Document that you shouldn't explicitly set --target to the default

[lilith]

Using a --target value that matches rustup show causes a range of bugs.

1. Two different crates with name 'X' error only when using --target=  #3665
2. MSVC: Spurious PDB error on the bots rust#33145 (comment)

cargo 0.18.0-nightly (5db6d64 2017-03-03)
rustc 1.17.0-nightly (b1e31766d 2017-03-03)

Both target/$PROFILE/ and target/$TARGET/$PROFILE folders are populated with binaries, nearly all of which are duplicates.

Issues seem to be more common in projects with a sufficiently large dependency graph. And when compiling for windows targets.

[alexcrichton]

I... is there a bug here? Sorry I'm not really following what would be necessary to close this issue...

[lilith]

There's a theme; it's possible the two bugs have a shared root cause. Ideas as to where to look would be helpful.

I would also suggest warning users away from --target=[default] in the meantime.

[alexcrichton]

Ah ok, would you be ok if I closed this issue in favor of those two then? This issue may not itself facilitate the fix much and we can otherwise continue discussion of the various bugs in those respective places.

[lilith]

I would suggest documentation that using --target with the default target is not tested/supported yet.

[dwijnand]

Is that still worth documenting, (almost) 2 years in?