Evaluator: `transplant_instance_variables` copies Minitest-internal ivars into class-scoped bindings

[jcouball]

Summary

Evaluator#transplant_instance_variables copies every instance variable from the spec instance — including Minitest-internal ones such as @__name__, @assertions, @__io__, and @time — into the class-scoped binding used to evaluate example code. Any example code that reads or writes those variable names gets or overwrites Minitest's own state, producing silent assertion count corruption or subtler test-isolation bugs.

Location

lib/yard_example_test/example/evaluator.rb — transplant_instance_variables

def transplant_instance_variables(ctx)
  @instance_variables.each do |ivar, value|
    local = "__yard_example_test__#{ivar.to_s.delete('@')}"
    ctx.local_variable_set(local, value)
    ctx.eval("#{ivar} = #{local}")
  end
end

@instance_variables is built from instance_variable_hash on the spec instance:

def instance_variable_hash
  instance_variables.to_h do |ivar|
    [ivar, instance_variable_get(ivar)]
  end
end

Minitest::Test sets several instance variables on itself before running each test. instance_variables enumerates all of them, so the snapshot includes at minimum:

Variable	Purpose
@__name__	The test method name
@assertions	Running assertion count
@__io__	I/O buffer for test output
@time	Elapsed time accumulator

Consequences

1. Assertion count corruption — if example code sets @assertions (e.g. a class method called assertions= that happens to write to @assertions in the binding), the counter is silently reset.
2. Unexpected failures — if example code reads @__name__ expecting it to come from user-defined setup, it gets a Minitest method name instead.
3. Any before-hook variable that collides with a Minitest internal name is silently overwritten by the snapshot taken after the hook runs — and then transplanted, potentially shadowing the internal value.

Proposed solution

Filter the snapshot to include only variables that were explicitly set by user-defined before hooks, not the full instance_variables set. One approach: snapshot instance variables before running hooks, then diff:

# in Example (spec instance method)
def instance_variable_hash
  minitest_ivars = Set[:@__name__, :@assertions, :@__io__, :@time]
  instance_variables.reject { |ivar| minitest_ivars.include?(ivar) }.to_h do |ivar|
    [ivar, instance_variable_get(ivar)]
  end
end

Alternatively, snapshot instance_variables before and after calling each before hook and only propagate the difference.

Acceptance criteria

• Minitest-internal instance variables (@__name__, @assertions, etc.) are not transplanted into class-scoped bindings
• Instance variables set by user before hooks are still available in evaluated code
• The assertion counter reported by Minitest is not affected by example evaluation
• All existing cucumber scenarios continue to pass