io::Error::new(_, string literal) allocates Box<Box<String>> and copies the string

[m-ou-se]

It's very common to use io::Error::new(kind, "some message"), including in many places in std. However, that will create a io::Error containing a Repr::Custom containing a Box<Custom> containing a Box<dyn Error + ..> containing a StringError containing a String containing a copy of the message. 🙃

Three allocations to just return an error message which is in some cases not even more helpful than just the ErrorKind, is not great. We should find a way to improve this.

[jyn514]

I think this is a duplicate of #82812 (or vice versa).

[m-ou-se]

The usages in std have been fixed by #83353

However, we still need to find a way to improve this for io::Errors created in other crates. We can't fix io::Error::new as explained here, so that'd have to be some new function or type, unfortunately. :(

[joshtriplett]

Should we consider providing a stable io::Error::new_const, or something similar to it?

[m-ou-se]

It'd have to be const fn new_const<const MSG: &str>() or const fn new_const(msg: &'static &'static str), so that's not very convenient.

[joboet]

This API now exists in the form of the io::const_error! macro (tracking issue).