Suggest reborrowing unsized value when borrowed trait object wanted #93596
Description
Given the following code:
use std::ffi::{OsStr, OsString}; use std::path::Path; fn check(p: &dyn AsRef<Path>) { let m = std::fs::metadata(&p); println!("{:?}", &m); } fn main() { let s: OsString = ".".into(); let s: &OsStr = &s; check(s); }The current output is:
error[E0277]: the size for values of type `[u8]` cannot be known at compilation time --> src/main.rs:12:11 | 12 | check(s); | ----- ^ doesn't have a size known at compile-time | | | required by a bound introduced by this call | = help: within `OsStr`, the trait `Sized` is not implemented for `[u8]` = note: required because it appears within the type `OsStr` = note: required for the cast to the object type `dyn AsRef<Path>` Ideally the output would suggest borrowing, since this:
check(&s); compiles and works just fine.
The problem is that you can't make an object which is a wide pointer directly into a trait object refernce, because it would have to be a triple-sized pointer. But often, you can just reborrow instead.