test: reorg #15698

Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua

test/functional/vimscript/let_spec.lua

@@ -0,0 +1,93 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local clear = helpers.clear
+local command = helpers.command
+local eval = helpers.eval
+local meths = helpers.meths
+local redir_exec = helpers.redir_exec
+local source = helpers.source
+local nvim_dir = helpers.nvim_dir
+
+before_each(clear)
+
+describe(':let', function()
+  it('correctly lists variables with curly-braces', function()
+    meths.set_var('v', {0})
+    eq('\nv                     [0]', redir_exec('let {"v"}'))
+  end)
+
+  it('correctly lists variables with subscript', function()
+    meths.set_var('v', {0})
+    eq('\nv[0]                  #0', redir_exec('let v[0]'))
+    eq('\ng:["v"][0]            #0', redir_exec('let g:["v"][0]'))
+    eq('\n{"g:"}["v"][0]        #0', redir_exec('let {"g:"}["v"][0]'))
+  end)
+
+  it(":unlet self-referencing node in a List graph #6070", function()
+    -- :unlet-ing a self-referencing List must not allow GC on indirectly
+    -- referenced in-scope Lists. Before #6070 this caused use-after-free.
+    source([=[
+      let [l1, l2] = [[], []]
+      echo 'l1:' . id(l1)
+      echo 'l2:' . id(l2)
+      echo ''
+      let [l3, l4] = [[], []]
+      call add(l4, l4)
+      call add(l4, l3)
+      call add(l3, 1)
+      call add(l2, l2)
+      call add(l2, l1)
+      call add(l1, 1)
+      unlet l2
+      unlet l4
+      call garbagecollect(1)
+      call feedkeys(":\e:echo l1 l3\n:echo 42\n:cq\n", "t")
+    ]=])
+  end)
+
+  it("multibyte env var #8398 #9267", function()
+    command("let $NVIM_TEST = 'AìaB'")
+    eq('AìaB', eval('$NVIM_TEST'))
+    command("let $NVIM_TEST = 'AaあB'")
+    eq('AaあB', eval('$NVIM_TEST'))
+    local mbyte = [[\p* .ม .ม .ม .ม่ .ม่ .ม่ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ ֹֻ
+                    .ֹֻ .ֹֻ .ֹֻ ֹֻ ֹֻ ֹֻ .ֹֻ .ֹֻ .ֹֻ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ
+                    .ֹֻ .ֹֻ .ֹֻ a a a ca ca ca à à à]]
+    command("let $NVIM_TEST = '"..mbyte.."'")
+    eq(mbyte, eval('$NVIM_TEST'))
+  end)
+
+  it("multibyte env var to child process #8398 #9267",  function()
+    local cmd_get_child_env = "let g:env_from_child = system(['"..nvim_dir.."/printenv-test', 'NVIM_TEST'])"
+    command("let $NVIM_TEST = 'AìaB'")
+    command(cmd_get_child_env)
+    eq(eval('$NVIM_TEST'), eval('g:env_from_child'))
+
+    command("let $NVIM_TEST = 'AaあB'")
+    command(cmd_get_child_env)
+    eq(eval('$NVIM_TEST'), eval('g:env_from_child'))
+
+    local mbyte = [[\p* .ม .ม .ม .ม่ .ม่ .ม่ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ ֹֻ
+                    .ֹֻ .ֹֻ .ֹֻ ֹֻ ֹֻ ֹֻ .ֹֻ .ֹֻ .ֹֻ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹ ֹ ֹ .ֹ .ֹ .ֹ ֹֻ ֹֻ
+                    .ֹֻ .ֹֻ .ֹֻ a a a ca ca ca à à à]]
+    command("let $NVIM_TEST = '"..mbyte.."'")
+    command(cmd_get_child_env)
+    eq(eval('$NVIM_TEST'), eval('g:env_from_child'))
+  end)
+
+  it("release of list assigned to l: variable does not trigger assertion #12387, #12430", function()
+    source([[
+      func! s:f()
+        let l:x = [1]
+        let g:x = l:
+      endfunc
+      for _ in range(2)
+        call s:f()
+      endfor
+      call garbagecollect()
+      call feedkeys('i', 't')
+    ]])
+    eq(1, eval('1'))
+  end)
+end)