I was looking for the same feature, especially I'd like to trace the invokation of implicit rules, it would be really usueful to see the implicit rule invoked and the file where it is defined.

As an use case I'm currently debugging this (make -d):

         Trying implicit prerequisite `assert.cxx'.
         Found an implicit rule for `/home/stefano/src/reilabs/flash2sip/contrib/src/ptlib_2.2.1/lib/obj_linux_x86_r/assert.o'.
           File `/home/stefano/include/ptlib.h' does not exist.
           Looking for an implicit rule for `/home/stefano/include/ptlib.h'.
           Trying pattern rule with stem `ptlib.h'.
           Trying implicit prerequisite `/home/stefano/include/ptlib.h,v'.
           Trying pattern rule with stem `ptlib.h'.
           Trying implicit prerequisite `/home/stefano/include/RCS/ptlib.h,v'.
           Trying pattern rule with stem `ptlib.h'.
           Trying implicit prerequisite `/home/stefano/include/RCS/ptlib.h'.
           Trying pattern rule with stem `ptlib.h'.
           Trying implicit prerequisite `/home/stefano/include/s.ptlib.h'.
           Trying pattern rule with stem `ptlib.h'.
           Trying implicit prerequisite `/home/stefano/include/SCCS/s.ptlib.h'.
           No implicit rule found for `/home/stefano/include/ptlib.h'.
          Must remake target `/home/stefano/include/ptlib.h'.
make[2]: Entering directory `/home/stefano/src/reilabs/flash2sip/contrib/src/ptlib_2.2.1/src/ptlib/unix'
make[2]: * No rule to make target `/home/stefano/include/ptlib.h', needed by `/home/stefano/src/reilabs/flash2sip/contrib/src/ptlib_2.2.1/lib/obj_linux_x86_r/assert.o'.  Stop.

and I don't know where to hit my head.

It doesn't tell which the implicit rule is and where the rule is defined, so it isn't that useful, with that information I wouldn't have to figure out which is the rule applied of all the possible implicit rules.

Regards and thanks for your attention.

You may try looking at http://lists.gnu.org/archive/html/help-make/2006-10/msg00033.html.
Using the -P (preprocessing) feature shows partial expansions of $(eval) invocations.

It usually takes a lot of time to debug makefiles written by others. This is quite frequent when one downloads a program, tries to build it and gets errors. Perhaps its something small, but tracing what variable or rule is ultimately responsible for a compilation error is unnecessarily difficult. Because makefiles can invoke or include other makefiles.

So here I am proposing several flags to make that task more easy.

First: a flag to trace rule invocation.

make --debug=r

when a rule is invoked, the filename of the Makefile, the line number of the rule, and the name of the target are printed.

For instance:

Invoking rule at Makefile:83 for target 'clean'

Sometimes it might be difficult to know in what directory is the makefile. For these cases the flag "d" (directory) is proposed.

For instance, the hacker Miguel is compiling the package guess-lottery which guesses lottery results. The package has been untarred in directory /home/miguel/guess-lottery/

make --debug=r,d

When the package is compiling invoke-god.c in directory src/engine, it will print

>  Invoking rule at /home/miguel/guess-lottery/src/engine/Makefile:83 for target 'invoke-god.o'

In case the makefile is defined in file rules.mak in directory rules of the package, make would print something like:

> Invoking rule at /home/miguel/guess-lottery/rules/rules.mak:41 included from /home/miguel/guess-lottery/src/engine/Makefile:10 for target 'invoke-god.o'

Sometimes make uses implicit rules. For these cases, it should print something enough to identify the rule. Using the previous example:

> Invoking builting implicit rule for "%c: %o" for target invoke-god.o

Another additional source of problems is assignment of variables.  Even if one knows the rule that is miscompling something, it might be because of compilation flags set in a variable. But where is that variable defined? Since makefile variables are inherited across makefile invocation, and makefiles can include other makefiles, the problem can be messy.

Therefore, an additional debugging flag is proposed for tracing variable invocations. Unfortunately, the debugging flag "v" is already used for "verbose". So the letter "p" (parameter) is proposed.

The proposed behaviour is that make prints all variable assignments when this flag is set. In addition, the invoking makefile and line number are printed as well. For example:

setting CFLAGS to "-Wall -fguess-bugs" (compiler.mak:34 included from rules.mak:2 included from Makefile:10)

make debug