`Name` trait methods should return `Cow<'static, str>`

[carlocorradini]

Name trait methods should return Cow<'static, str>.

I came across the Name trait and found it extremely useful, especially when printing debug information or handling encode/decode errors.

What bothers me is that the provided methods, full_name and type_url, always return a newly allocated String, even though their values are mostly, if not always, the same.

• Cow<'static, str>
  Always return a clone-on-write around str.
  This provides maximum flexibility for both us and the user, as it allows choosing whether the value is borrowed from a string literal (&'static str) or owned as a newly allocated String:

  pub trait Name: Message {
      const NAME: &'static str;
  
      const PACKAGE: &'static str;
  
      fn full_name() -> Cow<'static, str> {
          if Self::PACKAGE.is_empty() {
              Cow::Borrowed(Self::NAME)
          } else {
              Cow::Owned(format!("{}.{}", Self::PACKAGE, Self::NAME))
          }
      }
  
      fn type_url() -> Cow<'static, str> {
          Cow::Owned(format!("/{}", Self::full_name()))
      }
  }

  However, since we control prost-build and it is almost always used, we can take advantage of this by having the generated code always return a borrowed value from a hardcoded string literal (this is already how it is implemented):

  impl Name for MyMessage {
      const NAME: &'static str = "MyMessage";
  
      const PACKAGE: &'static str = "hello.world.v1";
  
      fn full_name() -> Cow<'static, str> {
          Cow::Borrowed("hello.world.v1.MyMessage")
      }
  
      fn type_url() -> Cow<'static, str> {
          Cow::Borrowed("/hello.world.v1.MyMessage")
      }
  }

  I understand this is a breaking change, but using Cow is extremely helpful for avoiding unnecessary allocations. Moreover, since Cow implements Deref and other useful traits, the migration should be mostly seamless (take this with a grain of salt 🧂).

• &'static str
  A dream...
  Always return a string literal.
  The concatenation of NAME and PACKAGE constants can be done using the const_format crate or an ad-hoc solution.
  Unfortunately, this solution cannot be used at the moment due to the following error: “can't use Self from outer item a const is a separate item from the item that contains it” and other const trait features not been stabilized yet.

  pub trait Name: Message {
      const NAME: &'static str;
  
      const PACKAGE: &'static str;
  
      fn full_name() -> &'static str {
          if Self::PACKAGE.is_empty() {
              Self::NAME
          } else {
              const_format::formatcp!("{}.{}", Self::PACKAGE, Self::NAME)
          }
      }
  
      fn type_url() -> &'static str {
          const_format::formatcp!("/{}", Self::full_name())
      }
  }