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rustlings/exercises/19_smart_pointers/cow1.rs

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// This exercise explores the `Cow` (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.
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use std::borrow::Cow;
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fn abs_all(input: &mut Cow<[i32]>) {
for ind in 0..input.len() {
let value = input[ind];
if value < 0 {
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// Clones into a vector if not already owned.
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input.to_mut()[ind] = -value;
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}
}
}
fn main() {
// You can optionally experiment here.
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
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fn reference_mutation() {
// Clone occurs because `input` needs to be mutated.
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let vec = vec![-1, 0, 1];
let mut input = Cow::from(&vec);
abs_all(&mut input);
assert!(matches!(input, Cow::Owned(_)));
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}
#[test]
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fn reference_no_mutation() {
// No clone occurs because `input` doesn't need to be mutated.
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let vec = vec![0, 1, 2];
let mut input = Cow::from(&vec);
abs_all(&mut input);
// TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
assert!(matches!(input, todo!()));
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}
#[test]
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fn owned_no_mutation() {
// We can also pass `vec` without `&` so `Cow` owns it directly. In this
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// case, no mutation occurs (all numbers are already absolute) and thus
// also no clone. But the result is still owned because it was never
// borrowed or mutated.
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let vec = vec![0, 1, 2];
let mut input = Cow::from(vec);
abs_all(&mut input);
// TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
assert!(matches!(input, todo!()));
}
#[test]
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fn owned_mutation() {
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// Of course this is also the case if a mutation does occur (not all
// numbers are absolute). In this case, the call to `to_mut()` in the
// `abs_all` function returns a reference to the same data as before.
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let vec = vec![-1, 0, 1];
let mut input = Cow::from(vec);
abs_all(&mut input);
// TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
assert!(matches!(input, todo!()));
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}
}