diff --git a/amaranth/back/rtlil.py b/amaranth/back/rtlil.py
index 34e4db0..f495eb2 100644
--- a/amaranth/back/rtlil.py
+++ b/amaranth/back/rtlil.py
@@ -988,8 +988,8 @@ class EmptyModuleChecker:
         return module_idx in self.empty
 
 
-def convert_fragment(fragment, ports, name="top", *, emit_src=True, **kwargs):
-    assert isinstance(fragment, _ir.Fragment)
+def convert_fragment(fragment, ports=(), name="top", *, emit_src=True, **kwargs):
+    assert isinstance(fragment, (_ir.Fragment, _ir.Design))
     name_map = _ast.SignalDict()
     netlist = _ir.build_netlist(fragment, ports=ports, name=name, **kwargs)
     empty_checker = EmptyModuleChecker(netlist)
diff --git a/amaranth/build/plat.py b/amaranth/build/plat.py
index 1088201..12d2bd3 100644
--- a/amaranth/build/plat.py
+++ b/amaranth/build/plat.py
@@ -9,8 +9,8 @@ import jinja2
 from .. import __version__
 from .._toolchain import *
 from ..hdl import *
-from ..hdl._ir import IOBufferInstance
-from ..hdl._xfrm import DomainLowerer
+from ..hdl._ir import IOBufferInstance, Design
+from ..hdl._xfrm import DomainLowerer, AssignmentLegalizer
 from ..lib.cdc import ResetSynchronizer
 from ..back import rtlil, verilog
 from .res import *
@@ -164,11 +164,13 @@ class Platform(ResourceManager, metaclass=ABCMeta):
             if pin.dir == "io":
                 add_pin_fragment(pin, self.get_diff_input_output(pin, port, attrs, invert))
 
-        ports = list(self.iter_ports())
-        return self.toolchain_prepare(fragment, name, ports=ports, **kwargs)
+        ports = [(None, signal, None) for signal in self.iter_ports()]
+        fragment = AssignmentLegalizer()(fragment)
+        fragment = Design(fragment, ports, hierarchy=(name,))
+        return self.toolchain_prepare(fragment, name, **kwargs)
 
     @abstractmethod
-    def toolchain_prepare(self, fragment, name, *, ports, **kwargs):
+    def toolchain_prepare(self, fragment, name, **kwargs):
         """
         Convert the ``fragment`` and constraints recorded in this :class:`Platform` into
         a :class:`BuildPlan`.
@@ -291,7 +293,7 @@ class TemplatedPlatform(Platform):
                 continue
             yield net_signal, port_signal, frequency
 
-    def toolchain_prepare(self, fragment, name, *, ports, emit_src=True, **kwargs):
+    def toolchain_prepare(self, fragment, name, *, emit_src=True, **kwargs):
         # Restrict the name of the design to a strict alphanumeric character set. Platforms will
         # interpolate the name of the design in many different contexts: filesystem paths, Python
         # scripts, Tcl scripts, ad-hoc constraint files, and so on. It is not practical to add
@@ -307,8 +309,8 @@ class TemplatedPlatform(Platform):
         # and to incorporate the Amaranth version into generated code.
         autogenerated = f"Automatically generated by Amaranth {__version__}. Do not edit."
 
-        rtlil_text, self._name_map = rtlil.convert_fragment(fragment, ports=ports, name=name,
-                                                            emit_src=emit_src)
+        rtlil_text, self._name_map = rtlil.convert_fragment(fragment, name=name,
+                                                            emit_src=emit_src, propagate_domains=False)
 
         # Retrieve an override specified in either the environment or as a kwarg.
         # expected_type parameter is used to assert the type of kwargs, passing `None` will disable
diff --git a/amaranth/hdl/_ir.py b/amaranth/hdl/_ir.py
index 16c1371..733e276 100644
--- a/amaranth/hdl/_ir.py
+++ b/amaranth/hdl/_ir.py
@@ -386,16 +386,17 @@ class Fragment:
 
         return new_ports
 
-    def prepare(self, ports=(), *, hierarchy=("top",), legalize_assignments=False, missing_domain=lambda name: _cd.ClockDomain(name)):
+    def prepare(self, ports=(), *, hierarchy=("top",), legalize_assignments=False, missing_domain=lambda name: _cd.ClockDomain(name), propagate_domains=True):
         from ._xfrm import DomainLowerer
 
         ports = self._prepare_ports(ports)
 
-        new_domains = self._propagate_domains(missing_domain)
-        for domain in new_domains:
-            ports.append((None, domain.clk, PortDirection.Input))
-            if domain.rst is not None:
-                ports.append((None, domain.rst, PortDirection.Input))
+        if propagate_domains:
+            new_domains = self._propagate_domains(missing_domain)
+            for domain in new_domains:
+                ports.append((None, domain.clk, PortDirection.Input))
+                if domain.rst is not None:
+                    ports.append((None, domain.rst, PortDirection.Input))
 
         def resolve_signal(signal):
             if isinstance(signal, _ast.ClockSignal):
@@ -1369,8 +1370,11 @@ def _compute_ports(netlist: _nir.Netlist):
         top_module.ports[name] = (value, _nir.ModuleNetFlow.Output)
 
 
-def build_netlist(fragment, ports, *, name="top", **kwargs):
-    design = fragment.prepare(ports=ports, hierarchy=(name,), legalize_assignments=True, **kwargs)
+def build_netlist(fragment, ports=(), *, name="top", **kwargs):
+    if isinstance(fragment, Design):
+        design = fragment
+    else:
+        design = fragment.prepare(ports=ports, hierarchy=(name,), legalize_assignments=True, **kwargs)
     netlist = _nir.Netlist()
     _emit_netlist(netlist, design)
     netlist.resolve_all_nets()