Auto merge of #34133 - m-decoster:master, r=GuillaumeGomez

Add explanations for E0503 and E0508.

(cannot use `..` because it was mutably borrowed, cannot move out of type `..`, a non-copy fixed-size array)

Part of #32777.

src/librustc_borrowck/diagnostics.rs

@@ -532,6 +532,62 @@ For more information on the rust ownership system, take a look at
 https://doc.rust-lang.org/stable/book/references-and-borrowing.html.
 "##,
 
+E0503: r##"
+A value was used after it was mutably borrowed.
+
+Example of erroneous code:
+
+```compile_fail
+fn main() {
+    let mut value = 3;
+    // Create a mutable borrow of `value`. This borrow
+    // lives until the end of this function.
+    let _borrow = &mut value;
+    let _sum = value + 1; // error: cannot use `value` because
+                          //        it was mutably borrowed
+}
+```
+
+In this example, `value` is mutably borrowed by `borrow` and cannot be
+used to calculate `sum`. This is not possible because this would violate
+Rust's mutability rules.
+
+You can fix this error by limiting the scope of the borrow:
+
+```
+fn main() {
+    let mut value = 3;
+    // By creating a new block, you can limit the scope
+    // of the reference.
+    {
+        let _borrow = &mut value; // Use `_borrow` inside this block.
+    }
+    // The block has ended and with it the borrow.
+    // You can now use `value` again.
+    let _sum = value + 1;
+}
+```
+
+Or by cloning `value` before borrowing it:
+
+```
+fn main() {
+    let mut value = 3;
+    // We clone `value`, creating a copy.
+    let value_cloned = value.clone();
+    // The mutable borrow is a reference to `value` and
+    // not to `value_cloned`...
+    let _borrow = &mut value;
+    // ... which means we can still use `value_cloned`,
+    let _sum = value_cloned + 1;
+    // even though the borrow only ends here.
+}
+```
+
+You can find more information about borrowing in the rust-book:
+http://doc.rust-lang.org/stable/book/references-and-borrowing.html
+"##,
+
 E0504: r##"
 This error occurs when an attempt is made to move a borrowed variable into a
 closure.
@@ -914,6 +970,50 @@ You can find more information about borrowing in the rust-book:
 http://doc.rust-lang.org/stable/book/references-and-borrowing.html
 "##,
 
+E0508: r##"
+A value was moved out of a non-copy fixed-size array.
+
+Example of erroneous code:
+
+```compile_fail
+struct NonCopy;
+
+fn main() {
+    let array = [NonCopy; 1];
+    let _value = array[0]; // error: cannot move out of type `[NonCopy; 1]`,
+                           //        a non-copy fixed-size array
+}
+```
+
+The first element was moved out of the array, but this is not
+possible because `NonCopy` does not implement the `Copy` trait.
+
+Consider borrowing the element instead of moving it:
+
+```
+struct NonCopy;
+
+fn main() {
+    let array = [NonCopy; 1];
+    let _value = &array[0]; // Borrowing is allowed, unlike moving.
+}
+```
+
+Alternatively, if your type implements `Clone` and you need to own the value,
+consider borrowing and then cloning:
+
+```
+#[derive(Clone)]
+struct NonCopy;
+
+fn main() {
+    let array = [NonCopy; 1];
+    // Now you can clone the array element.
+    let _value = array[0].clone();
+}
+```
+"##,
+
 E0509: r##"
 This error occurs when an attempt is made to move out of a value whose type
 implements the `Drop` trait.
@@ -1014,7 +1114,5 @@ fn main() {
 register_diagnostics! {
     E0385, // {} in an aliasable location
     E0388, // {} in a static location
-    E0503, // cannot use `..` because it was mutably borrowed
-    E0508, // cannot move out of type `..`, a non-copy fixed-size array
     E0524, // two closures require unique access to `..` at the same time
 }