feat(arc1): Add more details to description and hint (rust-lang#710)

Co-authored-by: bmacer <bmacer@cisco.com>
Co-authored-by: marisa <mokou@fastmail.com>
Co-authored-by: Roberto Vidal <vidal.roberto.j@gmail.com>

exercises/standard_library_types/arc1.rs

@@ -1,7 +1,21 @@
 // arc1.rs
+// In this exercise, we are given a Vec of u32 called "numbers" with values ranging
+// from 0 to 99 -- [ 0, 1, 2, ..., 98, 99 ]
+// We would like to use this set of numbers within 8 different threads simultaneously.
+// Each thread is going to get the sum of every eighth value, with an offset.
+// The first thread (offset 0), will sum 0, 8, 16, ...
+// The second thread (offset 1), will sum 1, 9, 17, ...
+// The third thread (offset 2), will sum 2, 10, 18, ...
+// ...
+// The eighth thread (offset 7), will sum 7, 15, 23, ...
+
+// Because we are using threads, our values need to be thread-safe.  Therefore,
+// we are using Arc.  We need to make a change in each of the two TODOs.
+
+
 // Make this code compile by filling in a value for `shared_numbers` where the
-// TODO comment is and create an initial binding for `child_numbers`
-// somewhere. Try not to create any copies of the `numbers` Vec!
+// first TODO comment is, and create an initial binding for `child_numbers`
+// where the second TODO comment is. Try not to create any copies of the `numbers` Vec!
 // Execute `rustlings hint arc1` for hints :)
 
 // I AM NOT DONE
@@ -16,6 +30,7 @@ fn main() {
     let mut joinhandles = Vec::new();
 
     for offset in 0..8 {
+        let child_numbers = // TODO
         joinhandles.push(thread::spawn(move || {
             let mut i = offset;
             let mut sum = 0;

info.toml

@@ -679,7 +679,12 @@ to avoid creating a copy of `numbers`, you'll need to create `child_numbers`
 inside the loop but still in the main thread.
 
 `child_numbers` should be a clone of the Arc of the numbers instead of a
-thread-local copy of the numbers."""
+thread-local copy of the numbers.
+
+This is a simple exercise if you understand the underlying concepts, but if this
+is too much of a struggle, consider reading through all of Chapter 16 in the book:
+https://doc.rust-lang.org/stable/book/ch16-00-concurrency.html
+"""
 
 [[exercises]]
 name = "iterators1"