I'm revisiting my include_dir crate to give it a proper include_dir!() macro with proc_macro_hack and encountered an interesting cyclic generics problem.
Basically, I want include_dir!() to expand to some Dir which includes the bytes of each file via include_bytes!() as a &'static [u8]. However, for simplicity of implementation I'd like to use the same Dir type except with all the fields backed by a Vec so I can update a Dir in-place while writing the macro. In theory, this is exactly the use case for Cow<'a, [File<'a>]>.
#[derive(Debug, Clone, PartialEq)]
pub struct Dir<'a> {
path: Cow<'a, Path>,
files: Cow<'a, [File<'a>]>,
dirs: Cow<'a, [Dir<'a>]>,
}
#[derive(Debug, Clone, PartialEq)]
pub struct File<'a> {
pub path: Cow<'a, Path>,
pub contents: Cow<'a, [u8]>,
}
Trying to compile results in an error because resolving Dir<'a>: ToOwned requires checking [Dir<'a>]: ToOwned, which in turn requires Dir<'a>: ToOwned.
error[E0275]: overflow evaluating the requirement `<[dir::Dir<'_>] as std::borrow::ToOwned>::Owned`
|
= note: required because it appears within the type `dir::Dir<'_>`
= note: required because of the requirements on the impl of `std::borrow::ToOwned` for `[dir::Dir<'_>]`
= note: required because it appears within the type `dir::Dir<'_>`
error: aborting due to previous error
Does anyone have any ideas on what I can do to fix the cycle in trait resolution? I could always create two parallel sets of File and Dir, one for compile time which stores its data on the heap and another which is 'static, but I'd prefer not to if possible.
The end goal is being able to generate something like this:
static Foo: Dir<'static> = Dir {
path: "foo/bar/",
files: Cow::Borrowed(&'static [
File {
path: "foo/bar/baz.txt",
contents: Cow::Borrowed(&'static [1, 2, 3, 4, ... ]),
},
...
]),
dirs: Cow::Borrowed(&[ ... ]),
};