[K0nserv]

Rust needs to know the exact size, layout, and alignment of every argument passed to a function to determine how it gets passed(register(s) or spilled to stack) and used. For example PathBuf and String can both be turned into a reference to a Path, and while they have the same size their layout and implementation of `as_ref` differ.

As for `impl`,

    fn foo(a: impl ToString)

is syntactic sugar for

    fn foo<S: ToString>(a: S)

The reason the standard library doesn't use this is because the code predates the introduction of `impl` in argument position.

The reason the function takes `AsRef<Path>` instead of `&Path` is callsite ergonomics. If it took `&Path` all callsites need to be turned into `read(path.as_ref())` or equivalent. With `AsRef<Path>` it transparently works with any type that can be turned into a `&Path` including `&Path` itself.

[jiwangcdi]

Then if Path is not about abstraction, why not use a raw byte slice like &[u8]

[K0nserv]

That's orthogonal. If the type was `&[u8]` instead of `Path` the type signature would be:

    pub fn read<P: AsRef<[u8]>>(path: P) -> Result<Vec<u8>>

The reasons for it to be generic and us `AsRef` remain. The reason for Path over &[u8] is, AFAIK, because not all byte slices are valid paths on all OSs, but also because a dedicated type lets the standard library add methods such as `Path::join`

[jiwangcdi]

So abstraction is still a point, but Rust cares about memory layout as well.

[K0nserv]

Sort of, this API enables static dispatch, but it could also have been written as

   pub fn read<P: AsRef<Path>>(path: &dyn P) -> Result<Vec<u8>>

In which case the size of `path` is always the same(two words[0]) and the same machine code could be used regardless of how the function is called. Rust still cares about the memory layout but because `&dyn AsRef<Path>` has the same layout for all implementations of `AsRef<Path>` there's no need for monomorphization[1].

0: https://doc.rust-lang.org/1.80.1/reference/types/trait-objec...

1: https://rustc-dev-guide.rust-lang.org/backend/monomorph.html...