diff --git a/runtime/lua/vim/shared.lua b/runtime/lua/vim/shared.lua
new file mode 100644
index 0000000000..9c49d93596
--- /dev/null
+++ b/runtime/lua/vim/shared.lua
@@ -0,0 +1,39 @@
+--- Shared functions
+--    - Used by Nvim and tests
+--    - Can run in vanilla Lua (do not require a running instance of Nvim)
+
+
+--- Merge map-like tables.
+--
+--@see |extend()|
+--
+-- behavior: Decides what to do if a key is found in more than one map:
+--           "error": raise an error
+--           "keep":  skip
+--           "force": set the item again
+local function tbl_extend(behavior, ...)
+  if (behavior ~= 'error' and behavior ~= 'keep' and behavior ~= 'force') then
+    error('invalid "behavior": '..tostring(behavior))
+  end
+  local ret = {}
+  for i = 1, select('#', ...) do
+    local tbl = select(i, ...)
+    if tbl then
+      for k, v in pairs(tbl) do
+        if behavior ~= 'force' and ret[k] ~= nil then
+          if behavior == 'error' then
+            error('key found in more than one map: '..k)
+          end  -- Else behavior is "keep".
+        else
+          ret[k] = v
+        end
+      end
+    end
+  end
+  return ret
+end
+
+local module = {
+  tbl_extend = tbl_extend,
+}
+return module
diff --git a/src/nvim/lua/vim.lua b/src/nvim/lua/vim.lua
index b0d0bfc74b..1a7aec6cc6 100644
--- a/src/nvim/lua/vim.lua
+++ b/src/nvim/lua/vim.lua
@@ -1,3 +1,39 @@
+-- Nvim-Lua stdlib: the `vim` module (:help lua-stdlib)
+--
+-- Lua code lives in one of three places:
+--    1. The runtime (`runtime/lua/vim/`). For "nice to have" features, e.g.
+--       the `inspect` and `lpeg` modules.
+--    2. The `vim.shared` module: code shared between Nvim and its test-suite.
+--    3. Compiled-into Nvim itself (`src/nvim/lua/`).
+--
+-- Guideline: "If in doubt, put it in the runtime".
+--
+-- Most functions should live directly on `vim.`, not sub-modules. The only
+-- "forbidden" names are those claimed by legacy `if_lua`:
+--    $ vim
+--    :lua for k,v in pairs(vim) do print(k) end
+--    buffer
+--    open
+--    window
+--    lastline
+--    firstline
+--    type
+--    line
+--    eval
+--    dict
+--    beep
+--    list
+--    command
+--
+-- Reference (#6580):
+--    - https://github.com/luafun/luafun
+--    - https://github.com/rxi/lume
+--    - http://leafo.net/lapis/reference/utilities.html
+--    - https://github.com/torch/paths
+--    - https://github.com/bakpakin/Fennel (pretty print, repl)
+--    - https://github.com/howl-editor/howl/tree/master/lib/howl/util
+
+
 -- Internal-only until comments in #8107 are addressed.
 -- Returns:
 --    {errcode}, {output}
@@ -187,10 +223,14 @@ deepcopy = function(orig)
   return deepcopy_funcs[type(orig)](orig)
 end
 
-local function __index(table, key)
-  if key == "inspect" then
-    table.inspect = require("vim.inspect")
-    return table.inspect
+local function __index(t, key)
+  if key == 'inspect' then
+    t.inspect = require('vim.inspect')
+    return t.inspect
+  elseif require('vim.shared')[key] ~= nil then
+    -- Expose all `vim.shared` functions on the `vim` module.
+    t[key] = require('vim.shared')[key]
+    return t[key]
   end
 end
 
diff --git a/test/functional/helpers.lua b/test/functional/helpers.lua
index 00acd12247..ae11455b3e 100644
--- a/test/functional/helpers.lua
+++ b/test/functional/helpers.lua
@@ -846,7 +846,7 @@ local module = {
   window = window,
   winmeths = winmeths,
 }
-module = global_helpers.map_extend('error', module, global_helpers)
+module = global_helpers.tbl_extend('error', module, global_helpers)
 
 return function(after_each)
   if after_each then
diff --git a/test/helpers.lua b/test/helpers.lua
index 966b7b408f..e987fa69db 100644
--- a/test/helpers.lua
+++ b/test/helpers.lua
@@ -1,4 +1,5 @@
 require('vim.compat')
+local shared = require('vim.shared')
 local assert = require('luassert')
 local luv = require('luv')
 local lfs = require('lfs')
@@ -435,34 +436,6 @@ local function concat_tables(...)
   return ret
 end
 
--- Concat map-like tables.
---
--- behavior: Decides what to do if a key is found in more than one map:
---           "error": raise an error
---           "keep":  skip
---           "force": set the item again
-local function map_extend(behavior, ...)
-  if (behavior ~= 'error' and behavior ~= 'keep' and behavior ~= 'force') then
-    error('invalid "behavior": '..tostring(behavior))
-  end
-  local ret = {}
-  for i = 1, select('#', ...) do
-    local tbl = select(i, ...)
-    if tbl then
-      for k, v in pairs(tbl) do
-        if behavior ~= 'force' and ret[k] ~= nil then
-          if behavior == 'error' then
-            error('key found in more than one map: '..k)
-          end  -- Else behavior is "keep".
-        else
-          ret[k] = v
-        end
-      end
-    end
-  end
-  return ret
-end
-
 local function dedent(str, leave_indent)
   -- find minimum common indent across lines
   local indent = nil
@@ -801,7 +774,6 @@ local module = {
   intchar2lua = intchar2lua,
   isCI = isCI,
   map = map,
-  map_extend = map_extend,
   matches = matches,
   mergedicts_copy = mergedicts_copy,
   near = near,
@@ -816,6 +788,7 @@ local module = {
   shallowcopy = shallowcopy,
   sleep = sleep,
   table_contains = table_contains,
+  tbl_extend = shared.tbl_extend,
   table_flatten = table_flatten,
   tmpname = tmpname,
   uname = uname,
@@ -823,6 +796,6 @@ local module = {
   which = which,
   write_file = write_file,
 }
-module = map_extend('error', module, Paths)
+module = shared.tbl_extend('error', module, Paths)
 
 return module
diff --git a/test/unit/helpers.lua b/test/unit/helpers.lua
index c42eddf7f3..e634b7296e 100644
--- a/test/unit/helpers.lua
+++ b/test/unit/helpers.lua
@@ -865,7 +865,7 @@ local module = {
   ptr2key = ptr2key,
   debug_log = debug_log,
 }
-module = global_helpers.map_extend('error', module, global_helpers)
+module = global_helpers.tbl_extend('error', module, global_helpers)
 return function()
   return module
 end