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).

1 files changed, 25 insertions, 1 deletions

diff --git a/test/functional/eval/null_spec.lua b/test/functional/eval/null_spec.lua
index fa8f7d873f..d403fbc878 100644
--- a/test/functional/eval/null_spec.lua
+++ b/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)