Build failure should print the command line that failed

[catamorphism]

Far too often, I've done something like:

$ make check
(wait)
compile_and_link: x86_64-apple-darwin/test/coretest.stage2-x86_64-apple-darwin
*** build error of some sort...

Then I want to run the command that failed by itself, but I don't have it; I can either remember what it is, or run VERBOSE=1 make check and wait. It would be nice if a build failure printed out just the command line that caused the last error. I could run with VERBOSE all the time, but that does get very verbose.

[nikomatsakis]

It turns out that make has a built-in facility for supressing commands (make -s). This facility will printout the command iff it fails. But our makefiles don't seem to be compatible with it.

[msullivan]

It would not be hard to get our makefiles to work properly with make -s, but I'm not sure whether we want to require users to specify it to get the currently default quiet behavior.

There are hacks I am considering that might make this work...

[msullivan]

Oh, although it doesn't look like -s actually prints the command line if it failed...

[msullivan]

One way to do this would be to replace $(Q) with a function that we call. This function would run the command and echo it if it failed. This is an easy but large change, I think.

[emberian]

Seems relevant, but will be obsolete with #5363

[pnkfelix]

Assigning P-low.

[huonw]

part of #8058

[steveklabnik]

Triage: no change

[steveklabnik]

Triage: obsoleted by rustbuild, though rustbuild is not yet in place.

[alexcrichton]

Done!