Option to have silent failure

[SiegeLord]

As far as I can tell, any call to fail!() prints a diagnostic message to stderr. This is less than ideal for processes which would like to have tight control over the contents of stderr. There should be some task-local option to enable silent failure. Some possible ways this option might be implemented:

1. Having an option for this in the TaskBuilder.
2. Having a function to set it direction in the calling task. This is more convenient, but you might worry about a rogue library turning it on after you turned it off.
3. Instead of disabling it, perhaps have an option to set a special failure writer instead of stderr.

In particular, I think that task::try() should fail silently as in principle the failure message is returned by that function.

[alexcrichton]

The set_stderr function as well as the stderr builder method are intended for redirection, would those work for you?

[SiegeLord]

The use case here is to be able to use stderr for non-failure diagnostics. Unless I'm misunderstanding that function, it will affect the return value of std::io::stderr, which I imagine is what you'd use when you want to write to stderr. I suppose I could set the task stderr to be some null writer, and use the system stderr manually (perhaps storing it in a task-local variable for convenience). Is this the pattern you're suggesting? It seems to require a bit of cooperation between 3rd party libraries, but I guess it's not too horrible.

[alexcrichton]

The set_stderr function only applies to the failure message, the stderr function unconditionally opens a handle to file descriptor 2.

[SiegeLord]

Ok, so I guess this is merely a documentation/pebcak issue. I see now, upon reading it closely, that it says 'of the current task/of the current process' but perhaps this should be reflected in the function name (i.e. it should be called set_task_stderr). It's also not entirely clear how you'd obtain the task-local stderr once you set it.

Those are separate issues however, so I'll close this.