Function handle_reservoir_remaining_gas 

pub const fn handle_reservoir_remaining_gas(
    instruction_result: InstructionResult,
    parent_gas: &mut Gas,
    child_gas: &mut Gas,
)

Settles a returning child frame’s gas and merges it into the parent (EIP-8037 reservoir model).

First the child settles its own gas: a failing frame (revert or halt) rolls its state-gas charges back in last-in-first-out order ([Gas::rollback_state_gas]) — crediting the spilled portion back to its remaining and restoring the reservoir to the value it inherited — and drops its execution refund counter; an exceptional halt additionally consumes the child’s regular gas.

Then the parent merges the settled child:

• unused regular gas (remaining, including any spill returned on revert) flows back to the parent on success or revert; a halt consumes it.
• the reservoir, a shared state-gas pool the child inherited at call time, is always adopted from the child (restored to the inherited value on revert/halt).
• net state gas, its spilled portion, and the refund counter persist only on success; on revert/halt the child’s state changes roll back and contribute nothing.