[DSLX] Re-enable tuple-array loop tests and prefix_scan IR lowering.

docs_src/dslx_reference.md

@@ -1617,11 +1617,7 @@ ranging from 0 to 7 and the value at the index `e = x[i]`.
 ```
 fn prefix_scan_eq(x: u32[8]) -> u3[8] {
   let (_, _, result) =
-    // TODO(https://github.com/google/xls/issues/5125): Destructure `(i, e)` in
-    // the `for` loop header once supported during IR conversion.
-    for (item, (prior, count, result)) in std::enumerate(x) {
-      let (i, e) = item;
-      ...
+    for ((i, e), (prior, count, result)) in std::enumerate(x) {...
 ```
 
 ### `for` Expression

docs_src/tutorials/prefix_scan.md

@@ -80,11 +80,8 @@ Using tuples as the accumulator is a convenient way to model multiple
 loop-carried values:
 
 ```dslx-snippet
-    // TODO(https://github.com/google/xls/issues/5125): Destructure `(i, elem)`
-    // in the `for` loop header once supported during IR conversion.
-    for (item, (prior, count, result)): ((u32, u32), (u32, u3, u3[8]))
+    for ((i, elem), (prior, count, result)): ((u32, u32), (u32, u3, u3[8]))
           in std::enumerate(x) {
-      let (i, elem) = item;
 ```
 
 The iterable of this loop is created by `std::enumerate(x)`. On each iteration,

xls/dslx/ir_convert/ir_converter_match_test.cc

@@ -305,9 +305,7 @@ fn f(x: u2) -> u8 {
   ExpectIr(converted);
 }
 
-// TODO: github.com/google/xls/issues/1289 - Need to support this to enumerate
-// in a for-loop.
-TEST_F(IrConverterTest, DISABLED_LoopThroughTupleArray) {
+TEST_F(IrConverterTest, LoopThroughTupleArray) {
   constexpr std::string_view program = R"(
 pub fn make_bool_array() -> bool[2] {
     let enumerated = [(u32:0, u32:0), (u32:1, u32:1)];

xls/dslx/ir_convert/testdata/ir_converter_test_LoopThroughTupleArray.ir

@@ -0,0 +1,27 @@
+package test_module
+
+file_number 0 "test_module.x"
+
+fn ____test_module__make_bool_array_counted_for_0_body(__index: bits[32] id=9, x: bits[1][2] id=10, __indexable: (bits[32], bits[32])[2] id=11) -> bits[1][2] {
+  array_index.12: (bits[32], bits[32]) = array_index(__indexable, indices=[__index], id=12)
+  literal.15: bits[1] = literal(value=1, id=15)
+  i: bits[32] = tuple_index(array_index.12, index=1, id=14)
+  tuple_index.13: bits[32] = tuple_index(array_index.12, index=0, id=13)
+  ret array_update.16: bits[1][2] = array_update(x, literal.15, indices=[i], id=16)
+}
+
+fn __test_module__make_bool_array() -> bits[1][2] {
+  literal.1: bits[32] = literal(value=0, id=1)
+  literal.2: bits[32] = literal(value=0, id=2)
+  literal.4: bits[32] = literal(value=1, id=4)
+  literal.5: bits[32] = literal(value=1, id=5)
+  tuple.3: (bits[32], bits[32]) = tuple(literal.1, literal.2, id=3)
+  tuple.6: (bits[32], bits[32]) = tuple(literal.4, literal.5, id=6)
+  literal.8: bits[1][2] = literal(value=[0, 0], id=8)
+  enumerated: (bits[32], bits[32])[2] = array(tuple.3, tuple.6, id=7)
+  ret counted_for.17: bits[1][2] = counted_for(literal.8, trip_count=2, stride=1, body=____test_module__make_bool_array_counted_for_0_body, invariant_args=[enumerated], id=17)
+}
+
+top fn __test_module__main() -> bits[1][2] {
+  ret invoke.18: bits[1][2] = invoke(to_apply=__test_module__make_bool_array, id=18)
+}

xls/examples/dslx_intro/BUILD

@@ -56,15 +56,22 @@ xls_dslx_opt_ir_test(
     dep = ":crc32_one_byte_inferred",
 )
 
-# TODO: Remove `lower_to_ir` flag once the support for std::enumerate
-#       is merged: https://github.com/google/xls/pull/3816
+xls_dslx_opt_ir(
+    name = "prefix_scan_equality",
+    srcs = ["prefix_scan_equality.x"],
+    dslx_top = "prefix_scan_eq",
+    ir_conv_args = {"lower_to_proc_scoped_channels": "true"},
+)
+
+xls_dslx_opt_ir_test(
+    name = "prefix_scan_equality_test",
+    dep = ":prefix_scan_equality",
+)
+
 xls_dslx_test(
     name = "prefix_scan_equality_dslx_test",
     srcs = ["prefix_scan_equality.x"],
-    dslx_test_args = {
-        "compare": "none",
-        "lower_to_ir": "false",
-    },
+    dslx_test_args = {"compare": "jit"},
 )
 
 filegroup(

xls/examples/dslx_intro/prefix_scan_equality.x

@@ -18,11 +18,8 @@ import std;
 
 fn prefix_scan_eq(x: u32[8]) -> u3[8] {
   let (_, _, result) =
-    // TODO(https://github.com/google/xls/issues/5125): Destructure `(i, elem)`
-    // in the `for` loop header once supported during IR conversion.
-    for (item, (prior, count, result)): ((u32, u32), (u32, u3, u3[8]))
+    for ((i, elem), (prior, count, result)): ((u32, u32), (u32, u3, u3[8]))
           in std::enumerate(x) {
-      let (i, elem) = item;
       let (to_place, new_count): (u3, u3) = match (i == u32:0, prior == elem) {
         // The first iteration always places 0 and propagates seen count of 1.
         (true, _) => (u3:0, u3:1),