Upgrade to Rust nightly

[ArtemGr]

Rust-smtp fails to compile on the fresh Rust nightly.
I did a quick upgrade: https://github.com/ArtemGr/rust-smtp/commit/f7e205e637eb31bc295849b561153abda9e5b247
Not creating a pull request because you might want to fix things differently.

[schickling]

I'm just curious: Why are you rewriting every to_string() with the format! macro? Is this considered to be a best practice?

[ArtemGr]

The reason I did it was because to_string() no longer wanted to compile. Maybe it was some kind of glitch and it works again, or maybe it was just removed from Prelude and should have been imported explicitly.

And yes, AFAIK, format! is the way to go.
to_string() is implemented as (a variation of) format! - http://doc.rust-lang.org/src/collections/string.rs.html#937-946

[schickling]

Thanks for that information @ArtemGr

[ArtemGr]

NB, for strings format! is temporarily slower than String::from_str, rust-lang/rust#20400

[ArtemGr]

FYI, an updated patch landed at https://github.com/ArtemGr/rust-smtp/commit/aaf2147117da119fba87986e10ea012d9fe6c74c

[amousset]

Merged, thank you. I am updating the tests.

[ArtemGr]

Cool! )

BTW, I'm not sure if the features vector will be formatted correctly, had to use the {:?} as a quick fix because it wouldn't format with {} for some reason.