Bug 1895358 - Part 1: Improve formatting of long multiline argument lists, r=ipc-reviewers,mccr8

This changes the way that StmtCode handles the pattern `$,{list}` alone on a
line, adjusting it such that each item on the list is printed onto its own
line, and then indented. This helps the formatting of large lists such as the
ones generated in part 2.

Differential Revision: https://phabricator.services.mozilla.com/D209854

ipc/ipdl/ipdl/cxx/code.py

@@ -153,9 +153,18 @@ def _line(raw, skip_indent, lineno, context):
         inline = match.span() != (0, len(line))
 
         for idx, value in enumerate(values):
+            # When using `,` as a list mode in a non-inline environment, put
+            # each expression on its own line.
+            multiline_list = list_chr == "," and not inline
+
             # If we're using ',' as list mode, put a comma between each node.
             if idx > 0 and list_chr == ",":
-                children.append(VerbatimNode(", "))
+                children.append(VerbatimNode(",\n" if multiline_list else ", "))
+
+            # Expr entries such as those used in multiline lists do not indent,
+            # so add a node to handle indenting on each line.
+            if multiline_list:
+                children.append(VerbatimNode("", indent=True))
 
             # If our value isn't a node, turn it into one. Verbatim should be
             # inline unless indent isn't being skipped, and the match isn't