would be nice to have a standard method of converting to a "C compatible" representation and perhaps even a nice convenient macro.
It would be awesome if the
std::ffimodule contained utilities which serves as common conversions from Rust types to C types in FFI functions. For example in git2-rs I have a macro which will call a function but convert each argument to it's "C equivalent" before doing so.The conversion trait is pretty ad-hoc at the moment, but it encompasses a number of useful conversions such as:
&T=>*const T&mut T=>*mut TCString=>*const c_charOption<CString>=>*const c_char(nullable)Option<&T>=>*const T(nullable)bool=>c_intThese sorts of conversions may not be appropriate for Rust at large, but the general idea is that types like
Option<CString>are pretty painful to convert to a nullable pointer today:This issue is also motivated by rust-lang/rust#16035 because it would serve two purposes:
as_ptrin favor ofas_ref. This would break code such as the line above (becauseas_ptrwould return a reference that would not be coerced to a raw pointer) but the old behavior could be opted into with this conversion trait.as_ptrin the first place actually largely goes away as you can in theory pass the&CStringto the C call directly (when using a macro like shown above).More broadly this sort of helper or trait plays into the story of writing unsafe code in Rust (and improving the ergonomics thereof) and may have other considerations. I'm sure there are also quite a few variants of the trait I've got already floating around in other crates as well :). Regardless, it would be nice to have a standard method of converting to a "C compatible" representation and perhaps even a nice convenient macro.