[llvm-dev] RFC: FileEdit utility

[Zachary Turner via llvm-dev]

I've put up D34764 <https://reviews.llvm.org/D34764> for review.  I'll
paste the review summary message below for the purposes of having this
message be self-contained.  Comments welcome, either here or on the review
thread.

-----------


This introduces a new utility, which I'm calling FileEdit, which is
intended to increase portability in tests across platforms and shells,
while adding some additional functionality that enables us to write more
concise, easily-understandable, and easily-creatable tests.

To elaborate on the first point, a (somewhat) common pattern in tests is to
run sed against the test file, replacing certain fixed strings in the check
statements with values that are known only during the run line. For
example, you might compile something and get some debug info with a path
relative to the test output directory, and you want to write a check
statement that checks the path is correct. Or you might want to run the
same command multiple times where only the value of a single command line
argument is changed, which would cause some small sectino of the output to
be different but the test to otherwise be identical. In these and other
cases, you may wish to use the same CHECK prefix with a replacement
variable.

sed solves this, but a) it's needlessly complicated and not everyone is a
Level 99 sed wizard, and b) There are real portability issues when it comes
to escaping and slashes that are not easily solvable.

For the second point, lit tests are convenient to write when only have 1
input file to whatever tool we wish to exercise, because we can inline the
text of the input file within the test file itself, meaning a test is
entirely self contained.

This is not true anymore when a test requires multiple input files (for
example, consider the case where you want to compile 2 .cpp files into
separate object files and link them together. It would be nice we could
have the same level of test-isolation for these kinds of tests as well.

This patch addresses both of the above issues, but one could easily imagine
other use cases for a tool such as this, so it has potential for a larger
feature set.

It addresses the first by having a command line option --replace=<string>,
where <string> is of the form s/pattern/replace/ and behaves in a mostly
intuitive way -- it replaces all occurrences of pattern (quirk: pattern is
not a regex, it is a string literal).

It addresses the second by scanning lines for a directive that identifies
the beginning of a file, and it copies everything that follows (until the
next directive) to the corresponding file. For example,

{!-- Foo.cpp
int main(int argc, char **argv) {
  return 0;
}

would create a file named Foo.cpp with the aforementioned text, which can
then be referenced in a run line.
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