Support for format args capture

[dtolnay]

Required in order for rust-lang/rust#90473 to be usable inside the macro argument.

let var = ...;
let error = anyhow!("interpolate {var}");  // equivalent to anyhow!("interpolate {var}", var=var)

[m-ou-se]

Using as_str().is_none() like this might become a bit of an issue in the future: It would subtly break when things like rust-lang/rust#78356 get implemented. That issue is about making things like format_args!("hello {}", "world") return Some("hello world") from as_str() by evaluating as much of a format string as possible at compile time. If we do more advanced const/inlining stuff, format_args!("hello {SOME_CONST}").as_str() might also return Some("hello world"). But then anyhow!("hello {SOME_CONST}") would use the message "hello {SOME_CONST}" instead of "hello world", since it uses the original string literal instead of the result from as_str().

(And right now it also means that anyhow!("{") is an error, but escaping it like anyhow!("{{") produces two braces instead of one.)

Also thanks for making and maintaining this crate! It's great.

[dtolnay]

Good call — fixed in #184 + #185.