[word_eval refactor] Add comments and extract variable

Author: Andy C

core/runtime.asdl

@@ -40,9 +40,12 @@ module runtime
            List[str] argv, List[CompoundWord] arg_locs,
            List[AssignArg] pairs)
 
-  # - Single or double quoted parts get neither split or globbed.
-  # - Bare words like echo or *.py are globbed, but NOT split with IFS.
-  # - Unquoted Substitutions are split and globbed.
+  # A Piece is a part of a word that's been evaluated
+  #   Bare word      | echo *.py   | quoted=False  do_split=False
+  #   Unquoted subst | $x   ${x}   | quoted=False  do_split=True
+  #   Quoted string  | 'sq' "$x"   | quoted=True   do_split=False
+  # The 'quoted' flag is used to determine whether globbing will occur, as well
+  # as empty elision like x=''; $x$x versus "$x"$x
   Piece = (str s, bool quoted, bool do_split)
 
   # A parse-time word_part from syntax.asdl is evaluated to a runtime

osh/word_eval.py

@@ -1798,12 +1798,13 @@ def _TranslateExtGlob(self, part_vals, w, glob_parts, fnmatch_parts):
         We need both glob and fnmatch patterns.  _EvalExtGlob does the
         flattening.
         """
+        will_glob = not self.exec_opts.noglob()
         for i, part_val in enumerate(part_vals):
             UP_part_val = part_val
             with tagswitch(part_val) as case:
                 if case(part_value_e.String):
                     part_val = cast(Piece, UP_part_val)
-                    if part_val.quoted and not self.exec_opts.noglob():
+                    if will_glob and part_val.quoted:
                         s = glob_.GlobEscape(part_val.s)
                     else:
                         # e.g. the @( and | in @(foo|bar) aren't quoted