build.plat: add build_{{name}}.json output.

This is useful for custom build runners that don't run a shell; more
specifically, WebAssembly based tooling.

Author: whitequark

amaranth/build/plat.py

@@ -2,9 +2,12 @@
 from collections.abc import Iterable
 from abc import ABCMeta, abstractmethod
 import os
-import textwrap
 import re
+import json
+import shlex
 import jinja2
+import textwrap
+import warnings
 
 from .. import __version__
 from .._toolchain import *
@@ -202,6 +205,12 @@ class TemplatedPlatform(Platform):
             if defined {{platform._toolchain_env_var}} call "%{{platform._toolchain_env_var}}%"
             {{emit_commands("bat")}}
         """,
+        "build_{{name}}.json": """
+            {
+                "comment": "{{autogenerated}}",
+                "commands": {{emit_commands("json")}}
+            }
+        """,
     }
 
     def iter_signal_clock_constraints(self):
@@ -300,6 +309,8 @@ def emit_commands(syntax):
                     template = \
                         "if [!{env_var}!] equ [\"\"] set {env_var}=\n" \
                         "if [!{env_var}!] equ [] set {env_var}={name}"
+                elif syntax == "json":
+                    continue
                 else:
                     assert False
                 commands.append(template.format(env_var=env_var, name=name))
@@ -312,10 +323,15 @@ def emit_commands(syntax):
                     commands.append(command)
                 elif syntax == "bat":
                     commands.append(command + " || exit /b")
+                elif syntax == "json":
+                    commands.append(command)
                 else:
                     assert False
 
-            return "\n".join(commands) + "\n"
+            if syntax == "json":
+                return json.dumps([shlex.split(command) for command in commands])
+            else:
+                return "\n".join(commands) + "\n"
 
         @jinja2.pass_context
         def invoke_tool(context, name):
@@ -324,6 +340,8 @@ def invoke_tool(context, name):
                 return f"\"${env_var}\""
             elif context.parent["syntax"] == "bat":
                 return f"\"%{env_var}%\""
+            elif context.parent["syntax"] == "json":
+                return name
             else:
                 assert False