Improve E0025 error explanation

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -73,10 +73,39 @@ the enum.
 "##,
 
 E0025: r##"
-Each field of a struct can only be bound once in a pattern. Each occurrence of a
-field name binds the value of that field, so to fix this error you will have to
-remove or alter the duplicate uses of the field name. Perhaps you misspelt
-another field name?
+Each field of a struct can only be bound once in a pattern. Erroneous code
+example:
+
+```
+struct Foo {
+    a: u8,
+    b: u8,
+}
+
+fn main(){
+    let x = Foo { a:1, b:2 };
+ 
+    let Foo { a: x, a: y } = x;
+    // error: field `a` bound multiple times in the pattern
+}
+```
+
+Each occurrence of a field name binds the value of that field, so to fix this
+error you will have to remove or alter the duplicate uses of the field name.
+Perhaps you misspelled another field name? Example:
+
+```
+struct Foo {
+    a: u8,
+    b: u8,
+}
+
+fn main(){
+    let x = Foo { a:1, b:2 };
+ 
+    let Foo { a: x, b: y } = x; // ok!
+}
+```
 "##,
 
 E0026: r##"