Annotate the lifetime of Visit trait explicitly

[thynson]

Feature Request

Motivation

The current trait definition of Visit, is blocking visitors from being implemented without copying string in safe Rust, the tracing-log contains an example for it, see https://github.com/tokio-rs/tracing/blob/master/tracing-log/src/lib.rs#L525C1-L540C6

Proposal

Fixing this issue, unfortunately, requires breaking change.

To make it possible to achieve the goal, the definition of the trait must ensure the visitor itself cannot outlives fields.

As the result, the lifetime of functions in Visit from tracing-core should be explicit annotated, taking record_str as example,

pub trait Visit {
    //...
    fn record_str(&mut self, field: &Field, value: &str);
}

should be modified to

pub trait Visit<'a> {
    //...
    fn record_str<'f>(&mut self, field: &'f Field, value: &'f str) where 'f: 'a;
}

Here is a snippets to prove that it should work.

https://godbolt.org/z/6s8P6Tv14