build: rebuild an output another configuration left in the build directory - #7236

A build directory records what was built, not what it was built with, so two
configs that share a build directory and a build name share the objects, and the
second one silently gets the first one's compiler flags.

MRuby::Command::Compiler#all_flags (lib/mruby/build/command.rb:95-101 on
master) assembles the defines at the moment of the call and writes them nowhere
rake will read again. The .d file beside an object records header
dependencies only, and MRUBY_CONFIG is a dependency by path, whose mtime does
not move when another config takes the directory over. Every dependency the
object has says it is up to date, so nothing is compiled, and the binary carries
the defines of whichever config came first.

The name is what brings two configs together: CrossBuild#initialize generates
its minimal build under the name host, which is the name
build_config/default.rb uses too. Two runs of the same build name from
different build configs collide the same way.

Before

The same tree and the same command twice,
rake test MRUBY_CONFIG=build_config/no-float.rb, differing only in what was in
the build directory beforehand: nothing, or a default rake build.

With a directory of its own, the generated host build carries MRB_NO_FLOAT,
and its mrbc refuses the float literal where it is written:

$ MRUBY_BUILD_DIR=/tmp/nofloat-build rake -m test MRUBY_CONFIG=build_config/no-float.rb
...
        (79 compile lines for the generated `host` build)
...
test/t/array.rb:65: Not implemented: PM_FLOAT_NODE
test/t/array.rb:0:0: generator error, Not implemented: PM_FLOAT_NODE
rake aborted!

Over a directory a plain rake has populated, not one compile line for host.
The float-enabled mrbc of the earlier build is reused, writes IREP_TT_FLOAT
pool entries, and src/load.c refuses them on the MRB_NO_FLOAT target: the
pre-#7230 failure, on a tree that carries #7230.

$ rake -m                                          # populates build/host
$ rake test MRUBY_CONFIG=build_config/no-float.rb
...
 Output Directory: build/host                      # the only mention of build/host in the run
...
TEST for no-float
mrbtest - Embeddable Ruby Test

.........trace (most recent call last):
test/t/argumenterror.rb:34: irep load error (ScriptError)
rake aborted!

MRB_NO_FLOAT is the loud case, because the answer changes shape. MRB_INT32 /
MRB_INT64, MRB_GC_FIXED_ARENA and the boxing modes are the quiet ones, and a
measurement taken in a shared directory can report that a merged fix did not
land.

After

Every compile records the command line it ran with, beside its output, the way
the .d file records the headers it read:

$ cat build/host/src/array.o.flags
command: gcc
options: -MMD -c %{flags} -o "%{outfile}" "%{infile}"
flags: -std=gnu99 -g -O3 -Wall ... -DMRB_USE_BIGINT ... -I"/mruby/include"

The record is compared when the rule for that output is resolved. An output the
record does not answer for is removed, so rake builds it again. The shared
directory of the second run above:

$ rake -m                                          # populates build/host
$ rake test MRUBY_CONFIG=build_config/no-float.rb
/mruby/build/host: output here was built by another configuration, rebuilding it
  flags added: -DMRB_NO_FLOAT -DMRB_NO_GEMS
  flags removed: -DMRC_TARGET_MRUBY -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET ...
...
        (79 compile lines for `host`)
...
test/t/array.rb:65: Not implemented: PM_FLOAT_NODE
test/t/array.rb:0:0: generator error, Not implemented: PM_FLOAT_NODE
rake aborted!

Only a compile that succeeded writes a record, so a build that stops halfway
leaves nothing claiming a configuration its output was not built with: what it
did not reach is still the earlier output, still without a record that answers
for it, and the next run removes that too.

Two things this rests on:

• The record is compared only through the rule that finds its source, which is
  the test rake itself applies before it uses a rule. Every compiler of a
  build defines a rule for the object names, differing in the extension of the
  source it looks for, and a compiler that does not build the output would
  otherwise judge it against flags no compile of it ever used.
• tasks/presym.rake now hands every target its generated header directory
  before the walk that resolves rules begins. That walk reaches past the target
  it runs for, since a build's products lead to the objects of the mrbc build
  it generated, and a rule resolved for those objects has to see the include
  paths the compile will use.

What it costs

One directory, the default gembox:

The first rake in a directory built before this change rebuilds it once, there
being no record to compare it against, and says so:
output here has no record of what built it, rebuilding it.

An object built by a file task written by hand rather than by a rule is not
compared, the check hanging off the rule. mrbgems/mruby-compiler is the only
such place in the tree: its Prism objects choose their flags inside the task
body, and they carry no dependency on MRUBY_CONFIG either.

Verification

Under build_config/, each run twice:

The last row is a config for the rules a build can define for one object name:
enable_cxx_exception, so that the cxx compiler defines rules for the object
names too, and two gems from examples/mrbgems, which are outside mrbgems/
and so use all four compilers. Overwriting the record of one core object and one
gem object in that build rebuilds those two and nothing else.

One directory, in this order:

Environment

No C source changes, so .text is unaffected in every build.

🤖 Generated with Claude Code

https://claude.ai/code/session_01ArcdfvwWWVJiJr1sWW4EQf

Summary by CodeRabbit

• Bug Fixes
  • Improved build reliability by detecting when generated outputs use missing or outdated compiler settings.
  • Prevented stale or incompatible build artifacts from being reused.
  • Added clearer warnings showing which configuration fields were added or removed.
  • Improved dependency handling and presymbol processing across build targets and bundled components.