build/plat: improve handling of get_override().

The existing functionality of get_override was poorly specified and
ill-purposed for boolean flags. This change extracts the core
variable retrieval logic to a helper function and adds a new handler
`get_override_flag` which special cases boolean flags.

The new behavior will also perform type checking on kwargs and inform
the user of the desired type expected.

amaranth/build/plat.py

@@ -315,6 +315,50 @@ class TemplatedPlatform(Platform):
 
         rtlil_text, self._name_map = rtlil.convert_fragment(fragment, name=name)
 
+        # 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
+        # type checking.
+        def _extract_override(var, *, expected_type):
+            deprecated_var_env = "NMIGEN_{}".format(var)
+            var_env = "AMARANTH_{}".format(var)
+            if deprecated_var_env in os.environ or var_env in os.environ:
+                # On Windows, there is no way to define an "empty but set" variable; it is tempting
+                # to use a quoted empty string, but it doesn't do what one would expect. Recognize
+                # this as a useful pattern anyway, and treat `set VAR=""` on Windows the same way
+                # `export VAR=` is treated on Linux.
+                if var_env in os.environ:
+                    var_env_value = os.environ[var_env]
+                elif deprecated_var_env in os.environ:
+                    var_env_value = os.environ[deprecated_var_env]
+                return re.sub(r'^\"\"$', "", var_env_value)
+            elif var in kwargs:
+                if not isinstance(kwargs[var], expected_type) and not expected_type is None:
+                    raise TypeError("Override '{}' must be a {}, not {!r}".format(var, expected_type.__name__, kwargs[var]))
+                else:
+                    return kwargs[var]
+            else:
+                return jinja2.Undefined(name=var)
+
+        def get_override(var):
+            value = _extract_override(var, expected_type=str)
+            return value
+
+        def get_override_flag(var):
+            value = _extract_override(var, expected_type=bool)
+            if isinstance(value, str):
+                value = value.lower()
+                if value in ("0", "no", "n", "false", ""):
+                    return False
+                if value in ("1", "yes", "y", "true"):
+                    return True
+                else:
+                    raise ValueError("Override '{}' must be one of "
+                                     "(\"0\", \"n\", \"no\", \"false\", \"\") "
+                                     "or "
+                                     "(\"1\", \"y\", \"yes\", \"true\"), not {!r}"
+                                     .format(var, value))
+            return value
+
         def emit_rtlil():
             return rtlil_text
 
@@ -354,27 +398,6 @@ class TemplatedPlatform(Platform):
 
             return "\n".join(commands)
 
-        def get_override(var):
-            deprecated_var_env = "NMIGEN_{}".format(var)
-            var_env = "AMARANTH_{}".format(var)
-            if deprecated_var_env in os.environ or var_env in os.environ:
-                # On Windows, there is no way to define an "empty but set" variable; it is tempting
-                # to use a quoted empty string, but it doesn't do what one would expect. Recognize
-                # this as a useful pattern anyway, and treat `set VAR=""` on Windows the same way
-                # `export VAR=` is treated on Linux.
-                if var_env in os.environ:
-                    var_env_value = os.environ[var_env]
-                elif deprecated_var_env in os.environ:
-                    var_env_value = os.environ[deprecated_var_env]
-                return re.sub(r'^\"\"$', "", var_env_value)
-            elif var in kwargs:
-                if isinstance(kwargs[var], str):
-                    return textwrap.dedent(kwargs[var]).strip()
-                else:
-                    return kwargs[var]
-            else:
-                return jinja2.Undefined(name=var)
-
         @jinja2.pass_context
         def invoke_tool(context, name):
             env_var = tool_env_var(name)
@@ -409,13 +432,13 @@ class TemplatedPlatform(Platform):
             return '"' + re.sub(r"([$[\\])", r"\\\1", string) + '"'
 
         def verbose(arg):
-            if get_override("verbose"):
+            if get_override_flag("verbose"):
                 return arg
             else:
                 return jinja2.Undefined(name="quiet")
 
         def quiet(arg):
-            if get_override("verbose"):
+            if get_override_flag("verbose"):
                 return jinja2.Undefined(name="quiet")
             else:
                 return arg
@@ -443,6 +466,7 @@ class TemplatedPlatform(Platform):
                 "syntax": syntax,
                 "invoke_tool": invoke_tool,
                 "get_override": get_override,
+                "get_override_flag": get_override_flag,
                 "verbose": verbose,
                 "quiet": quiet,
                 "autogenerated": autogenerated,