[qlt_ir] bugfix: always emit qcasts

qualtran/qlt_ir/_to_qlt_ir.py

@@ -573,18 +573,21 @@ def bloq_to_ast(
     # Signature
     qdb.add_signature(bloq.signature)
 
-    if force_extern:
-        return qdb.finalize_extern(reason='force_extern'), []
-
     # Detect bookkeeping / casting bloqs and emit as qcast.
     # Alloc and Free are _BookkeepingBloqs but are *not* casts.
+    # These checks come before `force_extern`: a cast has no body to hide, so it is
+    # always emitted as a `qcast` (and alloc/free are always `extern` anyways).
     from qualtran.bloqs.bookkeeping.allocate import Allocate
     from qualtran.bloqs.bookkeeping.free import Free
 
     if isinstance(bloq, (Allocate, Free)):
         return qdb.finalize_extern(reason='alloc/free'), []
     if _is_qcast_bloq(bloq):
         return qdb.finalize_qcast(), []
+
+    if force_extern:
+        return qdb.finalize_extern(reason='force_extern'), []
+
     if isinstance(bloq, qlt.CompositeBloq):
         cbloq = bloq
     else:
@@ -656,7 +659,7 @@ def add_bloqs(
             include_annotations: Whether to include visual display annotations derived
                 from `bloq.wire_symbol`.
             extern_only_from: Whether to only include a `from` clause for `extern` bloqs.
-            force_extern_pred: A predicate selecting bloqs to force to `extern`.
+            force_extern_pred: A predicate selecting non-cast bloqs to force to `extern`.
             max_depth: If provided, only decompose bloqs whose minimum depth below
                 `root` is less than `max_depth`, emitting bloqs at or beyond `max_depth`
                 as `extern qdef`. `None` (the default) decomposes fully.
@@ -760,7 +763,7 @@ def dump_qlt_ir(
         include_annotations: Whether to include visual display annotations derived
             from `bloq.wire_symbol` (defaults to `False`).
         extern_only_from: Whether to only include a `from` clause for `extern` bloqs.
-        force_extern_pred: A predicate selecting bloqs to force to `extern`.
+        force_extern_pred: A predicate selecting non-cast bloqs to force to `extern`.
         max_depth: If provided, only decompose bloqs whose minimum depth below
             `bloq` is less than `max_depth`, emitting bloqs at or beyond `max_depth`
             as `extern qdef`. `None` (the default) decomposes fully.

qualtran/qlt_ir/_to_qlt_ir_test.py

@@ -13,6 +13,7 @@
 #  limitations under the License.
 
 import io
+from typing import Any
 
 import pytest
 
@@ -567,3 +568,37 @@ def build_composite_bloq(self, bb, q):
     assert isinstance(by_key['_Shared'].qdef, QDefImplNode)
     assert by_key['T'].implemented is False
     assert isinstance(by_key['T'].qdef, QDefExternNode)
+
+
+def test_max_depth_and_force_extern_keep_qcasts():
+    """Casting bloqs have no body to hide, so they stay `qcast` below `max_depth`."""
+    from qualtran.bloqs.arithmetic import Add
+    from qualtran.bloqs.bookkeeping import Join, Split
+    from qualtran.qlt_ir import parse_module
+    from qualtran.qlt_ir.nodes import QCastNode
+
+    bloq = Add(qdt.QUInt(4))
+
+    all_mb_kwargs: list[dict[str, Any]] = [
+        dict(max_depth=1),
+        dict(force_extern_pred=lambda b: b != bloq),
+    ]
+    for mb_kwargs in all_mb_kwargs:
+        mb = QltModuleBuilder()
+        mb.add_bloqs(bloq, **mb_kwargs)
+        by_bloq = {q.bloq: q for q in mb.qdefs}
+        assert isinstance(by_bloq[bloq].qdef, QDefImplNode)
+        casts = [b for b in by_bloq if isinstance(b, (Split, Join))]
+        assert casts, 'Add(QUInt(4)) decomposition should contain splits/joins'
+        for b in casts:
+            assert isinstance(by_bloq[b].qdef, QCastNode), b
+        for b, q in by_bloq.items():
+            if b != bloq and not isinstance(b, (Split, Join)):
+                assert isinstance(q.qdef, QDefExternNode), b
+
+    txt = dump_root_qlt_ir(bloq)
+    assert txt is not None
+    assert 'extern qdef Split' not in txt
+    assert 'extern qdef Join' not in txt
+    mod = parse_module(txt)
+    assert any(isinstance(q, QCastNode) for q in mod.qdefs)