test(vim.text): add test cases for error conditions

runtime/lua/vim/text.lua

@@ -41,6 +41,9 @@ function M.hexdecode(enc)
   for i = 1, #enc, 2 do
     local u = lookup[enc:sub(i, i)] --[[@as integer]]
     local l = lookup[enc:sub(i + 1, i + 1)] --[[@as integer]]
+    if not u or not l then
+      return nil, 'string must contain only hex characters'
+    end
     str[#str + 1] = string.char(u * 16 + l)
   end
   return table.concat(str), nil

test/functional/lua/text_spec.lua

@@ -26,5 +26,21 @@ describe('vim.text', function()
       eq(output, vim.text.hexencode(input))
       eq(input, vim.text.hexdecode(output))
     end)
+
+    it('errors on invalid input', function()
+      -- Odd number of hex characters
+      do
+        local res, err = vim.text.hexdecode('ABC')
+        eq(nil, res)
+        eq('string must have an even number of hex characters', err)
+      end
+
+      -- Non-hexadecimal input
+      do
+        local res, err = vim.text.hexdecode('nothex')
+        eq(nil, res)
+        eq('string must contain only hex characters', err)
+      end
+    end)
   end)
 end)