Implement RFC 58: Core support for ValueCastable formatting.

amaranth/hdl/_ast.py

@@ -385,6 +385,27 @@ class ShapeCastable:
         """
         return super().__call__(*args, **kwargs) # :nocov:
 
+    def format(self, obj, spec):
+        """Format a value.
+
+        This method is called by the Amaranth language to implement formatting for custom
+        shapes. Whenever :py:`"{obj:spec}"` is encountered by :class:`Format`, and :py:`obj`
+        has a custom shape that has a :meth:`format` method, :py:`obj.shape().format(obj, "spec")`
+        is called, and the format specifier is replaced with the result.
+
+        The default :meth:`format` implementation is:
+
+        .. code::
+
+            def format(self, obj, spec):
+                return Format(f"{{:{spec}}}", Value.cast(obj))
+
+        Returns
+        -------
+        :class:`Format`
+        """
+        return Format(f"{{:{spec}}}", Value.cast(obj))
+
     # TODO: write an RFC for turning this into a proper interface method
     def _value_repr(self, value):
         return (_repr.Repr(_repr.FormatInt(), value),)
@@ -2576,6 +2597,9 @@ class Format:
             chunks.append(literal)
             if field_name is not None:
                 obj = get_field(field_name)
+                if conversion == "v":
+                    obj = Value.cast(obj)
+                else:
                     obj = fmt.convert_field(obj, conversion)
                 format_spec = subformat(format_spec)
                 if isinstance(obj, Value):
@@ -2583,7 +2607,16 @@ class Format:
                     self._parse_format_spec(format_spec, obj.shape())
                     chunks.append((obj, format_spec))
                 elif isinstance(obj, ValueCastable):
-                    raise TypeError("'ValueCastable' formatting is not supported")
+                    shape = obj.shape()
+                    if isinstance(shape, ShapeCastable):
+                        fmt = shape.format(obj, format_spec)
+                        if not isinstance(fmt, Format):
+                            raise TypeError(f"`ShapeCastable.format` must return a 'Format' instance, not {fmt!r}")
+                        chunks += fmt._chunks
+                    else:
+                        obj = Value.cast(obj)
+                        self._parse_format_spec(format_spec, obj.shape())
+                        chunks.append((obj, format_spec))
                 elif isinstance(obj, Format):
                     if format_spec != "":
                         raise ValueError(f"Format specifiers ({format_spec!r}) cannot be used for 'Format' objects")

docs/changes.rst

@@ -56,6 +56,7 @@ Implemented RFCs
 .. _RFC 51: https://amaranth-lang.org/rfcs/0051-const-from-bits.html
 .. _RFC 53: https://amaranth-lang.org/rfcs/0053-ioport.html
 .. _RFC 55: https://amaranth-lang.org/rfcs/0055-lib-io.html
+.. _RFC 58: https://amaranth-lang.org/rfcs/0058-valuecastable-format.html
 .. _RFC 59: https://amaranth-lang.org/rfcs/0059-no-domain-upwards-propagation.html
 .. _RFC 62: https://amaranth-lang.org/rfcs/0062-memory-data.html
 
@@ -68,6 +69,7 @@ Implemented RFCs
 * `RFC 50`_: ``Print`` statement and string formatting
 * `RFC 51`_: Add ``ShapeCastable.from_bits`` and ``amaranth.lib.data.Const``
 * `RFC 53`_: Low-level I/O primitives
+* `RFC 58`_: Core support for ``ValueCastable`` formatting
 * `RFC 59`_: Get rid of upwards propagation of clock domains
 * `RFC 62`_: The `MemoryData`` class
 

tests/test_hdl_ast.py

@@ -1412,6 +1412,79 @@ class MockValueCastable(ValueCastable):
         return self.dest
 
 
+class MockShapeCastableFormat(ShapeCastable):
+    def __init__(self, dest):
+        self.dest = dest
+
+    def as_shape(self):
+        return self.dest
+
+    def __call__(self, value):
+        return value
+
+    def const(self, init):
+        return Const(init, self.dest)
+
+    def from_bits(self, bits):
+        return bits
+
+    def format(self, value, format_desc):
+        return Format("_{}_{}_", Value.cast(value), format_desc)
+
+
+class MockValueCastableFormat(ValueCastable):
+    def __init__(self, dest):
+        self.dest = dest
+
+    def shape(self):
+        return MockShapeCastableFormat(Value.cast(self.dest).shape())
+
+    def as_value(self):
+        return self.dest
+
+
+class MockValueCastableNoFormat(ValueCastable):
+    def __init__(self, dest):
+        self.dest = dest
+
+    def shape(self):
+        return MockShapeCastable(Value.cast(self.dest).shape())
+
+    def as_value(self):
+        return self.dest
+
+
+class MockShapeCastableFormatWrong(ShapeCastable):
+    def __init__(self, dest):
+        self.dest = dest
+
+    def as_shape(self):
+        return self.dest
+
+    def __call__(self, value):
+        return value
+
+    def const(self, init):
+        return Const(init, self.dest)
+
+    def from_bits(self, bits):
+        return bits
+
+    def format(self, value, format_desc):
+        return Value.cast(value)
+
+
+class MockValueCastableFormatWrong(ValueCastable):
+    def __init__(self, dest):
+        self.dest = dest
+
+    def shape(self):
+        return MockShapeCastableFormatWrong(Value.cast(self.dest).shape())
+
+    def as_value(self):
+        return self.dest
+
+
 class MockValueCastableChanges(ValueCastable):
     def __init__(self, width=0):
         self.width = width
@@ -1567,6 +1640,21 @@ class FormatTestCase(FHDLTestCase):
         fmt = Format("sub: {}, c: {:4x}", subfmt, c)
         self.assertRepr(fmt, "(format 'sub: a: {:2x}, b: {:3x}, c: {:4x}' (sig a) (sig b) (sig c))")
 
+    def test_construct_valuecastable(self):
+        a = Signal()
+        b = MockValueCastable(a)
+        fmt = Format("{:x}", b)
+        self.assertRepr(fmt, "(format '{:x}' (sig a))")
+        c = MockValueCastableFormat(a)
+        fmt = Format("{:meow}", c)
+        self.assertRepr(fmt, "(format '_{}_meow_' (sig a))")
+        d = MockValueCastableNoFormat(a)
+        fmt = Format("{:x}", d)
+        self.assertRepr(fmt, "(format '{:x}' (sig a))")
+        e = MockValueCastableFormat(a)
+        fmt = Format("{!v:x}", e)
+        self.assertRepr(fmt, "(format '{:x}' (sig a))")
+
     def test_construct_wrong(self):
         a = Signal()
         b = Signal(signed(16))
@@ -1576,9 +1664,6 @@ class FormatTestCase(FHDLTestCase):
         with self.assertRaisesRegex(ValueError,
                 r"^cannot switch from automatic field numbering to manual field specification$"):
             Format("{}, {1}", a, b)
-        with self.assertRaisesRegex(TypeError,
-                r"^'ValueCastable' formatting is not supported$"):
-            Format("{}", MockValueCastable(Const(0)))
         with self.assertRaisesRegex(ValueError,
                 r"^Format specifiers \('s'\) cannot be used for 'Format' objects$"):
             Format("{:s}", Format(""))
@@ -1622,6 +1707,14 @@ class FormatTestCase(FHDLTestCase):
                 r"^Cannot specify '_' with format specifier 'c'$"):
             Format("{a:_c}", a=a)
 
+    def test_construct_valuecastable_wrong(self):
+        a = Signal()
+        b = MockValueCastableFormatWrong(a)
+        with self.assertRaisesRegex(TypeError,
+                r"^`ShapeCastable.format` must return a 'Format' instance, "
+                r"not \(sig a\)$"):
+            fmt = Format("{:x}", b)
+
     def test_plus(self):
         a = Signal()
         b = Signal()