test(nvim): trim one trailing newline in dedent helper

The `helpers.dedent()` now trims one trailing newline by default.
Additionally, it now ignores lines that contain only whitespaces.
This change allows to ignore the last line of a Lua literal string.
For example:
    helpers.dedent([[
      line1
      line2
    ]])
will now return `line1\nline2` by default. This behavior can be
disabled by setting `opts.trim = false`.

tests/nvim/filetypes/mediawiki_test.lua

@@ -166,18 +166,20 @@ T["fenced languages"]["refreshed after buffer reloading"] = function()
 
   -- Set initial buffer contents
   set_lines(dedent([[
-        <!-- vim: set ft=mediawiki: -->
-        == Heading ==
-        ]]))
+    <!-- vim: set ft=mediawiki: -->
+    == Heading ==
+
+  ]]))
   cmd.write(f)
   cmd.edit()
 
   -- Add new syntaxhighlight region
   set_lines(
     dedent([[
-          <syntaxhighlight lang="yaml">
-          key: 123
-          </syntaxhighlight>]]),
+      <syntaxhighlight lang="yaml">
+      key: 123
+      </syntaxhighlight>
+    ]]),
     { start = -1 }
   )
   cmd.write()

tests/nvim/runtime/lua/test/helpers.lua

@@ -4,30 +4,41 @@ local M = {}
 M.expect = require("test.expect")
 M.new_child = require("test.Child").new
 
---- Remove common indent from a given string
----@param str string
+---@class test.helpers.dedent.opts
+---@field trim? boolean Trim last trailing newline. Default: `true`.
+
+--- Remove common indent from a given string.
+--- One trailing newline is trimmed by default.
+--- Can be disabled with `opts.trim = false`.
+--- Empty lines (lines containing only whitespaces) are ignored.
+---@param str string Input string
+---@param opts? test.helpers.dedent.opts Options
 ---@return string
-function M.dedent(str)
+function M.dedent(str, opts)
+  opts = opts or {}
+  local trim = vim.F.if_nil(opts.trim, true)
+
   -- Find minimal indent
   local minimal_indent = math.huge
   for s in vim.gsplit(str, "\n", { plain = true, trimempty = true }) do
     if s ~= "" then
       local _, indent = s:find("^%s*")
       if indent == 0 then
         -- No indent, return early
-        return str
-      elseif indent and indent < minimal_indent then
+        return str:sub(-1) == "\n" and trim and str:sub(1, -2) or str
+      elseif indent and indent < minimal_indent and indent ~= #s then
         minimal_indent = indent
       end
     end
   end
 
-  return vim
+  local out = vim
     .iter(vim.gsplit(str, "\n", { plain = true }))
     :map(function(s)
       return s:sub(minimal_indent + 1)
     end)
     :join("\n")
+  return out:sub(-1) == "\n" and trim and out:sub(1, -2) or out
 end
 
 --- Wrap every value in the table into another table,

tests/nvim/skeletons_test.lua

@@ -40,7 +40,8 @@ local skeletons = {
 
 
 
-      return T]]),
+      return T
+    ]]),
     cursor_line = 17,
   },
 }

tests/nvim/test/helpers_test.lua

@@ -9,25 +9,37 @@ local T = new_set()
 T["dedent"] = new_set({
   parametrize = {
     -- Empty input
-    { "", "" },
+    { { "" }, "" },
     -- No indent
-    { "a\nb", "a\nb" },
+    { { "a\nb" }, "a\nb" },
     -- Common indent
-    { "  a\n  b", "a\nb" },
+    { { "  a\n  b" }, "a\nb" },
     -- Not all lines are indented
-    { "  a\nb", "  a\nb" },
-    { "a\n  b", "a\n  b" },
+    { { "  a\nb" }, "  a\nb" },
+    { { "a\n  b" }, "a\n  b" },
     -- Different indent
-    { "    a\n      b\n        c", "a\n  b\n    c" },
+    { { "    a\n      b\n        c" }, "a\n  b\n    c" },
     -- Empty lines
-    { "  a\n\n  b\n", "a\n\nb\n" },
+    { { "  a\n\n  b\n  c" }, "a\n\nb\nc" },
     -- Tabs
-    { "\ta\n\tb", "a\nb" },
-    { "\t\ta\n\tb", "\ta\nb" },
+    { { "\ta\n\tb" }, "a\nb" },
+    { { "\t\ta\n\tb" }, "\ta\nb" },
+    -- Ignore less-indented spaces
+    { { "    a\n  \n    b" }, "a\n\nb" },
+    -- Trims last newline
+    { { "a\nb\n" }, "a\nb" },
+    -- Trims only one newline
+    { { "  a\n\n  b\n\n\n" }, "a\n\nb\n\n" },
+    -- Last empty and less-indented line is trimmed
+    { { "    a\n      b\n    c\n  " }, "a\n  b\nc" },
+    { { "    a\n      b\n    c\n\n  " }, "a\n  b\nc\n" },
+    -- '{opts.trim = false}'
+    { { "a\nb\n", { trim = false } }, "a\nb\n" },
+    { { "  a\n  \n  b\n", { trim = false } }, "a\n\nb\n" },
   },
 }, {
   test = function(input, expectation)
-    eq(helpers.dedent(input), expectation)
+    eq(helpers.dedent(unpack(input)), expectation)
   end,
 })