impl fmt::Write for BufWriter

[lolbinarycat]

Proposal

Problem statement

If you have an implementor of io::Write and a function that accepts an argument of type fmt::Write, it is difficult to bridge the gap.

Currently, the only way to do it is going through a formatting trait like fmt::Debug, or via an intermediate String.

Motivating examples or use cases

A generic numeric type that has existing blanket implementations of fmt::Debug and fmt::Display, but wishes to provide an additional formatting option only when certain additional generic constraints are met.

Solution sketch

impl fmt::Write for io::BufWriter { ... }

Alternatives

• move the write_fmt method into a common supertrait.
• a seperate dedicated wrapper type that takes an implementor of io::Write and implements fmt::Write. This wrapper struct could potentially flush the internal writer on drop.
• an as_fmt_writer method on io::Writer
• add some other way to construct fmt::Formatter, possibly involving passing a closure.

Downsides

• may cause trait method resolution problems
• does not solve the problem for no_std users
• flushing must be done by the consumer

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

[programmerjake]

fmt::Write is problematic because it can't properly keep error information by itself, since the error type is a ZST.

I think it would be better to have a method like (the equivalent method could be added to fmt::Write too):

// in std::io
pub trait Write {
    // ...
    fn fmt_write_with<F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result>(&mut self, f: F) -> Result<()>
    where
        Self: Sized,
    {
        ...
    }
}