The nil seems wrong because it looks like it basically breaks the ObjectSpace.trace_object_allocations functionality, at least the allocation_class_path and allocation_method_id methods.
Though to be fair if those basically always return "Class" and :new they probably have little value.

Notably the spec no longer works on TruffleRuby because of the changed expectations due to this commit.
TruffleRuby also inlines Class#new, and has logic inside the implementation of Class#new that if allocation tracing is enabled then it records the relevant information directly on the object, while this information is available.
Maybe CRuby could do the same? Or maybe these methods should be deprecated?

What if CRuby would return "Class" for allocation_class_path and :new for allocation_method_id if the information is not there? Then we could revert this commit and avoid changing behavior.
Would it be incorrect? In which case?

@eregon We could probably do that, but to be honest I think we should delete these tests (or update them not to run inside anonymous blocks). As you noted, returning Class and :new is not very helpful information. I think the current behavior is actually more helpful. AFAICT, the reason it's returning nil in this case has something to do with how the test runner is set up (I think because everything is being executed in blocks?)

For example:

require "objspace"

class Foo
  def test
    ObjectSpace.trace_object_allocations do
      o = Object.new
      p ObjectSpace.allocation_class_path(o)
      p ObjectSpace.allocation_method_id(o)
    end
  end
end

Foo.new.test

1.times do
  ObjectSpace.trace_object_allocations do
    o = Object.new
    p ObjectSpace.allocation_class_path(o)
    p ObjectSpace.allocation_method_id(o)
  end
end

Output is this:

make runruby
RUBY_ON_BUG='gdb -x ./.gdbinit -p' ./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems  ./test.rb 
"Foo"
:test
nil
nil

The nil is odd, but there is no class or method name in that case. Also the first example is way more useful than just returning Class and :new.

Good point, we'll refactor those specs to move the trace_object_allocations usages inside named classes and methods and test that as it's more representative of a real usage of the feature.