Thank you all for the valuable hints and explanations. Starting electronics with rust may not be the easiest path, but it is a fun one... My project that I worked on yesterday was to get the display st7735 working - and because I sometimes was not sure whether the controller is in DFU mode (I already ordered an st-link) I thought I just turn on the LED that way I see... Whenever I connected an LED manually I connected it active high... that's why I never thought about connecting it active low… but hey, in the end I managed to get the display to work which is really great! And I learned so much just by connecting a display :) And rust did a very good job at guiding me. One issue that rust was not able to catch is an issue with delay. 
Initially I used ExclusiveDevice::new(NoDelay) which did not work.
```rust
    let mut disp = {
        let spi_device = embedded_hal_bus::spi::ExclusiveDevice::new_no_delay(spi, cs);

        st7735_lcd::ST7735::new(spi_device, dc, rst, true, false, 160, 128)
    };
```
later when initializing I need an actual delay:
```rust
    disp.init(&mut delay).unwrap();
```
I still need to read and learn about the different or are they the same `Delay` structs... each crate seems to provide one...