build: build the test objects through the rules - #7253

mrbtest.o, assert.o and the gem_test.o of every gem, the objects
mrbtest is made of, are declared by hand in
mrbgems/mruby-test/mrbgem.rake: file tasks without an action, with the
generated source as their only prerequisite. Rake finds the rule for such
a task, but only when the task runs, after it has already answered
needed? from that one file. The .d the compile writes and the flags
record beside the object are read by the prerequisite procs of the rule,
and for these objects those never ran. A header the object includes
changing, or a define reaching the build by a way that leaves the
generated source alone, left the old object in mrbtest. It is the same
gap #7250 closed for mrbgems/gem_init.o.

mrbtest.c reads mrb->gc.arena_idx around every gem's tests
(mrb_gc_arena_save, mrb_gc_arena_restore). Add a field at the top of
struct mrb_state and run rake test:

$ rake test                   # built once, all green
$ sed -i 's/^  struct mrb_jmpbuf \*jmp;$/&\n  void *probe_pad;/' include/mruby.h
$ rake test                   # 185 CC, none of the 52 test objects among them
mrbtest - Embeddable Ruby Test

.rake aborted!
Command failed with status (1): [build/host/bin/mrbtest]
$ rake test                   # nothing to build, the same failure

Every other object of the build is compiled again; mrbtest.o keeps the
old offset of gc.arena_idx, and mrbtest exits after the first test.

Why

The three tasks came with the test rakefiles (2014, 7201404; 2015,
c7d0f8e). Rake's Task#execute runs enhance_with_matching_rule for a
task without an action, so the rule's action does compile the object and
its prerequisite procs do run then; but that is after invoke decided
from the written prerequisite whether to execute at all. The rule from
Compiler#define_rules has paired an object under the build directory
with the generated source beside it since 2013 (ced80d2); mrblib.o
is built by it alone, and mrbgems/gem_init.o since #7250. It reads the
.d and discards an output whose recorded flags differ (#7236).

What this does

mrbgems/mruby-test/mrbgem.rake no longer declares the three kinds of
object. The rule of the compiler pairs each with its generated source,
and its .d and flags record take effect. The compile lines do not
change: the flags record of every one of the 271 objects of a rake test
build is the same as on master.

Verified

rake -m -j16 test, CCACHE_DISABLE=1, gcc; default gembox with
enable_bintest and enable_test; the build directory built once on
master (364d6dd) before each row, and the run after each row does
nothing in both columns.

Environment

gcc -MMD -c -std=gnu99 -g -O3 -Wall -Wundef -Werror-implicit-function-declaration -Wwrite-strings -DMRBTEST_COMPILER_PRISM -DMRBGEM_MRUBY_TEST_VERSION=0.0.0 -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DMRB_USE_COMPLEX -DMRB_USE_BIGINT -DMRB_USE_DEBUG_HOOK -I"include" -I"build/host/include" -o "build/host/mrbgems/mruby-test/mrbtest.o" "build/host/mrbgems/mruby-test/mrbtest.c"

No C source changes and the compile lines are the same, so .text is
unaffected in every build.

Summary by CodeRabbit

• Build Improvements
  • Streamlined test build dependency tracking by removing unnecessary generated-file dependencies.
  • Existing code-generation steps remain unchanged.