Don't refute "()" in let-bindings (fixes #3104).

src/rustc/middle/check_alt.rs

@@ -420,6 +420,7 @@ fn is_refutable(tcx: ty::ctxt, pat: @pat) -> bool {
         is_refutable(tcx, sub)
       }
       pat_wild | pat_ident(_, _, none) => { false }
+      pat_lit(@{node: expr_lit(@{node: lit_nil, _}), _}) => { false } // "()"
       pat_lit(_) | pat_range(_, _) => { true }
       pat_rec(fields, _) => {
         for fields.each |it| {

src/test/run-pass/irrefutable-unit.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let ((),()) = ((),());
+}