Support simple output format

[spenserblack]

First, thank you for making this crate!

I made a simple GitHub Action so that I could use this during workflows. One of the things I've noticed is that there doesn't seem to be simple output, but either a "pretty" output for humans, or multiple JSON outputs.

For the purposes of writing shell scripts to check the MSRV, I think it would be even beneficial to have an output format that does nothing but:

• Prints the version and exits 0
• Prints an error like "could not find version" to STDERR and exits 1

This would remove the need for a tool like jq to parse the output, and shell scripts that use cargo msrv could be simplified to something vaguely like

# obviously something more useful would happen than just cat to STDOUT :)
cargo msrv > version 2> error || cat error && cat version

[foresterre]

Sounds plausible! I'll see what I can do :)

[foresterre]

I'm currently working on implementing a new way to report output to the user. I hope to complete that process this or next week. After that process is complete, I'll add this, and then we're about ready for a new release as well.

[foresterre]

Hey @spenserblack, I have been working on this feature request. Currently, I'm printing all output to stderr*, but looking at your example of a shell script, I think it would be favourable for your use case to print a success to stdout and a failure to stderr. What do you think?

* Or to any W: Write, but it's limited to a single writable sink
** Using W: Write (or SW: Write, FW: Write) is favourable over Stdio and Stderr mostly for unit testing, where I can instead output to a byte buffer directly

[spenserblack]

I think it would be favourable for your use case to print a success to stdout and a failure to stderr.

Yes, that's exactly what I was thinking 😃
Besides my use case specifically, I think for scripting in general it's useful for expected outputs to go to stdout and unexpected (failures) to go to stderr. The only common exceptions I can think of right now are

• CLI help messages are often printed to stderr, even though cli-tool --help would be the correct usage
• When the tool is intended to have a machine-readable output, human-readable logs might all go to stderr, even success logs

[foresterre]

In #431 (merged), I added the minimal output mode, accessible via cargo msrv --output-format minimal.

Running your example script gives:

A successful run:

$ cargo msrv --output-format minimal > version 2> error || cat error && cat version
1.56.1

A failed run (i.e. no compatible version found):

$ cargo msrv --output-format minimal > version 2> error || cat error && cat version
none

Additional documentation can be found in the book here.

If you find a bug, or like to have this implementation changed (e.g. have more details about what went wrong, I would be happy to implement this as well).

[spenserblack]

This is awesome, thanks!