According to the reference manual RM0091 Rev 10 "Table 30. DMA requests for each channel on STM32F07x devices", Note 2:

DMA request mapped on this DMA channel only if the corresponding remapping bit is set in SYSCFG configuration register 1 (SYSCFG_CFGR1).

This bit is not set. After setting SYSCFG_CFGR1.I2C1_DMA_RMP = 1 it works.

Tested with embassy-stm32 0.1.0.

EDIT: This seems to be an issue for all STM32 devices which need remapping bits set or cleared in SYSCFG_CFGR1 to work with specific DMA channels. This affects at least STM32F0 and STM32F3.

I made this quick workaround for STM32F072 in my code:

const SYSCFG_CFGR1: *mut u32 = 0x40010000 as *mut u32; // address from datasheet Table 12 "Peripheral register boundary addresses"
...
unsafe {
    // remap I2C1 DMA to channel 6 and 7
    SYSCFG_CFGR1.write_volatile(SYSCFG_CFGR1.read_volatile() | 1 << 27);
}