Handle fail after return correctly in typestate

Previously, typestate would conclude that this function was
correctly diverging:

fn f() -> ! { ret; fail; }

even though it always returns to the caller. It wasn't handling the
i_diverge and i_return bits correctly in the fail case. Fixed it.

Closes #897

src/comp/middle/tstate/states.rs

@@ -48,6 +48,22 @@ fn handle_move_or_copy(fcx: fn_ctxt, post: poststate, rhs_path: @path,
  }
 }
 
+fn handle_fail(fcx: fn_ctxt, pres:prestate, post:poststate) {
+ // Remember what the old value of the "I return" trit was, so that
+ // we can avoid changing that (if it was true, there was a return
+ // that dominates this fail and the fail is unreachable)
+ if !promises(fcx, pres, fcx.enclosing.i_return)
+ // (only if we're in a diverging function -- you can fail when
+ // you're supposed to return, but not vice versa).
+ && fcx.enclosing.cf == noreturn {
+ kill_poststate_(fcx, fcx.enclosing.i_return, post);
+ } else {
+ // This code is unreachable (it's dominated by a return),
+ // so doesn't diverge.
+ kill_poststate_(fcx, fcx.enclosing.i_diverge, post);
+ }
+}
+
 fn seq_states(fcx: fn_ctxt, pres: prestate, bindings: [binding]) ->
  {changed: bool, post: poststate} {
  let changed = false;
@@ -189,6 +205,7 @@ fn find_pre_post_state_exprs(fcx: fn_ctxt, pres: prestate, id: node_id,
  alt cf {
  noreturn {
  let post = false_postcond(num_constraints(fcx.enclosing));
+ handle_fail(fcx, pres, post);
  changed |= set_poststate_ann(fcx.ccx, id, post);
  }
  _ { changed |= set_poststate_ann(fcx.ccx, id, rs.post); }
@@ -584,10 +601,7 @@ fn find_pre_post_state_expr(fcx: fn_ctxt, pres: prestate, e: @expr) -> bool {
  /* if execution continues after fail, then everything is true!
  woo! */
  let post = false_postcond(num_constrs);
- alt fcx.enclosing.cf {
- noreturn { kill_poststate_(fcx, fcx.enclosing.i_return, post); }
- _ { }
- }
+ handle_fail(fcx, pres, post);
  ret set_prestate_ann(fcx.ccx, e.id, pres) |
  set_poststate_ann(fcx.ccx, e.id, post) |
  option::maybe(false, maybe_fail_val, {|fail_val|

src/test/compile-fail/issue-897-2.rs

@@ -0,0 +1,4 @@
+// error-pattern: In non-returning function f, some control paths may return
+fn g() -> ! { fail; }
+fn f() -> ! { ret 42; g(); }
+fn main() { }

src/test/compile-fail/issue-897.rs

@@ -0,0 +1,3 @@
+// error-pattern: In non-returning function f, some control paths may return
+fn f() -> ! { ret 42; fail; }
+fn main() { }