write! and the like should probably require a Writer implementation

[SiegeLord]

Right now it just calls a method named write_fmt. This duck typing might lead to confusing error messages, and contradicts the documentation of the macro.

[alexcrichton]

Unfortunately this isn't currently possible (we tried to do this awhile ago). There are two contexts in which the write! macro is primarily called:

1. When using io::Writer
2. When using fmt::Formatter or fmt::Writer

Unfortunately these two cases call different write_fmt methods (different traits), so this will not necessarily be easy to resolve.

[alexcrichton]

Closing due to my previous comment, and also that this is stable now.