eval/string(): Share code between msgpackdump() and string()+:echo

runtime/doc/eval.txt

@@ -7809,7 +7809,20 @@ This does NOT work: >
 			postponed until you type something), force a redraw
 			with the |:redraw| command.  Example: >
 		:new | redraw | echo "there is a new window"
-<
+<							*:echo-self-refer*
+			When printing nested containers echo prints second 
+			occurrence of the self-referencing container using 
+			"[...@level]" (self-referencing |List|) or 
+			"{...@level}" (self-referencing |Dict|): >
+		:let l = []
+		:call add(l, l)
+		:let l2 = []
+		:call add(l2, [l2])
+		:echo l l2
+<			echoes "[[...@0]] [[[...@0]]]". Echoing "[l]" will 
+			echo "[[[...@1]]]" because l first occurs at second 
+			level.
+
 							*:echon*
 :echon {expr1} ..	Echoes each {expr1}, without anything added.  Also see
 			|:comment|.

runtime/doc/vim_diff.txt

@@ -75,6 +75,24 @@ are always available and may be used simultaneously in separate plugins.  The
 
 'encoding' cannot be changed after startup.
 
+|string()| and |:echo| behaviour changed:
+1. No maximum recursion depth limit is applied to nested container 
+   structures.
+2. |string()| fails immediately on nested containers, not when recursion limit 
+   was exceeded.
+2. When |:echo| encounters duplicate containers like >
+
+       let l = []
+       echo [l, l]
+<
+   it does not use "[...]" (was: "[[], [...]]", now: "[[], []]"). "..." is 
+   only used for recursive containers.
+3. |:echo| printing nested containers adds "@level" after "..." designating 
+   the level at which recursive container was printed: |:echo-self-refer|.  
+   Same thing applies to |string()| (though it uses construct like 
+   "{E724@level}"), but this is not reliable because |string()| continues to 
+   error out.
+
 ==============================================================================
 4. New Features						     *nvim-features-new*