Rollup merge of rust-lang#33384 - cramertj:E0506, r=arielb1

Add detailed error explanation for E0506

Part of rust-lang#32777

src/librustc_borrowck/diagnostics.rs

@@ -390,6 +390,90 @@ fn foo(a: &mut i32) {
 ```
 "##,
 
+E0506: r##"
+This error occurs when an attempt is made to assign to a borrowed value.
+
+Example of erroneous code:
+
+```compile_fail
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let mut fancy_num = FancyNum { num: 5 };
+    let fancy_ref = &fancy_num;
+    fancy_num = FancyNum { num: 6 };
+    // error: cannot assign to `fancy_num` because it is borrowed
+
+    println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
+}
+```
+
+Because `fancy_ref` still holds a reference to `fancy_num`, `fancy_num` can't
+be assigned to a new value as it would invalidate the reference.
+
+Alternatively, we can move out of `fancy_num` into a second `fancy_num`:
+
+```
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let mut fancy_num = FancyNum { num: 5 };
+    let moved_num = fancy_num;
+    fancy_num = FancyNum { num: 6 };
+
+    println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num);
+}
+```
+
+If the value has to be borrowed, try limiting the lifetime of the borrow using
+a scoped block:
+
+```
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let mut fancy_num = FancyNum { num: 5 };
+
+    {
+        let fancy_ref = &fancy_num;
+        println!("Ref: {}", fancy_ref.num);
+    }
+
+    // Works because `fancy_ref` is no longer in scope
+    fancy_num = FancyNum { num: 6 };
+    println!("Num: {}", fancy_num.num);
+}
+```
+
+Or by moving the reference into a function:
+
+```
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let mut fancy_num = FancyNum { num: 5 };
+
+    print_fancy_ref(&fancy_num);
+
+    // Works because function borrow has ended
+    fancy_num = FancyNum { num: 6 };
+    println!("Num: {}", fancy_num.num);
+}
+
+fn print_fancy_ref(fancy_ref: &FancyNum){
+    println!("Ref: {}", fancy_ref.num);
+}
+```
+"##,
+
 E0507: r##"
 You tried to move out of a value which was borrowed. Erroneous code example:
 
@@ -516,7 +600,6 @@ register_diagnostics! {
     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
     E0509, // cannot move out of type `..`, which defines the `Drop` trait
     E0524, // two closures require unique access to `..` at the same time