[PATCH, RFC] plugin to warn about surplus includes

[Bernhard Reutner-Fischer]

Hi,

A couple of days ago i was asking:
does any frontend support some sort of noting that a user-header was
included but no decl of it was ever referenced (i.e. superfluous
#include) ?

I could not find an appropriate tool to diagnose them, so i was thinking
about a gcc plugin to do that.

Would some plugin like the attached be suitable to put into svn if
cleaned up a bit?
If so, where would they live?

Observations while thinking about all this:
- libcpp does not record LC_PSEUDO_ENTER (or something like that),
  so pristine libcpp cannot detect duplicate includes, like iostream
  in attached sample input.cc
- should figure out how to print a help-text for the plugin
  Hints?
- should handle structs.

Thanks in advance for hints on the help-text printing or comments about
the overall idea of such a facility.
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GCC_SRC_DIR ?= /scratch/src/gcc-4.6.orig
GCC_OBJ_DIR ?= /scratch/obj.$(shell uname -m)/gcc-4.6.orig
CC = gcc-4.6.orig-HEAD

SRC_INCS := gcc libcpp/include include
OBJ_INCS := gcc
CPPFLAGS += $(patsubst %,-I$(GCC_SRC_DIR)/%,$(SRC_INCS)) \
		$(patsubst %,-I$(GCC_OBJ_DIR)/%,$(OBJ_INCS))
CFLAGS += -fPIC

warn_include.so: warn_include.c
	$(LINK.c) -shared -o $@ $^

.PHONY: clean
clean:
	rm -f *.o *.so

help:
	@echo gcc-4.6.orig-HEAD -fplugin=./warn_include.so -fplugin-arg-warn_include-missing -fplugin-arg-warn_include-surplus=0 -c -o /dev/null input.cc