mruby · matz · Aug 14, 2026 · Aug 14, 2026 · Aug 14, 2026 · Aug 14, 2026
@@ -50,17 +50,20 @@
   conf.enable_cxx_abi
 end
 
-MRuby::Build.new('default') do |conf|
+MRuby::Build.new('byte-string') do |conf|
   conf.toolchain
 
-  # The one build here on the default gembox. It leaves out mruby-encoding,
-  # which is what defines MRB_UTF8_STRING, so its strings index by byte. The
-  # tests written as the byte-indexed mirror of the UTF-8 ones (String#scrub
+  # The one build here whose strings index by byte. mruby-encoding is what
+  # defines MRB_UTF8_STRING, so dropping it is what makes "あ".length 3, and
+  # the tests written as the byte-indexed mirror of the UTF-8 ones (String#scrub
   # degrading to a no-op, the byte-counting halves of mruby-regexp and
   # mruby-string-ext) run nowhere else: every other build in CI, here and in
-  # ci/msvc, takes full-core. Tests only, since the binaries this gembox adds
-  # are the same ones the bintest above already covers.
-  conf.gembox 'default'
+  # ci/msvc, keeps the gem. Taking it out of full-core rather than reaching for
+  # a smaller gembox keeps the rest of the box on the byte-indexed side too.
+  # Tests only, since the binaries full-core adds are the same ones the bintest
+  # above already covers.
+  conf.gembox 'full-core'
+  conf.gems.delete 'mruby-encoding'
 
   conf.enable_test
 end
@@ -285,11 +285,30 @@ conf.gem :core => 'mruby-bin-mirb'
 
 # Integrate GemBox (set of Gems)
 conf.gembox "default"
+
+# ... and take one back out of it
+conf.gems.delete "mruby-socket"
 ```
 
 A GemBox is a set of Gems defined in `mrbgems/default.gembox` for example.
 It's just a set of `mrbgem` configurations.
 
+`conf.gems.delete` removes a Gem the configuration has already added, so a
+build can say "this GemBox, minus one" without restating the box. It has to
+come after the `gembox` line that brought the Gem in, and naming a Gem that
+is not in the build fails, so a misspelled name does not pass for a build
+that quietly keeps the Gem. `conf.gems.reject!` takes a block instead and
+removes every Gem it matches, returning `nil` when it matches none.
+
+A Gem that another Gem in the build declares as a dependency cannot be
+removed this way: dependency resolution loads it again, and reports
+
+```
+gem 'mruby-string-ext' can't be removed; mruby-regexp depends on it
+```
+
+Removing the Gem that depends on it as well is what makes it go.
+
 There is a `RubyGem` (gem for CRuby) named `mgem` that help you to
 manage `mrbgems`. Try `gem install mgem`. `mgem` can show you the list
 of registered `mrbgems`.

@@ -422,6 +422,7 @@ class List
 
       def initialize
         @ary = []
+        @removed = []
       end
 
       def each(&b)
@@ -442,6 +443,30 @@ def <<(gem)
         self
       end
 
+      # Remove the gem named +name+ and return it.  Naming a gem that
+      # is not in this build is a typo, not a no-op, so this fails;
+      # use +reject!+ when a predicate matching nothing is acceptable.
+      #
+      # A gem another gem declares as a dependency comes back during
+      # dependency resolution; setup_dependencies says so when it does.
+      def delete(name)
+        gem = self[name]
+        fail "Can't remove gem '#{name}'; it is not in this build" unless gem
+        @ary.delete(gem)
+        @removed << name
+        gem
+      end
+
+      # Remove every gem for which the block returns true.  Returns
+      # nil when nothing was removed, like Array#reject!.
+      def reject!(&block)
+        gone = @ary.select(&block)
+        return nil if gone.empty?
+        @ary -= gone
+        @removed.concat(gone.map(&:name))
+        self
+      end
+
       def empty?
         @ary.empty?
       end
@@ -492,20 +517,26 @@ def setup_dependencies(build)
         default_gems = {}
         each do |g|
           g.dependencies.each do |dep|
-            default_gems[dep[:gem]] ||= default_gem_params(dep)
+            default_gems[dep[:gem]] ||= default_gem_params(dep).merge(:required_by => g.name)
           end
         end
 
         until default_gems.empty?
           def_name, def_gem = default_gems.shift
           next if gem_table[def_name]
 
+          # The build config asked for this one to go, but a gem that
+          # stayed cannot be built without it.  Say so rather than
+          # letting the removal look like it took.
+          warn "gem '#{def_name}' can't be removed; #{def_gem[:required_by]} depends on it" if
+            @removed.include?(def_name)
+
           spec = gem_table[def_name] = build.gem(def_gem[:default])
           fail "Invalid gem name: #{spec.name} (Expected: #{def_name})" if spec.name != def_name
           spec.setup
 
           spec.dependencies.each do |dep|
-            default_gems[dep[:gem]] ||= default_gem_params(dep)
+            default_gems[dep[:gem]] ||= default_gem_params(dep).merge(:required_by => spec.name)
           end
         end