Extract out get_argsfiles_output function

Summary:
The existing `_get_argsfile_output` does two things:
1. creates the argsfile subtarget DefaultInfo provider
2. sets the `by_ext` attribute for use in xcode_data

Up the stack, #2 is replaced by the `CompileArgsfiles` record which contains a map of extension to `CompileArgsfile`.

Extracting out #1 allows us to restructure the code (later on) to
1. define the argsfile subtarget, DefaultInfo and action only for targets that want it
2. extract it out of compilation so that compilation code is simpler

Reviewed By: milend

Differential Revision: D46743454

fbshipit-source-id: 31a108410e49fb85851d91334ed598a10731e7d9

prelude/cxx/argsfiles.bzl

@@ -25,3 +25,16 @@ CompileArgsfiles = record(
     # Absolute path argsfiles used for extra outputs, mapped by extension.
     absolute = field({str.type: CompileArgsfile.type}, default = {}),
 )
+
+def get_argsfiles_output(ctx: "context", argsfile_by_ext: {str.type: CompileArgsfile.type}, summary_name: str.type) -> DefaultInfo.type:
+    argsfiles = []
+    argsfile_names = cmd_args()
+    dependent_outputs = []
+    for _, argsfile in argsfile_by_ext.items():
+        argsfiles.append(argsfile.file)
+        argsfile_names.add(cmd_args(argsfile.file).ignore_artifacts())
+        dependent_outputs.extend(argsfile.input_args)
+
+    argsfiles_summary = ctx.actions.write(summary_name, argsfile_names)
+
+    return DefaultInfo(default_outputs = [argsfiles_summary] + argsfiles, other_outputs = dependent_outputs)

prelude/cxx/compile.bzl

@@ -11,7 +11,7 @@ load(
     "@prelude//utils:utils.bzl",
     "flatten",
 )
-load(":argsfiles.bzl", "CompileArgsfile", "CompileArgsfiles")
+load(":argsfiles.bzl", "CompileArgsfile", "CompileArgsfiles", "get_argsfiles_output")
 load(":attr_selection.bzl", "cxx_by_language_ext")
 load(
     ":compiler.bzl",
@@ -268,20 +268,10 @@ def create_compile_cmds(
         )
 
 def _get_argsfile_output(ctx: "context", argsfile_by_ext: {str.type: CompileArgsfile.type}, summary_name: str.type) -> CxxCompileCommandArgsFiles.type:
-    argsfiles = []
-    argsfile_names = cmd_args()
-    dependent_outputs = []
-    argsfile_artifacts_by_ext = {}
-    for ext, argsfile in argsfile_by_ext.items():
-        argsfiles.append(argsfile.file)
-        argsfile_names.add(cmd_args(argsfile.file).ignore_artifacts())
-        dependent_outputs.extend(argsfile.input_args)
-        argsfile_artifacts_by_ext[ext] = argsfile.file
-
-    argsfiles_summary = ctx.actions.write(summary_name, argsfile_names)
+    argsfile_artifacts_by_ext = {ext: argsfile.file for ext, argsfile in argsfile_by_ext.items()}
 
     return CxxCompileCommandArgsFiles(
-        info = DefaultInfo(default_outputs = [argsfiles_summary] + argsfiles, other_outputs = dependent_outputs),
+        info = get_argsfiles_output(ctx, argsfile_by_ext, summary_name),
         by_ext = argsfile_artifacts_by_ext,
     )