[LLVMdev] lit: deprecating trailing \ in RUN lines

[Alp Toker]

I'd like to propose deprecating and shortly thereafter removing the lit 
test runner feature that concatenates RUN lines ending in a trailing \

*Rationale:**
*

  * Trailing \ has a special meaning in various language standards that
    we support nowadays. In the C preprocessor, for example, it's
    handled _before_ comments. Various compilers handle this differently
    and it introduces needless incompatibilities.
  * Forgetting the trailing \ between two RUN lines causes the lines to
    be run individually. People have checked in tests which they
    believed were getting run whereas the features being tested were
    actually silently broken. I've been committing fixes for some of
    these but it's exceptionally time-consuming to hunt them down after
    the fact.
  * Removing trailing \ will introduce the neat property that one RUN
    line corresponds precisely to one command that's executed. This is
    good for humans and will enable simplifications in the test runner.
  * Eliminating the trailing \ syntax will unblock work on improved
    failure source locations in lit. Right now, when the builders
    encounter a test RUN failure it's a matter of guesswork as to which
    RUN line is failing, and the cycle of commit-fix-and-watch-buildbots
    is laborious. We've all wasted time with this at some point and can
    totally do better.


*Considerations:*

  * Some run lines will now go over 80 columns. I suggest living with it
    as a fact of life, or simplifying the lines on a case by case basis
    -- exceptionally long RUN lines and pipe sequences can often be
    replaced with a more idiomatic sequence of commands that would help
    isolate test failures more rapidly.
  * There are external projects relying on RUN lines with a trailing \.
    It'll be difficult to fix them all in one go.


For these reasons, I propose fixing the core LLVM modules to begin and 
diagnosing with a lit warning for everyone else. That would be 
sufficient to switch on the enhanced lit diagnostics* with line 
information on the llvm.org builders which is where most time is wasted 
today.

* Separate patch, will post after this one is resolved.

Alp.

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