Macro slog_extlog::impl_value_wrapper

macro_rules! impl_value_wrapper {
    ($new:ident, ?($inner:tt)*) => { ... };
    ($new:ident, ($inner:tt)*) => { ... };
}

Generate a slog::Value trait implementation for a type we don't own but which implements std::fmt::Display or std::fmt::Debug.

This allows using types from third-party crates as values in external logs. The macro defines a new wrapper type to be used in external logs, which does implement slog::Value.

For example, if you want to use the (fictional) foo crate and log errors from it, then write:

#[macro_use]
extern crate slog_extlog;
extern crate foo;

impl_value_wrapper!(FooError, foo::Error);

Then anywhere you want to log a foo::Error, you can instead use FooError(foo::Error) - the logged value will use foo:Error's impl of Display.

For a type which implements Debug but not Display, then you can use a ? character:

impl_value_wrapper!(FooInternal, ?foo::InternalType);

Note: this macro can be deprecated once default impl is available and this issue is fixed.