eval: add v:_null_string

Replacement for Vim's test_null_string().
Vim uses it to verify that its codebase handles null strings.
Preparation for the Test_null_list() in patch v8.2.1822.

Use v:_null_string, not non-existent env var, for null string tests.

Mention v:_null_string in id() because id(v:_null_string) returns (nil).

runtime/doc/eval.txt

@@ -5563,13 +5563,14 @@ id({expr})						*id()*
 		Returns a |String| which is a unique identifier of the
 		container type (|List|, |Dict| and |Partial|). It is
 		guaranteed that for the mentioned types `id(v1) ==# id(v2)`
-		returns true iff `type(v1) == type(v2) && v1 is v2` (note:
+		returns true iff `type(v1) == type(v2) && v1 is v2`.
-		|v:_null_list| and |v:_null_dict| have the same `id()` with
+		Note that |v:_null_string|, |v:_null_list|, and |v:_null_dict|
-		different types because they are internally represented as
+		have the same `id()` with different types because they are
-		a NULL pointers). Currently `id()` returns a hexadecimal
+		internally represented as a NULL pointers.  `id()` returns a
-		representanion of the pointers to the containers (i.e. like
+		hexadecimal representanion of the pointers to the containers
-		`0x994a40`), same as `printf("%p", {expr})`, but it is advised
+		(i.e. like `0x994a40`), same as `printf("%p", {expr})`,
-		against counting on exact format of return value.
+		but it is advised against counting on the exact format of
+		return value.
 
 		It is not guaranteed that `id(no_longer_existing_container)`
 		will not be equal to some other `id()`: new containers may

src/nvim/eval.c

@@ -229,6 +229,7 @@ static struct vimvar {
   // Neovim
   VV(VV_STDERR,         "stderr",           VAR_NUMBER, VV_RO),
   VV(VV_MSGPACK_TYPES,  "msgpack_types",    VAR_DICT, VV_RO),
+  VV(VV__NULL_STRING,   "_null_string",     VAR_STRING, VV_RO),
   VV(VV__NULL_LIST,     "_null_list",       VAR_LIST, VV_RO),
   VV(VV__NULL_DICT,     "_null_dict",       VAR_DICT, VV_RO),
   VV(VV_LUA,            "lua",              VAR_PARTIAL, VV_RO),

src/nvim/eval.h

@@ -158,6 +158,7 @@ typedef enum {
     // Neovim
     VV_STDERR,
     VV_MSGPACK_TYPES,
+    VV__NULL_STRING,  // String with NULL value. For test purposes only.
     VV__NULL_LIST,  // List with NULL value. For test purposes only.
     VV__NULL_DICT,  // Dictionary with NULL value. For test purposes only.
     VV_LUA,

test/functional/eval/null_spec.lua

@@ -14,7 +14,8 @@ describe('NULL', function()
     clear()
     command('let L = v:_null_list')
     command('let D = v:_null_dict')
-    command('let S = $XXX_NONEXISTENT_VAR_XXX')
+    command('let S = v:_null_string')
+    command('let V = $XXX_NONEXISTENT_VAR_XXX')
   end)
   local tmpfname = 'Xtest-functional-viml-null'
   after_each(function()
@@ -129,6 +130,7 @@ describe('NULL', function()
     null_expr_test('is accepted by setloclist()', 'setloclist(1, L)', 0, 0)
     null_test('is accepted by :cexpr', 'cexpr L', 0)
     null_test('is accepted by :lexpr', 'lexpr L', 0)
+    null_expr_test('does not crash execute()', 'execute(L)', 0, '')
   end)
   describe('dict', function()
     it('does not crash when indexing NULL dict', function()
@@ -142,4 +144,26 @@ describe('NULL', function()
     null_expr_test('makes map() return v:_null_dict', 'map(D, "v:val") is# D', 0, 1)
     null_expr_test('makes filter() return v:_null_dict', 'filter(D, "1") is# D', 0, 1)
   end)
+  describe('string', function()
+    null_test('does not crash :echomsg', 'echomsg S', 0)
+    null_test('does not crash :execute', 'execute S', 0)
+    null_expr_test('does not crash execute()', 'execute(S)', 0, '')
+    null_expr_test('makes executable() error out', 'executable(S)', 'E928: String required', 0)
+    null_expr_test('does not crash filereadable()', 'filereadable(S)', 0, 0)
+    null_expr_test('does not crash filewritable()', 'filewritable(S)', 0, 0)
+    null_expr_test('does not crash fnamemodify()', 'fnamemodify(S, S)', 0, '')
+    null_expr_test('does not crash getfperm()', 'getfperm(S)', 0, '')
+    null_expr_test('does not crash getfsize()', 'getfsize(S)', 0, -1)
+    null_expr_test('does not crash getftime()', 'getftime(S)', 0, -1)
+    null_expr_test('does not crash getftype()', 'getftype(S)', 0, '')
+    null_expr_test('does not crash glob()', 'glob(S)', 0, '')
+    null_expr_test('does not crash globpath()', 'globpath(S, S)', 0, '')
+    null_expr_test('does not crash mkdir()', 'mkdir(S)', 0, 0)
+    null_expr_test('does not crash sort()', 'sort(["b", S, "a"])', 0, {'', 'a', 'b'})
+    null_expr_test('does not crash split()', 'split(S)', 0, {})
+
+    null_test('can be used to set an option', 'let &grepprg = S', 0)
+
+    null_expr_test('is equal to non-existent variable', 'S == V', 0, 1)
+  end)
 end)

test/functional/eval/writefile_spec.lua

@@ -59,6 +59,16 @@ describe('writefile()', function()
     eq('\n', read_file(fname))
   end)
 
+  it('writes list with a null string to a file', function()
+    eq(0, exc_exec(
+       ('call writefile([v:_null_string], "%s", "b")'):format(
+          fname)))
+    eq('', read_file(fname))
+    eq(0, exc_exec(('call writefile([v:_null_string], "%s")'):format(
+        fname)))
+    eq('\n', read_file(fname))
+  end)
+
   it('appends to a file', function()
     eq(nil, read_file(fname))
     eq(0, funcs.writefile({'abc', 'def', 'ghi'}, fname))