This one isn’t about your code. Turf reached a state that its earlier checks should have made impossible – a type that was never resolved, a field lookup on something that isn’t a struct – and stopped, rather than guess and risk producing a program that does the wrong thing.
It is a bug in the compiler, not a mistake you made, and it is worth reporting.
What helps most when you report it:
- the smallest piece of code that still prints this message (cut lines until it stops, then put the last one back),
- the full text of the error, including the line and column,
- the output of
turf --version.
In the meantime a small change to the code around the line it points to – naming an intermediate value with mut, adding a type annotation – often steps around it.
Run turf --explain E121 in a terminal to read this again.