Function derive_more::display::display_as_helper_struct

fn display_as_helper_struct() -> TokenStream

Create a helper struct that is required by some Display impls.

The struct is necessary in cases where Display is derived for an enum with an outer #[display(fmt = "...")] attribute and if that outer format-string contains a single placeholder. In that case, we have to format twice:

• we need to format each variant according to its own, optional format-string,
• we then need to insert this formatted variant into the outer format-string.

This helper struct solves this as follows:

• formatting the whole object inserts the helper struct into the outer format string,
• upon being formatted, the helper struct calls an inner closure to produce its formatted result,
• the closure in turn uses the inner, optional format-string to produce its result. If there is no inner format-string, it falls back to plain $trait::fmt().