Add E0513 error explanation

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -3766,6 +3766,45 @@ extern "platform-intrinsic" {
 ```
 "##,
 
+E0513: r##"
+The type of the variable couldn't be found out.
+
+Erroneous code example:
+
+```compile_fail,E0513
+use std::mem;
+
+unsafe {
+    let size = mem::size_of::<u32>();
+    mem::transmute_copy::<u32, [u8; size]>(&8_8);
+    // error: no type for local variable
+}
+```
+
+To fix this error, please use a constant size instead of `size`. To make
+this error more obvious, you could run:
+
+```compile_fail,E0080
+use std::mem;
+
+unsafe {
+    mem::transmute_copy::<u32, [u8; mem::size_of::<u32>()]>(&8_8);
+    // error: constant evaluation error
+}
+```
+
+So now, you can fix your code by setting the size directly:
+
+```
+use std::mem;
+
+unsafe {
+    mem::transmute_copy::<u32, [u8; 4]>(&8_8);
+    // `u32` is 4 bytes so we replace the `mem::size_of` call with its size
+}
+```
+"##,
+
 E0516: r##"
 The `typeof` keyword is currently reserved but unimplemented.
 Erroneous code example:
@@ -4064,7 +4103,6 @@ register_diagnostics! {
     E0399, // trait items need to be implemented because the associated
            // type `{}` was overridden
     E0436, // functional record update requires a struct
-    E0513, // no type for local variable ..
     E0521, // redundant default implementations of trait
     E0533, // `{}` does not name a unit variant, unit struct or a constant
     E0562, // `impl Trait` not allowed outside of function