Add detailed error explanation for E0504

Removed unnecessary use of threads from E0504

Cleaned up line ending on E0504

Added more examples for E0504

Changed to erroneous code wording

Switched Rc example to thread/Arc example

Added comments describing why errors no longer occur

Author: cramertj

src/librustc_borrowck/diagnostics.rs

@@ -386,6 +386,109 @@ fn foo(a: &mut i32) {
     let bar = || {
         inside_closure(a)
     };
+```
+"##,
+
+E0504: r##"
+This error occurs when an attempt is made to move a borrowed variable into a
+closure.
+
+Example of erroneous code:
+
+```compile_fail
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let fancy_num = FancyNum { num: 5 };
+    let fancy_ref = &fancy_num;
+
+    let x = move || {
+        println!("child function: {}", fancy_num.num);
+        // error: cannot move `fancy_num` into closure because it is borrowed
+    };
+
+    x();
+    println!("main function: {}", fancy_ref.num);
+}
+```
+
+Here, `fancy_num` is borrowed by `fancy_ref` and so cannot be moved into
+the closure `x`. There is no way to move a value into a closure while it is
+borrowed, as that would invalidate the borrow.
+
+If the closure can't outlive the value being moved, try using a reference
+rather than moving:
+
+```
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let fancy_num = FancyNum { num: 5 };
+    let fancy_ref = &fancy_num;
+
+    let x = move || {
+        // fancy_ref is usable here because it doesn't move `fancy_num`
+        println!("child function: {}", fancy_ref.num);
+    };
+
+    x();
+
+    println!("main function: {}", fancy_num.num);
+}
+```
+
+If the value has to be borrowed and then moved, try limiting the lifetime of
+the borrow using a scoped block:
+
+```
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let fancy_num = FancyNum { num: 5 };
+
+    {
+        let fancy_ref = &fancy_num;
+        println!("main function: {}", fancy_ref.num);
+        // `fancy_ref` goes out of scope here
+    }
+
+    let x = move || {
+        // `fancy_num` can be moved now (no more references exist)
+        println!("child function: {}", fancy_num.num);
+    };
+
+    x();
+}
+```
+
+If the lifetime of a reference isn't enough, such as in the case of threading,
+consider using an `Arc` to create a reference-counted value:
+
+```
+use std::sync::Arc;
+use std::thread;
+
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let fancy_ref1 = Arc::new(FancyNum { num: 5 });
+    let fancy_ref2 = fancy_ref1.clone();
+
+    let x = thread::spawn(move || {
+        // `fancy_ref1` can be moved and has a `'static` lifetime
+        println!("child thread: {}", fancy_ref1.num);
+    });
+
+    x.join().expect("child thread should finish");
+    println!("main thread: {}", fancy_ref2.num);
 }
 ```
 "##,
@@ -514,7 +617,6 @@ register_diagnostics! {
     E0500, // closure requires unique access to `..` but .. is already borrowed
     E0502, // cannot borrow `..`.. as .. because .. is also borrowed as ...
     E0503, // cannot use `..` because it was mutably borrowed
-    E0504, // cannot move `..` into closure because it is borrowed
     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