Add detailed error explanation for E0509

Edited the error explanation for E0509 to clarify dropping of moved fields

Edited the error explanation for E0509 to clarify move out of Drop value language

Fixed typeo in last commit to E0509

Switched to erroneous code wording

Author: cramertj

src/librustc_borrowck/diagnostics.rs

@@ -429,6 +429,101 @@ You can find more information about borrowing in the rust-book:
 http://doc.rust-lang.org/stable/book/references-and-borrowing.html
 "##,
 
+E0509: r##"
+This error occurs when an attempt is made to move out of a value whose type
+implements the `Drop` trait.
+
+Example of erroneous code:
+
+```compile_fail
+struct FancyNum {
+    num: usize
+}
+
+struct DropStruct {
+    fancy: FancyNum
+}
+
+impl Drop for DropStruct {
+    fn drop(&mut self) {
+        // Destruct DropStruct, possibly using FancyNum
+    }
+}
+
+fn main() {
+    let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
+    let fancy_field = drop_struct.fancy; // Error E0509
+    println!("Fancy: {}", fancy_field.num);
+    // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
+}
+```
+
+Here, we tried to move a field out of a struct of type `DropStruct` which
+implements the `Drop` trait. However, a struct cannot be dropped if one or
+more of its fields have been moved.
+
+Structs implementing the `Drop` trait have an implicit destructor that gets
+called when they go out of scope. This destructor may use the fields of the
+struct, so moving out of the struct could make it impossible to run the
+destructor. Therefore, we must think of all values whose type implements the
+`Drop` trait as single units whose fields cannot be moved.
+
+This error can be fixed by creating a reference to the fields of a struct,
+enum, or tuple using the `ref` keyword:
+
+```
+struct FancyNum {
+    num: usize
+}
+
+struct DropStruct {
+    fancy: FancyNum
+}
+
+impl Drop for DropStruct {
+    fn drop(&mut self) {
+        // Destruct DropStruct, possibly using FancyNum
+    }
+}
+
+fn main() {
+    let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
+    let ref fancy_field = drop_struct.fancy; // No more errors!
+    println!("Fancy: {}", fancy_field.num);
+    // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
+}
+```
+
+Note that this technique can also be used in the arms of a match expression:
+
+```
+struct FancyNum {
+    num: usize
+}
+
+enum DropEnum {
+    Fancy(FancyNum)
+}
+
+impl Drop for DropEnum {
+    fn drop(&mut self) {
+        // Destruct DropEnum, possibly using FancyNum
+    }
+}
+
+fn main() {
+    // Creates and enum of type `DropEnum`, which implements `Drop`
+    let drop_enum = DropEnum::Fancy(FancyNum{num: 10});
+    match drop_enum {
+        // Creates a reference to the inside of `DropEnum::Fancy`
+        DropEnum::Fancy(ref fancy_field) => // No error!
+            println!("It was fancy-- {}!", fancy_field.num),
+    }
+    // implicit call to `drop_enum.drop()` as drop_enum goes out of scope
+}
+```
+"##,
+
 }
 
 register_diagnostics! {
@@ -443,6 +538,5 @@ register_diagnostics! {
     E0505, // cannot move out of `..` because it is borrowed
     E0506, // cannot assign to `..` because it is borrowed
     E0508, // cannot move out of type `..`, a non-copy fixed-size array
-    E0509, // cannot move out of type `..`, which defines the `Drop` trait
     E0524, // two closures require unique access to `..` at the same time
 }