Minor FileCheck proposal: CHECK-UNIQUE for labels to improve error messages

Hi,

Apologies if this has been proposed before; I couldn't find anything
in basic searching.

I've been writing tests lately and noticed that the error messages are
not very helpful in cases where a check is incorrect but matches
something that occurs in a later block: the checker continues assuming
that the matched line is correct (no matter how much farther ahead it
occurs) and then provides a misleading error message some time later.

The problem is exacerbated if many similar tests are put in one file,
since many checks can end up succeeding with unintended matches before
something ends up failing.

I propose a simple addition to FileCheck syntax to improve error
messages in this case: a CHECK-UNIQUE directive which indicates that a
given check is a unique match that should only occur once (and only
once) in a given file and can only be matched with the given
CHECK-UNIQUE; this would be used for labels or any other unique
identifier lines.

The CHECK-UNIQUE lines can then allow a better error message to be
produced, by basically treating each section between CHECK-UNIQUE
lines separately, so the actual line causing the error can be
pinpointed more accurately.

Anyone else thing this is a good idea? I'm happy to work on this if so.

-Stephen

I like this idea. There are many places in the test cases where this idiom
is needed and currently CHECK followed by a CHECK-NOT is used, which is
cumbersome.

Eli