use field_with to print "<locked>" and "<uninit>" in Debug

[joboet]

This avoids having to create a new Arguments just for printing a string.

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[hanna-kruppe]

If the motivation here is reducing code size, note that field_with consist of quite a lot of code (also for DebugStruct) that'll get monomorphized for each distinct closure type.

A possible alternative is d.field($(name,)? &fmt::from_fn(|f| f.write_str(...))). Although this will have a separate Debug vtable for each call site, so it's not obvious to me that it's better than Arguments construction for code size. The most economic but most annoying option would probably be to have a single monomorphic type that holds a string and implements Debug as f.write_str(self.the_string).

[joboet]

Right, fair enough!