Add explanation for E0607

Author: estebank

src/librustc_typeck/check/cast.rs

@@ -281,10 +281,31 @@ impl<'a, 'gcx, 'tcx> CastCheck<'tcx> {
                                 .emit();
             }
             CastError::SizedUnsizedCast => {
-                type_error_struct!(fcx.tcx.sess, self.span, self.expr_ty, E0607,
-                                 "cannot cast thin pointer `{}` to fat pointer `{}`",
-                                 self.expr_ty,
-                                 fcx.ty_to_string(self.cast_ty)).emit();
+                let mut err = type_error_struct!(
+                    fcx.tcx.sess,
+                    self.span,
+                    self.expr_ty,
+                    E0607,
+                    "cannot cast thin pointer `{}` to fat pointer `{}`",
+                    self.expr_ty,
+                    fcx.ty_to_string(self.cast_ty)
+                );
+                if fcx.tcx.sess.opts.debugging_opts.explain {
+                    err.note(
+                        "Thin pointers are \"simple\" pointers: they are purely a reference to a \
+                         memory address.\n\n\
+                         Fat pointers are pointers referencing \"Dynamically Sized Types\" (also \
+                         called DST). DST don't have a statically known size, therefore they can \
+                         only exist behind some kind of pointers that contain additional \
+                         information. Slices and trait objects are DSTs. In the case of slices, \
+                         the additional information the fat pointer holds is their size.");
+                    err.note("to fix this error, don't try to cast directly between thin and fat \
+                              pointers");
+                    err.help("for more information about casts, take a look at [The Book]\
+                              (https://doc.rust-lang.org/book/first-edition/\
+                              casting-between-types.html)");
+                }
+                err.emit();
             }
             CastError::UnknownCastPtrKind |
             CastError::UnknownExprPtrKind => {