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// cow1.rs
//
// This exercise explores the Cow, or Clone-On-Write type. Cow is a
// clone-on-write smart pointer. It can enclose and provide immutable access to
// borrowed data, and clone the data lazily when mutation or ownership is
// required. The type is designed to work with general borrowed data via the
// Borrow trait.
//
// This exercise is meant to show you what to expect when passing data to Cow.
// Fix the unit tests by checking for Cow::Owned(_) and Cow::Borrowed(_) at the
// TODO markers.
//
// Execute `rustlings hint cow1` or use the `hint` watch subcommand for a hint.
// I AM NOT DONE
use std::borrow::Cow;
fn abs_all<'a, 'b>(input: &'a mut Cow<'b, [i32]>) -> &'a mut Cow<'b, [i32]> {
for i in 0..input.len() {
let v = input[i];
if v < 0 {
// Clones into a vector if not already owned.
input.to_mut()[i] = -v;
}
}
input
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn reference_mutation() -> Result<(), &'static str> {
// Clone occurs because `input` needs to be mutated.
let slice = [-1, 0, 1];
let mut input = Cow::from(&slice[..]);
match abs_all(&mut input) {
Cow::Owned(_) => Ok(()),
_ => Err("Expected owned value"),
}
}
#[test]
fn reference_no_mutation() -> Result<(), &'static str> {
// No clone occurs because `input` doesn't need to be mutated.
let slice = [0, 1, 2];
let mut input = Cow::from(&slice[..]);
match abs_all(&mut input) {
// TODO
}
}
#[test]
fn owned_no_mutation() -> Result<(), &'static str> {
// We can also pass `slice` without `&` so Cow owns it directly. In this
// case no mutation occurs and thus also no clone, but the result is
// still owned because it was never borrowed or mutated.
let slice = vec![0, 1, 2];
let mut input = Cow::from(slice);
match abs_all(&mut input) {
// TODO
}
}
#[test]
fn owned_mutation() -> Result<(), &'static str> {
// Of course this is also the case if a mutation does occur. In this
// case the call to `to_mut()` in the abs_all() function returns a
// reference to the same data as before.
let slice = vec![-1, 0, 1];
let mut input = Cow::from(slice);
match abs_all(&mut input) {
// TODO
}
}
}
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