`SharedString` cannot be converted into `std::borrow::Cow<'static, str>'

[vstojkovic-mongodb]

Although the conversion is implemented, the compiler won't let me use it because str is not Sized.

For example, the following code:

struct MetricMetadata {
    description: SharedString,
    unit: Option<Unit>,
}

impl MetricMetadata {
    fn description(&self) -> std::borrow::Cow<'static, str> {
        self.description.into()
    }
}

refuses to compile with the following error:

error[E0277]: the size for values of type `str` cannot be known at compilation time
  --> src/metadata.rs:38:26
   |
38 |         self.description.into()
   |                          ^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `str`
   = note: required for `std::borrow::Cow<'_, str>` to implement `std::convert::From<metrics::cow::Cow<'_, str>>`
   = note: required for `metrics::cow::Cow<'_, str>` to implement `std::convert::Into<std::borrow::Cow<'_, str>>`

[tobz]

Hmmm, yeah: looks like we need to add a ?Sized constraint to T for that From implementation.

Happy to accept a PR for that if this is a priority to you.