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## Motivation In `tokio-trace`, field values may be recorded as either a subset of Rust primitive types or as `fmt::Display` and `fmt::Debug` implementations. Currently, `tokio-trace` provides the `field::display` and `field::debug` functions which wrap a type with a type that implements `Value` using the wrapped type's `fmt::Display` or `fmt::Debug` implementation. However, importing and using these functions adds unnecessary boilerplate. In #1081, @jonhoo suggested adding shorthand syntax to the macros, similar to that used by the `slog` crate, as a solution for the wordiness of the current API. ## Solution This branch adds `?` and `%` sigils to field values in the span and event macros, which expand to the `field::debug` and `field::display` wrappers, respectively. The shorthand sigils may be used in any position where the macros take a field value. For example: ```rust trace_span!("foo", my_field = ?something, ...); // shorthand for `debug` info!(foo = %value, bar = false, ...) // shorthand for `display` ``` Adding this shorthand required a fairly large change to how field key-value pairs are handled by the macros --- since `%foo` and `%foo` are not valid Rust expressions, we can no longer match repeated `$ident = $expr` patterns, and must now match field lists as repeated token trees. The inner helper macros for constructing `FieldSet`s and `ValueSet`s have to parse the token trees recursively. This added a decent chunk of complexity, but fortunately we have a large number of compile tests for the macros and I'm quite confident that all existing invocations will still work. Closes #1081 Signed-off-by: Eliza Weisman <eliza@buoyant.io>