Why do the SPI/I2C transaction slices have to be mutable?

[Dominaezzz]

In the SPI transaction API and the I2C transaction API a mutable slice of operations is taken but I don't see why this is needed.

See Rust Playground example showing mut isn't necessary.

[Rahix]

Your playground example isn't complete as you are missing the immutable reference. Try putting the for loop into a function which takes &[...] and you'll notice it does not work anymore: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=510670b534ad0a4fb665cd5aa2831362

The reason is that in your example the for loop operates on the owned array of operations ([Operation; 2]) while the function takes a reference to a slice (&[Operation]). In the latter case, even though there are mutable references inside the slice, the outer & prevents mutation of the data.

[Dominaezzz]

Ahhhh I see, thanks for the explanation.