[Rust] See through type aliases for borrowing optimizations

[rylev]

As noted in https://github.com/bytecodealliance/wit-bindgen/issues/769 and https://github.com/bytecodealliance/wit-bindgen/pull/675, type aliases cause certain optimizations around borrowing to not longer be applied.

For example take the following semantically equivalent functions in wit:

borrow: func(param: list<u8>);

type param = list<u8>;
no-borrow: func(param: param);

Those functions will be generated as the following:

fn borrow(param: &[u8]);

type Param = Vec<u8>;
fn no_borrow(param: Param);

Unfortunately, we want to preserve the structure of the wit files in the generated code meaning that generating the no_borrow function with a &[u8] param instead of the Param alias would violate that tenant. It is unclear which should take precedent, the borrowing optimization or the desire to maintain wit structure.

[esoterra]

Shouldn't we just generate something like this?

fn borrow(param: &[u8]);

type Param<'call> = &'call [u8];
fn no_borrow<'call>(param: Param<'call>);

Rust Playground

[rylev]

How do we handle the following then:

type param = list<u8>;
no-borrow: func(param: param);

record my-record {
  field: param
}

This would require two types: a borrowed type used as a function parameter and a type used in owned contexts such as struct fields. This is doable, but it violates the goal of having the generated code mirror the wit declaration.