Spirit of the law? It's in the 1040 instructions:

What's New
Economic impact payments—EIP 1 and EIP 2. Any economic impact payments you received are not taxable for federal income tax purposes, but they reduce your recovery rebate credit.
Recovery rebate credit. This credit is figured like last year's economic impact payment, except eligibility and the amount of the credit are based on your tax year 2020 information. See the instructions for line 30 and the Recovery Rebate Credit Worksheet to figure your credit amount.