Teach rustc to use the result of if expressions

src/comp/middle/trans.rs

@@ -3037,21 +3037,30 @@ fn trans_if(@block_ctxt cx, @ast.expr cond,
     auto then_res = trans_block(then_cx, thn);
 
     auto else_cx = new_scope_block_ctxt(cx, "else");
-    auto else_res = res(else_cx, C_nil());
 
+    auto else_res;
+    auto expr_llty;
     alt (els) {
         case (some[@ast.expr](?elexpr)) {
             else_res = trans_expr(else_cx, elexpr);
+
+            // If we have an else expression, then the entire
+            // if expression can have a non-nil type.
+            // FIXME: Handle dynamic type sizes
+            auto expr_ty = ty.expr_ty(elexpr);
+            expr_llty = type_of(else_res.bcx.fcx.ccx, expr_ty);
+        }
+        case (_) {
+            else_res = res(else_cx, C_nil());
+            expr_llty = T_nil();
         }
-        case (_) { /* fall through */ }
     }
 
     cond_res.bcx.build.CondBr(cond_res.val,
                               then_cx.llbb,
                               else_cx.llbb);
 
-    // FIXME: use inferred type when available.
-    ret join_results(cx, T_nil(),
+    ret join_results(cx, expr_llty,
                      vec(then_res, else_res));
 }
 

src/test/run-pass/expr-if.rs

@@ -0,0 +1,98 @@
+// xfail-boot
+// -*- rust -*-
+
+// Tests for if as expressions
+
+fn test_if() {
+  let bool res = if (true) { true } else { false };
+  check (res);
+}
+
+fn test_else() {
+  let bool res = if (false) { false } else { true };
+  check (res);
+}
+
+fn test_elseif1() {
+  let bool res = if (true) {
+    true
+  } else if (true) {
+    false
+  } else {
+    false
+  };
+  check (res);
+}
+
+fn test_elseif2() {
+  let bool res = if (false) {
+    false
+  } else if (true) {
+    true
+  } else {
+    false
+  };
+  check (res);
+}
+
+fn test_elseif3() {
+  let bool res = if (false) {
+    false
+  } else if (false) {
+    false
+  } else {
+    true
+  };
+  check (res);
+}
+
+fn test_inferrence() {
+  auto res = if (true) { true } else { false };
+  check (res);
+}
+
+fn test_if_as_if_condition() {
+  auto res1 = if (if (false) { false } else { true }) {
+    true
+  } else {
+    false
+  };
+  check (res1);
+
+  auto res2 = if (if (true) { false } else { true }) {
+    false
+  } else {
+    true
+  };
+  check (res2);
+}
+
+fn test_if_as_block_result() {
+  auto res = if (true) {
+    if (false) {
+      false
+    } else {
+      true
+    }
+  } else {
+    false
+  };
+  check (res);
+}
+
+fn test_str() {
+  auto res = if (true) { "happy" } else { "sad" };
+  check (res == "happy");
+}
+
+fn main() {
+  test_if();
+  test_else();
+  test_elseif1();
+  test_elseif2();
+  test_elseif3();
+  test_inferrence();
+  test_if_as_if_condition();
+  test_if_as_block_result();
+  test_str();
+}