hdl: make all public Value classes other than Record final.

In some cases, nMigen uses type() instead of isinstance() to dispatch
on types. Make sure all such uses of type() are robust; in addition,
make it clear that nMigen AST classes are not meant to be subclassed.
(Record is an exception.)

Fixes #65.

nmigen/hdl/ast.py

@@ -210,6 +210,7 @@ class Value(metaclass=ABCMeta):
     __hash__ = None
 
 
+@final
 class Const(Value):
     """A constant, literal integer value.
 
@@ -283,16 +284,19 @@ class AnyValue(Value, DUID):
         return ValueSet()
 
 
+@final
 class AnyConst(AnyValue):
     def __repr__(self):
         return "(anyconst {}'{})".format(self.nbits, "s" if self.signed else "")
 
 
+@final
 class AnySeq(AnyValue):
     def __repr__(self):
         return "(anyseq {}'{})".format(self.nbits, "s" if self.signed else "")
 
 
+@final
 class Operator(Value):
     def __init__(self, op, operands, src_loc_at=0):
         super().__init__(src_loc_at=1 + src_loc_at)
@@ -387,6 +391,7 @@ def Mux(sel, val1, val0):
     return Operator("m", [sel, val1, val0], src_loc_at=1)
 
 
+@final
 class Slice(Value):
     def __init__(self, value, start, end):
         if not isinstance(start, int):
@@ -424,6 +429,7 @@ class Slice(Value):
         return "(slice {} {}:{})".format(repr(self.value), self.start, self.end)
 
 
+@final
 class Part(Value):
     def __init__(self, value, offset, width):
         if not isinstance(width, int) or width < 0:
@@ -447,6 +453,7 @@ class Part(Value):
         return "(part {} {} {})".format(repr(self.value), repr(self.offset), self.width)
 
 
+@final
 class Cat(Value):
     """Concatenate values.
 
@@ -495,6 +502,7 @@ class Cat(Value):
         return "(cat {})".format(" ".join(map(repr, self.parts)))
 
 
+@final
 class Repl(Value):
     """Replicate a value
 
@@ -534,6 +542,7 @@ class Repl(Value):
         return "(repl {!r} {})".format(self.value, self.count)
 
 
+@final
 class Signal(Value, DUID):
     """A varying integer value.
 
@@ -649,6 +658,7 @@ class Signal(Value, DUID):
         return "(sig {})".format(self.name)
 
 
+@final
 class ClockSignal(Value):
     """Clock signal for a clock domain.
 
@@ -680,6 +690,7 @@ class ClockSignal(Value):
         return "(clk {})".format(self.domain)
 
 
+@final
 class ResetSignal(Value):
     """Reset signal for a clock domain.
 
@@ -802,6 +813,7 @@ class Array(MutableSequence):
                                        ", ".join(map(repr, self._inner)))
 
 
+@final
 class ArrayProxy(Value):
     def __init__(self, elems, index):
         super().__init__(src_loc_at=1)
@@ -836,6 +848,7 @@ class ArrayProxy(Value):
         return "(proxy (array [{}]) {!r})".format(", ".join(map(repr, self.elems)), self.index)
 
 
+@final
 class Sample(Value):
     """Value from the past.
 
@@ -899,6 +912,7 @@ class Statement:
                 raise TypeError("Object '{!r}' is not an nMigen statement".format(obj))
 
 
+@final
 class Assign(Statement):
     def __init__(self, lhs, rhs):
         self.lhs = Value.wrap(lhs)
@@ -940,14 +954,17 @@ class Property(Statement):
         return "({} {!r})".format(self._kind, self.test)
 
 
+@final
 class Assert(Property):
     _kind = "assert"
 
 
+@final
 class Assume(Property):
     _kind = "assume"
 
 
+# @final
 class Switch(Statement):
     def __init__(self, test, cases):
         self.test  = Value.wrap(test)
@@ -981,6 +998,7 @@ class Switch(Statement):
         return "(switch {!r} {})".format(self.test, " ".join(cases))
 
 
+@final
 class Delay(Statement):
     def __init__(self, interval=None):
         self.interval = None if interval is None else float(interval)
@@ -995,6 +1013,7 @@ class Delay(Statement):
             return "(delay {:.3}us)".format(self.interval * 1e6)
 
 
+@final
 class Tick(Statement):
     def __init__(self, domain="sync"):
         self.domain = str(domain)
@@ -1006,6 +1025,7 @@ class Tick(Statement):
         return "(tick {})".format(self.domain)
 
 
+@final
 class Passive(Statement):
     def _rhs_signals(self):
         return ValueSet()

nmigen/hdl/rec.py

@@ -62,6 +62,7 @@ class Layout:
             yield (name, shape, dir)
 
 
+# Unlike most Values, Record *can* be subclassed.
 class Record(Value):
     def __init__(self, layout, name=None):
         if name is None:

nmigen/tools.py

@@ -6,7 +6,7 @@ from collections.abc import Iterable
 from contextlib import contextmanager
 
 
-__all__ = ["flatten", "union", "log2_int", "bits_for", "memoize", "deprecated"]
+__all__ = ["flatten", "union", "log2_int", "bits_for", "memoize", "final", "deprecated"]
 
 
 def flatten(i):
@@ -57,6 +57,14 @@ def memoize(f):
     return g
 
 
+def final(cls):
+    def init_subclass():
+        raise TypeError("Subclassing {}.{} is not supported"
+                        .format(cls.__module__, cls.__name__))
+    cls.__init_subclass__ = init_subclass
+    return cls
+
+
 def deprecated(message, stacklevel=2):
     def decorator(f):
         @functools.wraps(f)