write! breakage in newest nightly

[sfackler]

Compiling rust-postgres/rust-postgres@1fa5941 comes up with this on the current nightly, but not yesterday's:

<std macros>:4:35: 4:50 error: type `&mut [_]` does not implement any method in scope named `write_fmt`
<std macros>:4         format_args!(|args| { dst.write_fmt(args) }, $($arg)*)
                                                 ^~~~~~~~~~~~~~~
<std macros>:4:35: 4:50 error: type `&mut [u8]` does not implement any method in scope named `write_fmt`
<std macros>:4         format_args!(|args| { dst.write_fmt(args) }, $($arg)*)
                                                 ^~~~~~~~~~~~~~~
<std macros>:4:35: 4:50 error: type `&mut [_]` does not implement any method in scope named `write_fmt`
<std macros>:4         format_args!(|args| { dst.write_fmt(args) }, $($arg)*)
                                                 ^~~~~~~~~~~~~~~
<std macros>:4:35: 4:50 error: type `&mut [u8]` does not implement any method in scope named `write_fmt`
<std macros>:4         format_args!(|args| { dst.write_fmt(args) }, $($arg)*)
                                                 ^~~~~~~~~~~~~~~
error: aborting due to 4 previous errors

It's also a bit weird/concerning that there are no expansion traces.

[sfackler]

After building the --pretty expanded version, the write! calls in this function are the ones generating the errors: https://github.com/sfackler/rust-postgres/blob/1fa5941562739104d4a5fc39826f6ab7d27eb210/src/lib.rs#L518-L543

[japaric]

Just a hunch but this may be related to #19153 and #19142. Namely that write!(f) ICEs, and must be change to write!(&mut f), probably in your case f has type &mut [_]

[alexcrichton]

What may be happening here is that you're calling write!(vec, ...) when it should be write!(&mut vec, ..) which could mean that the compiler is deref'ing vec to a slice and not finding the write_fmt method.

[sfackler]

Yep, that appears to have fixed it. Is the lack of expansion traces a known issue as well?

[sfackler]

@alexcrichton This line in write! appears to be the culprit. Why does it exist?

let dst = &mut *$dst;

[alexcrichton]

The purpose of the write! macro was to take a &mut Writer object at the front as opposed just any plain old Writer. It emphasizes that the macro is not consuming the sink and explains why it must be declared mutable locally.