RFC: Merge hints

[Marius Bakke <marius@devup.no>]

Hi,

It would be convenient if git had a facility for adding "merge hints",
both for automatic conflict resolution, as well as giving clues to
persons inheriting or merging a branch.

The main use case here is aiding long-running branches.  Imagine a
feature branch where all uses of FOO_STR is replaced with STR.  For
various reasons this branch cannot be merged to master, yet it needs to
be kept up to date, and new uses of FOO_STR keeps getting added.

If you are lucky, your compiler will complain that FOO_STR is undefined
when running whatever tests you have after a merge.  But perhaps the new
code is for a different platform and not reached during a test run.  A
diligent developer might also run 'git grep FOO_STR' *if* they know
about the change.  Fixing these trivial problems is boring and manual.

A more tricky case is where...

  def foo(a,b)

has become

  def foo(a,b,c)

...and c is a required positional argument.  Any new uses of 'foo' will
need to be adjusted during a merge.  This function can be multi line and
not easily greppable.

The diligent developer is in no shortage of tools: "git log -G foo
..master" will reveal any potential problems, even before the merge.
Yet, they still have to 1) know about the change; and 2) fix up the uses
manually.

It seems to me that git could offer better tools to deal with this.
Perhaps a ".mergehints", where you could provide a regular expression
either for automatic merge resolution, or just have git complain loudly
when a given pattern is merged (say, introduce conflict markers even
though there were no "real" conflicts).  I.e.:

[hint1]
files = *.c
pattern = FOO_STR
transform = s/FOO_STR/STR

[hint2]
files = *.py
pattern = 'foo('
conflict = true

Thoughts?  Are there other tools at the diligent developers disposal
that can aid with these kinds of problems?