Some answers from Discord:

1. stm32l1xx-hal already depends on stm32l1 as seen here. There isn't any need to import it twice. It is enough to add to file Cargo.toml:

   [dependencies.stm32l1xx-hal]
   version = "0.1.0"
   default-features = false
   features = ["stm32l151", "rt"]
   
   

• Note that the default-features = false is optional, but without it the compiler was giving me an error.

2. The syntaxes are equivalent, but as I said above, I only need to add the HAL one. You can add curly braces {} in the first style to add the options, such as:

   stm32l1xx-hal = { version = "0.1.0", features = ["stm32l151", "rt"]}
   
   

3. The right code for doing the blinky (which was on PB6, not PB4) was:

   #![no_main]
   #![no_std]
   
   use panic_halt as _;
   
   use cortex_m_rt::entry;
   use stm32l1xx_hal::delay::Delay;
   use stm32l1xx_hal::gpio::GpioExt;
   use stm32l1xx_hal::hal::digital::v2::OutputPin;
   use stm32l1xx_hal::rcc::{Config, RccExt};
   use stm32l1xx_hal::stm32::Peripherals;
   use stm32l1xx_hal::stm32::CorePeripherals;
   use stm32l1xx_hal::time::MicroSeconds;
   
   
   #[entry]
   fn main() -> ! {
   
       let p = Peripherals::take().unwrap();
       let cp = CorePeripherals::take().unwrap();
   
       // Get LED pin PB6
       let gpiob = p.GPIOB.split();
       let mut led = gpiob.pb6.into_push_pull_output();
   
       // Set up a delay
       let rcc = p.RCC.freeze(Config::default());
       let mut delay = Delay::new(cp.SYST, rcc.clocks);
   
       loop {
   
           // Turn LED On
           led.set_high().unwrap();
   
           delay.delay(MicroSeconds(1_000_000_u32));
   
           // Turn LED Off
           led.set_low().unwrap();
   
           delay.delay(MicroSeconds(1_000_000_u32));
       }
   }
   
   

• For me, the key was to understand that the split method can be called from the Peripherals because stm32l1xx-hal implemented split for a struct defined in stm32l1. In other words, the HAL crate is not only defining new structs, but also extending functionality for existing structs. I need to wrap my head around trait design patterns.