`./miri run --dep` seems to be buffering stderr

[RalfJung]

If I add something like this at a place in Miri that gets hit during execution

eprintln!("hi!");
loop {}

and then I use ./miri run --dep, I see no output:

    Finished `test` profile [optimized + debuginfo] target(s) in 0.06s
     Running tests/ui.rs (target/debug/deps/ui-1730b7ed90e55bbf)

Is it possible that ui_test is capturing the entire output of the program, and only then dumping it to stderr all at once at the end? That would be not great for debugging...

Cc @oli-obk

[RalfJung]

Hm actually we do the command spawning in Miri. And we inherit stderr. I'm not sure where that output is ending up...

[RalfJung]

Oh, I figured it out... when building dependencies we actually run the tests/deps crate, and that is what hangs.

[RalfJung]

But there is still something very strange going on: the -Zmiri flags passed to ./miri run are actually forwarded to that miri run invocation that is supposed to just build the dependencies...?

[RalfJung]

Ah, it invokes Miri with all those flags to obtain the cfgs here.