Why should I use SendStr (over &str or String) for a message property on an Error struct in Rust?

Question

I found this excellent blog post about error handling in Rust.

It defines an error struct as such:

struct ProgramError {
    kind: ProgramErrorKind,
    message: SendStr
}

For the message it uses SendStr which is an type alias for MaybeOwned<'static> . I understand that this can either hold an &str or an String but the documentation is a bit vague about its use case.

This can be useful as an optimization when an allocation is sometimes needed but not always.

I would like to understand why one would like to choose SendStr over &str or String in the case of defining a ProgramError class.

Answer 1

SendStr is String or &'static str , allowing it to be efficient in a common case of using a string literal (no need to allocate a new value each time), while being able to use a dynamic value if necessary.

Why not use &str all the time? Well, either you'll need to make it static strings only, or the user will need to store the string somewhere else in the case of dynamic values, and so passing an error around (most notably, up the stack) will be more difficult.