DebugTuple builder needs more options controlling its behaviour

[nagisa]

DebugTuple should have options controlling how to output tuples with 0 and 1 element.

Currently if 0 fields are provided, only the name passed into debug_tuple is printed. Would be nice if there was option to print {name}().

Currently if only 1 field is provided, trailing comma is not printed. There should be an option to add one: (value,).

These could be added as option setters on the DebugTuple builder. E.g. fn empty_tuple(bool) and fn trailing_comma(bool).

That would make debug_tuple actually usable to print tuples.

[nagisa]

cc @tsion

[solson]

This explanation might be redundant, but I see two separate issues:

• A named tuple with no elements prints Name rather than Name(). Admittedly, the latter doesn't currently exist in Rust, but it might be added later like struct Name {} was, and the former Name isn't a tuple struct at all. This might be considered an actual bug.
• An un-named tuple with 0 or 1 elements prints nothing or (elem) (with no trailing comma), respectively. This is basically a feature request to make debug_tuple work for anonymous tuples.

[sfackler]

Tuples already have debug implementations - what external implementations would exist?

[nagisa]

@sfackler we’re trying to use in pretty-printing MIR representation for tuple aggregates. That’s at least one use case.