Rollup merge of rust-lang#45082 - jacwah:explain-E0382, r=steveklabnik

Mention Clone and refs in --explain E0382

I followed the discussion in rust-lang#42446 and came up with these additions.

- Mention references before going into traits. They're probably more likely solutions.
- Mention `Clone` before `Copy`. Cloning has wider applicability and `#derive[Copy, Clone]` makes more sense after learning about `Clone`.

The language is not great, any suggestions there would be appreciated ✨

Author: kennytm

src/librustc_mir/diagnostics.rs

@@ -320,20 +320,68 @@ Since `MyStruct` is a type that is not marked `Copy`, the data gets moved out
 of `x` when we set `y`. This is fundamental to Rust's ownership system: outside
 of workarounds like `Rc`, a value cannot be owned by more than one variable.
 
-If we own the type, the easiest way to address this problem is to implement
-`Copy` and `Clone` on it, as shown below. This allows `y` to copy the
-information in `x`, while leaving the original version owned by `x`. Subsequent
-changes to `x` will not be reflected when accessing `y`.
+Sometimes we don't need to move the value. Using a reference, we can let another
+function borrow the value without changing its ownership. In the example below,
+we don't actually have to move our string to `calculate_length`, we can give it
+a reference to it with `&` instead.
+
+```
+fn main() {
+    let s1 = String::from("hello");
+
+    let len = calculate_length(&s1);
+
+    println!("The length of '{}' is {}.", s1, len);
+}
+
+fn calculate_length(s: &String) -> usize {
+    s.len()
+}
+```
+
+A mutable reference can be created with `&mut`.
+
+Sometimes we don't want a reference, but a duplicate. All types marked `Clone`
+can be duplicated by calling `.clone()`. Subsequent changes to a clone do not
+affect the original variable.
+
+Most types in the standard library are marked `Clone`. The example below
+demonstrates using `clone()` on a string. `s1` is first set to "many", and then
+copied to `s2`. Then the first character of `s1` is removed, without affecting
+`s2`. "any many" is printed to the console.
+
+```
+fn main() {
+    let mut s1 = String::from("many");
+    let s2 = s1.clone();
+    s1.remove(0);
+    println!("{} {}", s1, s2);
+}
+```
+
+If we control the definition of a type, we can implement `Clone` on it ourselves
+with `#[derive(Clone)]`.
+
+Some types have no ownership semantics at all and are trivial to duplicate. An
+example is `i32` and the other number types. We don't have to call `.clone()` to
+clone them, because they are marked `Copy` in addition to `Clone`.  Implicit
+cloning is more convienient in this case. We can mark our own types `Copy` if
+all their members also are marked `Copy`.
+
+In the example below, we implement a `Point` type. Because it only stores two
+integers, we opt-out of ownership semantics with `Copy`. Then we can
+`let p2 = p1` without `p1` being moved.
 
 ```
 #[derive(Copy, Clone)]
-struct MyStruct { s: u32 }
+struct Point { x: i32, y: i32 }
 
 fn main() {
-    let mut x = MyStruct{ s: 5u32 };
-    let y = x;
-    x.s = 6;
-    println!("{}", x.s);
+    let mut p1 = Point{ x: -1, y: 2 };
+    let p2 = p1;
+    p1.x = 1;
+    println!("p1: {}, {}", p1.x, p1.y);
+    println!("p2: {}, {}", p2.x, p2.y);
 }
 ```