If you can have "one mutable" and "more than one non-mutable" reference (not sure if this is the case), you still cannot safely do that optimization, as foo() a

If you can have "one mutable" and "more than one non-mutable" reference (not sure if this is the case), you still cannot safely do that optimization, as foo() and bar() could in principle be reading f.a and the optimized version would then not have the correct values of f.a when foo() and bar() are called. Note, I genuinely don't know if this caveat applies to Rust. But, m general, not only mutable references need to be considered.