Merge pull request #1103 from exdx/feat/cow

feat: add cow1.rs exercise
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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.
// 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
}
fn main() {
// 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) {
Cow::Borrowed(_) => println!("I borrowed the slice!"),
_ => panic!("expected borrowed value"),
}
// 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(_) => println!("I modified the slice and now own it!"),
_ => panic!("expected owned value"),
}
// No clone occurs because `input` is already owned.
let slice = vec![-1, 0, 1];
let mut input = Cow::from(slice);
match abs_all(&mut input) {
// TODO
Cow::Borrowed(_) => println!("I own this slice!"),
_ => panic!("expected borrowed value"),
}
}

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@ -932,6 +932,17 @@ is too much of a struggle, consider reading through all of Chapter 16 in the boo
https://doc.rust-lang.org/stable/book/ch16-00-concurrency.html
"""
[[exercises]]
name = "cow1"
path = "exercises/standard_library_types/cow1.rs"
mode = "compile"
hint = """
Since the vector is already owned, the `Cow` type doesn't need to clone it.
Checkout https://doc.rust-lang.org/std/borrow/enum.Cow.html for documentation
on the `Cow` type.
"""
# THREADS
[[exercises]]