indexing - Implementing Index trait to return a value that is not a reference -

i have simple struct implement index for, newcomer rust i'm having number of troubles borrow checker. struct pretty simple, i'd have store start , step value, when indexed usize should return start + idx * step:

pub struct mystruct {     pub start: f64,     pub step: f64, } 

my intuition i'd able take signature of index , plug in types:

impl index<usize> mystruct {     type output = f64;      fn index(&self, idx: usize) -> &f64 {         self.start + (idx f64) * self.step     } } 

this gives error mismatched types saying expected type &f64, found type f64. has yet understand how rust's type system works, tried slapping & on expression:

fn index(&self, idx: usize) -> &f64 {     &(self.start + (idx f64) * self.step) } 

this tells me borrowed value not live long enough, maybe needs lifetime variable?

fn index<'a>(&self, idx: usize) -> &'a f64 {     &(self.start + (idx f64) * self.step) } 

the error same, note gives lifetime 'a instead of lifetime #1, guess that's not necessary, @ point feel i'm stuck. i'm confused such simple exercise languages has become difficult implement in rust, since want return computation function happens behind reference. how should go implementing index simple structure value calculated on demand?

the index trait meant return borrowed pointer member of self (e.g. item in vec). signature of index method index trait makes impractical implement have behavior described, you'd have store every value returned index in self , ensure pointers remain valid until mystruct dropped.