Remove need for &format!(...) or &&"" dances in span_label calls

[oli-obk]

These were always a thorn in my eye. Note that this will monomorphize to two impls, one for String and one for &str. But I think that cost is worth the ergonomics at the call sites that can be seen throughout this PR.

[oli-obk]

r? @jonathandturner

[sophiajt]

Thanks! I always had a mental note to do that but then forgot.

[sophiajt]

@bors r+

[bors]

📌 Commit b18117a has been approved by jonathandturner

[bors]

⌛ Testing commit b18117a with merge 0c85b2b...

[bors]

💔 Test failed - status-travis

[sophiajt]

Looking at Travis, seems like a few places might have been missed:

[00:03:00] error[E0277]: the trait bound `std::string::String: std::convert::From<&&str>` is not satisfied
[00:03:00] --> /checkout/src/libsyntax/parse/parser.rs:2711:21
[00:03:00] |
[00:03:00] 2711 | err.span_label(span_of_tilde, &"did you mean `!`?");
[00:03:00] | ^^^^^^^^^^ the trait `std::convert::From<&&str>` is not implemented for `std::string::String`
[00:03:00] |
[00:03:00] = help: the following implementations were found:
[00:03:00] <std::string::String as std::convert::From<&'a str>>
[00:03:00] <std::string::String as std::convert::From<std::boxed::Box<str>>>
[00:03:00] <std::string::String as std::convert::From<std::borrow::Cow<'a, str>>>
[00:03:00] = note: required because of the requirements on the impl of `std::convert::Into<std::string::String>` for `&&str`
[00:03:00] 

[oli-obk]

Nothing missed. It just bitrots very fast since it touches every single span_labelb. Will fix later

[GuillaumeGomez]

Well, since @jonathandturner already r+ed and you didn't change much, I'll just r+ as well.

@bors: r=jonathandturner

[bors]

📌 Commit dd87eab has been approved by jonathandturner

[bors]

⌛ Testing commit dd87eab with merge 198917b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: jonathandturner
Pushing 198917b to master...