error-stack: implementing conditional fmt::Display behaviour based on ColorMode for an attachment

[mkatychev]

I am adding an attachment called Type to my error reporting library:

/// wrapper attachment that is used to refer to the type of an object
/// rather than the value
pub struct Type(&'static str);

impl Type {
    // const fn when `std::any::type_name` is const fn in stable
    pub fn of<T>() -> Self {
        Self(std::any::type_name::<T>())
    }
}

impl std::fmt::Display for Type {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "<{}>", self.0)
    }
}

impl std::fmt::Debug for Type {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_tuple("Type").field(&self.0).finish()
    }
}

I would like to evaluate the colour mode for each attachment that I display:

const GREEN_LT: &'static str = '<'.green();
// const ...
impl std::fmt::Display for Type {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match somehow_get_color_mode() {
            ColorMode::None => write!(f, "<{}>", self.0),
            ColorMode::Color => write!(f, "{GREEN_LT}{}{GREEN_GT}"}, self.0),
            ColorMode::Emphasis => write!(f, "{BOLD_LT}{}{BOLD_GT}", self.0),
        }
    }
}

The suggested approach for dealing with conditional formatting is use HookContext data from within Report::install_debug_hook::<Type>, the issue here is that it requires an explicit invocation for each attachment type anytime you might use the attachment.

Location, Backtrace and SpanTrace do not need to declare debug hooks since they have a prelude that calls the hooks for them error_stack::fmt::hook::default::install_builtin_hooks:

hash/libs/error-stack/src/fmt/hook.rs

Lines 497 to 507 in 1c930a0

	INSTALL_BUILTIN.call_once(|| {
	INSTALL_BUILTIN_RUNNING.store(true, Ordering::Release);
	Report::install_debug_hook::<Location>(location);
	#[cfg(all(feature = "std", rust_1_65))]
	Report::install_debug_hook::<Backtrace>(backtrace);
	#[cfg(feature = "spantrace")]
	Report::install_debug_hook::<SpanTrace>(span_trace);
	});

I'm curious if there's a good way to allow attachments to define their own conditional formatting and have them be instantiated into the prelude along with the builtin types.