Add span_help for E0002

Author: GuillaumeGomez

src/librustc/diagnostics.rs

@@ -44,7 +44,7 @@ E0002: r##"
 This error indicates that an empty match expression is invalid because the type
 it is matching on is non-empty (there exist values of this type). In safe code
 it is impossible to create an instance of an empty type, so empty match
-expressions are almost never desired.  This error is typically fixed by adding
+expressions are almost never desired. This error is typically fixed by adding
 one or more cases to the match expression.
 
 An example of an empty type is `enum Empty { }`. So, the following will work:

src/librustc/middle/check_match.rs

@@ -218,6 +218,9 @@ fn check_expr(cx: &mut MatchCheckCtxt, ex: &hir::Expr) {
                     span_err!(cx.tcx.sess, ex.span, E0002,
                               "non-exhaustive patterns: type {} is non-empty",
                               pat_ty);
+                    span_help!(cx.tcx.sess, ex.span,
+                        "Please ensure that all possible cases are being handled; \
+                         possibly adding wildcards or more match arms.");
                 }
                 // If the type *is* empty, it's vacuously exhaustive
                 return;