Add error explanation for E0070

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -124,8 +124,8 @@ be specified exactly one time.
 
 E0063: r##"
 This error indicates that during an attempt to build a struct or struct-like
-enum variant, one of the fields was not provided. Each field should be specified
-exactly once.
+enum variant, one of the fields was not provided. Each field should be
+specified exactly once.
 "##,
 
 E0066: r##"
@@ -141,7 +141,8 @@ and [RFC 809] for more details.
 E0067: r##"
 The left-hand side of an assignment operator must be an lvalue expression. An
 lvalue expression represents a memory location and includes item paths (ie,
-namespaced variables), dereferences, indexing expressions, and field references.
+namespaced variables), dereferences, indexing expressions, and field
+references.
 
 ```
 use std::collections::LinkedList;
@@ -170,6 +171,53 @@ Since `return;` is just like `return ();`, there is a mismatch between the
 function's return type and the value being returned.
 "##,
 
+E0070: r##"
+The left-hand side of an assignment operator must be an lvalue expression. An
+lvalue expression represents a memory location and can be a variable (with
+optional namespacing), a dereference, an indexing expression or a field
+reference.
+
+More details can be found here:
+https://doc.rust-lang.org/reference.html#lvalues,-rvalues-and-temporaries
+
+Now, we can go further. Here are some bad examples:
+```
+struct SomeStruct {
+    x: i32,
+    y: i32
+}
+const SOME_CONST : i32 = 12;
+
+fn some_other_func() {}
+
+fn some_function() {
+    SOME_CONST = 14; // error : a constant value cannot be changed!
+    1 = 3; // error : 1 isn't a valid lvalue!
+    some_other_func() = 4; // error : we can't assign value to a function!
+    SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
+                       // like a variable!
+}
+```
+
+And now let's give good examples:
+
+```
+struct SomeStruct {
+    x: i32,
+    y: i32
+}
+let mut s = SomeStruct {x: 0, y: 0};
+
+s.x = 3; // that's good !
+
+// ...
+
+fn some_func(x: &mut i32) {
+    *x = 12; // that's good !
+}
+```
+"##,
+
 E0072: r##"
 When defining a recursive struct or enum, any use of the type being defined
 from inside the definition must occur behind a pointer (like `Box` or `&`).
@@ -779,7 +827,6 @@ register_diagnostics! {
     E0060,
     E0061,
     E0068,
-    E0070,
     E0071,
     E0074,
     E0075,