Use stderr for watch output

[tmat]

Fixes #50768

TODO:

• Currently tests merge both stdout and stderr into a single stream of lines. Updating all tests to distinguish between app output and watch output is desirable but a lot of effort. Will leave that for later: Split application and watch output testing #50873

[baronfel]

We're probably going to need a breaking change notice/Tactics for this for 10 - watch is less load-bearing from a scripting perspective than the other CLI commands but it's still a major change in behavior.

[tmat]

@baronfel The change redirects all output from watch to stderr. However, dotnet watch invokes dotnet run to run the app and the run command does output strings to stdout, e.g. here:

https://github.com/dotnet/sdk/blob/main/src/Cli/dotnet/Commands/Run/RunCommand.cs#L222

[baronfel]

Yep - I don't think we can change run entirely to change to stderr this late in the cycle though. It's one of the first things we were going to change for 11.

cc @marcpopMSFT

[marcpopMSFT]

People rely on the run output being predictable. I think it's too late for run and I'd do that early in 11 and see if we got feedback.

[jaredpar]

@baronfel is anything othre than reviews blocking this?

[baronfel]

@jaredpar breaking change notice emails/docs?

[jaredpar]

@baronfel who is intended to do this work: @tmat, you, etc ...? Getting late into .NET 10 so want to make sure that we understand responsibilities.

[baronfel]

It's on me - I've finally cleared out some other blocking work.

[baronfel]

Doc issue written at dotnet/docs#48628.