[makefile] Define make variable at rule execution time

In my GNUmakefile, I would like to have a rule that uses a temporary directory. For example:

out.tar: TMP := $(shell mktemp -d)
        echo hi $(TMP)/hi.txt
        tar -C $(TMP) cf $@ .
        rm -rf $(TMP)

As written, the above rule creates the temporary directory at the time that the rule is parsed. This means that, even I don't make out.tar all the time, many temporary directories get created. I would like to avoid my /tmp being littered with unused temporary directories.

Is there a way to cause the variable to only be defined when the rule is fired, as opposed to whenever it is defined?

My main thought is to dump the mktemp and tar into a shell script but that seems somewhat unsightly.

Another possibility is to use separate lines to set up Make variables when a rule fires.

For example, here is a makefile with two rules. If a rule fires, it creates a temp dir and sets TMP to the temp dir name.

PHONY = ruleA ruleB display

all: ruleA

ruleA: TMP = $(shell mktemp -d testruleA_XXXX)
ruleA: display

ruleB: TMP = $(shell mktemp -d testruleB_XXXX)
ruleB: display

display:
    echo ${TMP}

Running the code produces the expected result:

$ ls
Makefile
$ make ruleB
echo testruleB_Y4Ow
testruleB_Y4Ow
$ ls
Makefile  testruleB_Y4Ow

A relatively easy way of doing this is to write the entire sequence as a shell script.

out.tar:
   set -e ;\
   TMP=$$(mktemp -d) ;\
   echo hi $$TMP/hi.txt ;\
   tar -C $$TMP cf $@ . ;\
   rm -rf $$TMP ;\

I have consolidated some related tips here: https://stackoverflow.com/a/29085684/86967

I dislike "Don't" answers, but... don't.

make's variables are global and are supposed to be evaluated during makefile's "parsing" stage, not during execution stage.

In this case, as long as the variable local to a single target, follow @nobar's answer and make it a shell variable.

Target-specific variables, too, are considered harmful by other make implementations: kati, Mozilla pymake. Because of them, a target can be built differently depending on if it's built standalone, or as a dependency of a parent target with a target-specific variable. And you won't know which way it was, because you don't know what is already built.