Work around #3215/#3217 use-after-free in typeck::check::alt

Author: bblum

src/rustc/middle/typeck/check/alt.rs

@@ -235,7 +235,10 @@ fn check_pat(pcx: pat_ctxt, pat: @ast::pat, expected: ty::t) {
       ast::pat_struct(path, fields, etc) => {
         // Grab the class data that we care about.
         let class_fields, class_id, substitutions;
-        match structure_of(fcx, pat.span, expected) {
+        // FIXME(#3217) If this were "match structure_of...", this bug causes
+        // it to not live long enough for 'substitutions'.
+        let structure = structure_of(fcx, pat.span, expected);
+        match structure {
             ty::ty_class(cid, ref substs) => {
                 class_id = cid;
                 substitutions = substs;