diff --git a/scripts/lintcommit.lua b/scripts/lintcommit.lua
index 7f3dffd22c..f9a4631b7e 100644
--- a/scripts/lintcommit.lua
+++ b/scripts/lintcommit.lua
@@ -88,7 +88,7 @@ local function validate_commit(commit_message)
   -- Check if type is correct
   local type = vim.split(before_colon, "(", {plain = true})[1]
   local allowed_types = {'build', 'ci', 'docs', 'feat', 'fix', 'perf', 'refactor', 'revert', 'test', 'vim-patch'}
-  if not vim.list_contains(allowed_types, type) then
+  if not vim.tbl_contains(allowed_types, type) then
     return string.format(
       [[Invalid commit type "%s". Allowed types are:
       %s.
diff --git a/test/helpers.lua b/test/helpers.lua
index 34c2036798..8f06311a3c 100644
--- a/test/helpers.lua
+++ b/test/helpers.lua
@@ -20,12 +20,16 @@ local module = {
   REMOVE_THIS = {},
 }
 
+--- @param p string
+--- @return string
 local function relpath(p)
   p = vim.fs.normalize(p)
   local cwd = luv.cwd()
   return p:gsub("^" .. cwd)
 end
 
+--- @param path string
+--- @return boolean
 function module.isdir(path)
   if not path then
     return false
@@ -37,6 +41,8 @@ function module.isdir(path)
   return stat.type == 'directory'
 end
 
+--- @param path string
+--- @return boolean
 function module.isfile(path)
   if not path then
     return false
@@ -48,6 +54,7 @@ function module.isfile(path)
   return stat.type == 'file'
 end
 
+--- @return string
 function module.argss_to_cmd(...)
   local cmd = ''
   for i = 1, select('#', ...) do
@@ -462,6 +469,7 @@ function module.check_cores(app, force) -- luacheck: ignore
   end
 end
 
+--- @return string?
 function module.repeated_read_cmd(...)
   for _ = 1, 10 do
     local stream = module.popen_r(...)
@@ -561,6 +569,9 @@ function module.concat_tables(...)
   return ret
 end
 
+--- @param str string
+--- @param leave_indent? boolean
+--- @return string
 function module.dedent(str, leave_indent)
   -- find minimum common indent across lines
   local indent = nil
diff --git a/test/unit/formatc.lua b/test/unit/formatc.lua
index 2fd37c599a..c94f0d88f7 100644
--- a/test/unit/formatc.lua
+++ b/test/unit/formatc.lua
@@ -154,6 +154,8 @@ local C_keywords = set {  -- luacheck: ignore
 --
 --  The first one will have a lot of false positives (the line '{' for
 --  example), the second one is more unique.
+--- @param string
+--- @return string
 local function formatc(str)
   local toks = TokeniseC(str)
   local result = {}
diff --git a/test/unit/helpers.lua b/test/unit/helpers.lua
index 708929ad9f..10b7594a88 100644
--- a/test/unit/helpers.lua
+++ b/test/unit/helpers.lua
@@ -14,20 +14,15 @@ local map = global_helpers.tbl_map
 local eq = global_helpers.eq
 local trim = global_helpers.trim
 
--- C constants.
-local NULL = ffi.cast('void*', 0)
-
-local OK = 1
-local FAIL = 0
-
-local cimport
-
 -- add some standard header locations
 for _, p in ipairs(Paths.include_paths) do
   Preprocess.add_to_include_path(p)
 end
 
-local child_pid = nil
+local child_pid = nil --- @type integer
+--- @generic F: function
+--- @param func F
+--- @return F
 local function only_separate(func)
   return function(...)
     if child_pid ~= 0 then
@@ -36,9 +31,20 @@ local function only_separate(func)
     return func(...)
   end
 end
-local child_calls_init = {}
-local child_calls_mod = nil
-local child_calls_mod_once = nil
+
+--- @class ChildCall
+--- @field func function
+--- @field args any[]
+
+--- @class ChildCallLog
+--- @field func string
+--- @field args any[]
+--- @field ret any?
+
+local child_calls_init = {} --- @type ChildCall[]
+local child_calls_mod = nil --- @type ChildCall[]
+local child_calls_mod_once = nil --- @type ChildCall[]?
+
 local function child_call(func, ret)
   return function(...)
     local child_calls = child_calls_mod or child_calls_init
@@ -53,16 +59,16 @@ end
 
 -- Run some code at the start of the child process, before running the test
 -- itself. Is supposed to be run in `before_each`.
+--- @param func function
 local function child_call_once(func, ...)
   if child_pid ~= 0 then
-    child_calls_mod_once[#child_calls_mod_once + 1] = {
-      func=func, args={...}}
+    child_calls_mod_once[#child_calls_mod_once + 1] = { func = func, args = {...} }
   else
     func(...)
   end
 end
 
-local child_cleanups_mod_once = nil
+local child_cleanups_mod_once = nil --- @type ChildCall[]?
 
 -- Run some code at the end of the child process, before exiting. Is supposed to
 -- be run in `before_each` because `after_each` is run after child has exited.
@@ -125,8 +131,9 @@ local pragma_pack_id = 1
 
 -- some things are just too complex for the LuaJIT C parser to digest. We
 -- usually don't need them anyway.
+--- @param body string
 local function filter_complex_blocks(body)
-  local result = {}
+  local result = {} --- @type string[]
 
   for line in body:gmatch("[^\r\n]+") do
     if not (string.find(line, "(^)", 1, true) ~= nil
@@ -153,18 +160,20 @@ typedef struct { char bytes[16]; } __attribute__((aligned(16))) __uint128_t;
 typedef struct { char bytes[16]; } __attribute__((aligned(16))) __float128;
 ]]
 
-local preprocess_cache_init = {}
+local preprocess_cache_init = {} --- @type table<string,string>
 local previous_defines_mod = ''
-local preprocess_cache_mod = nil
+local preprocess_cache_mod = nil --- @type table<string,string>
 
 local function is_child_cdefs()
-  return (os.getenv('NVIM_TEST_MAIN_CDEFS') ~= '1')
+  return os.getenv('NVIM_TEST_MAIN_CDEFS') ~= '1'
 end
 
 -- use this helper to import C files, you can pass multiple paths at once,
 -- this helper will return the C namespace of the nvim library.
-cimport = function(...)
-  local previous_defines, preprocess_cache, cdefs
+local function cimport(...)
+  local previous_defines --- @type string
+  local preprocess_cache --- @type table<string,string>
+  local cdefs
   if is_child_cdefs() and preprocess_cache_mod then
     preprocess_cache = preprocess_cache_mod
     previous_defines = previous_defines_mod
@@ -180,7 +189,7 @@ cimport = function(...)
       path = './' .. path
     end
     if not preprocess_cache[path] then
-      local body
+      local body --- @type string
       body, previous_defines = Preprocess.preprocess(previous_defines, path)
       -- format it (so that the lines are "unique" statements), also filter out
       -- Objective-C blocks
@@ -202,6 +211,7 @@ cimport = function(...)
         -- (they are needed in the right order with the struct definitions,
         -- otherwise luajit has wrong memory layouts for the sturcts)
         if line:match("#pragma%s+pack") then
+          --- @type string
           line = line .. " // " .. pragma_pack_id
           pragma_pack_id = pragma_pack_id + 1
         end
@@ -229,20 +239,21 @@ cimport = function(...)
   return lib
 end
 
-local cimport_immediate = function(...)
+local function cimport_immediate(...)
   local saved_pid = child_pid
   child_pid = 0
   local err, emsg = pcall(cimport, ...)
   child_pid = saved_pid
   if not err then
-    emsg = tostring(emsg)
-    io.stderr:write(emsg .. '\n')
+    io.stderr:write(tostring(emsg) .. '\n')
     assert(false)
   else
     return lib
   end
 end
 
+--- @param preprocess_cache table<string,string[]>
+--- @param path string
 local function _cimportstr(preprocess_cache, path)
   if imported:contains(path) then
     return lib
@@ -265,12 +276,14 @@ end
 
 local function alloc_log_new()
   local log = {
-    log={},
-    lib=cimport('./src/nvim/memory.h'),
-    original_functions={},
+    log={}, --- @type ChildCallLog[]
+    lib=cimport('./src/nvim/memory.h'), --- @type table<string,function>
+    original_functions={}, --- @type table<string,function>
     null={['\0:is_null']=true},
   }
+
   local allocator_functions = {'malloc', 'free', 'calloc', 'realloc'}
+
   function log:save_original_functions()
     for _, funcname in ipairs(allocator_functions) do
       if not self.original_functions[funcname] then
@@ -278,13 +291,16 @@ local function alloc_log_new()
       end
     end
   end
+
   log.save_original_functions = child_call(log.save_original_functions)
+
   function log:set_mocks()
     for _, k in ipairs(allocator_functions) do
       do
         local kk = k
         self.lib['mem_' .. k] = function(...)
-          local log_entry = {func=kk, args={...}}
+          --- @type ChildCallLog
+          local log_entry = { func = kk, args = {...} }
           self.log[#self.log + 1] = log_entry
           if kk == 'free' then
             self.original_functions[kk](...)
@@ -305,17 +321,21 @@ local function alloc_log_new()
       end
     end
   end
+
   log.set_mocks = child_call(log.set_mocks)
+
   function log:clear()
     self.log = {}
   end
+
   function log:check(exp)
     eq(exp, self.log)
     self:clear()
   end
+
   function log:clear_tmp_allocs(clear_null_frees)
-    local toremove = {}
-    local allocs = {}
+    local toremove = {} --- @type integer[]
+    local allocs = {} --- @type table<string,integer>
     for i, v in ipairs(self.log) do
       if v.func == 'malloc' or v.func == 'calloc' then
         allocs[tostring(v.ret)] = i
@@ -338,26 +358,20 @@ local function alloc_log_new()
       table.remove(self.log, toremove[i])
     end
   end
-  function log:restore_original_functions()
-    -- Do nothing: set mocks live in a separate process
-    return
-    --[[
-       [ for k, v in pairs(self.original_functions) do
-       [   self.lib['mem_' .. k] = v
-       [ end
-       ]]
-  end
+
   function log:setup()
     log:save_original_functions()
     log:set_mocks()
   end
+
   function log:before_each()
-    return
   end
+
   function log:after_each()
-    log:restore_original_functions()
   end
+
   log:setup()
+
   return log
 end
 
@@ -374,98 +388,109 @@ local function to_cstr(string)
 end
 
 cimport_immediate('./test/unit/fixtures/posix.h')
-local sc = {
-  fork = function()
-    return tonumber(ffi.C.fork())
-  end,
-  pipe = function()
-    local ret = ffi.new('int[2]', {-1, -1})
-    ffi.errno(0)
-    local res = ffi.C.pipe(ret)
-    if (res ~= 0) then
-      local err = ffi.errno(0)
-      assert(res == 0, ("pipe() error: %u: %s"):format(
-      err, ffi.string(ffi.C.strerror(err))))
-    end
-    assert(ret[0] ~= -1 and ret[1] ~= -1)
-    return ret[0], ret[1]
-  end,
-  read = function(rd, len)
-    local ret = ffi.new('char[?]', len, {0})
-    local total_bytes_read = 0
-    ffi.errno(0)
-    while total_bytes_read < len do
-      local bytes_read = tonumber(ffi.C.read(
-      rd,
-      ffi.cast('void*', ret + total_bytes_read),
-      len - total_bytes_read))
-      if bytes_read == -1 then
-        local err = ffi.errno(0)
-        if err ~= ffi.C.kPOSIXErrnoEINTR then
-          assert(false, ("read() error: %u: %s"):format(
-          err, ffi.string(ffi.C.strerror(err))))
-        end
-      elseif bytes_read == 0 then
-        break
-      else
-        total_bytes_read = total_bytes_read + bytes_read
-      end
-    end
-    return ffi.string(ret, total_bytes_read)
-  end,
-  write = function(wr, s)
-    local wbuf = to_cstr(s)
-    local total_bytes_written = 0
-    ffi.errno(0)
-    while total_bytes_written < #s do
-      local bytes_written = tonumber(ffi.C.write(
-      wr,
-      ffi.cast('void*', wbuf + total_bytes_written),
-      #s - total_bytes_written))
-      if bytes_written == -1 then
-        local err = ffi.errno(0)
-        if err ~= ffi.C.kPOSIXErrnoEINTR then
-          assert(false, ("write() error: %u: %s ('%s')"):format(
-          err, ffi.string(ffi.C.strerror(err)), s))
-        end
-      elseif bytes_written == 0 then
-        break
-      else
-        total_bytes_written = total_bytes_written + bytes_written
-      end
-    end
-    return total_bytes_written
-  end,
-  close = ffi.C.close,
-  wait = function(pid)
-    ffi.errno(0)
-    local stat_loc = ffi.new('int[1]', {0})
-    while true do
-      local r = ffi.C.waitpid(pid, stat_loc, ffi.C.kPOSIXWaitWUNTRACED)
-      if r == -1 then
-        local err = ffi.errno(0)
-        if err == ffi.C.kPOSIXErrnoECHILD then
-          break
-        elseif err ~= ffi.C.kPOSIXErrnoEINTR then
-          assert(false, ("waitpid() error: %u: %s"):format(
-          err, ffi.string(ffi.C.strerror(err))))
-        end
-      else
-        assert(r == pid)
-      end
-    end
-    return stat_loc[0]
-  end,
-  exit = ffi.C._exit,
-}
 
-local function format_list(lst)
-  local ret = ''
-  for _, v in ipairs(lst) do
-    if ret ~= '' then ret = ret .. ', ' end
-    ret = ret .. assert:format({v, n=1})[1]
+local sc = {}
+
+function sc.fork()
+  return tonumber(ffi.C.fork())
+end
+
+function sc.pipe()
+  local ret = ffi.new('int[2]', {-1, -1})
+  ffi.errno(0)
+  local res = ffi.C.pipe(ret)
+  if (res ~= 0) then
+    local err = ffi.errno(0)
+    assert(res == 0, ("pipe() error: %u: %s"):format(
+    err, ffi.string(ffi.C.strerror(err))))
   end
-  return ret
+  assert(ret[0] ~= -1 and ret[1] ~= -1)
+  return ret[0], ret[1]
+end
+
+--- @return string
+function sc.read(rd, len)
+  local ret = ffi.new('char[?]', len, {0})
+  local total_bytes_read = 0
+  ffi.errno(0)
+  while total_bytes_read < len do
+    local bytes_read = tonumber(ffi.C.read(
+    rd,
+    ffi.cast('void*', ret + total_bytes_read),
+    len - total_bytes_read))
+    if bytes_read == -1 then
+      local err = ffi.errno(0)
+      if err ~= ffi.C.kPOSIXErrnoEINTR then
+        assert(false, ("read() error: %u: %s"):format(
+        err, ffi.string(ffi.C.strerror(err))))
+      end
+    elseif bytes_read == 0 then
+      break
+    else
+      total_bytes_read = total_bytes_read + bytes_read
+    end
+  end
+  return ffi.string(ret, total_bytes_read)
+end
+
+function sc.write(wr, s)
+  local wbuf = to_cstr(s)
+  local total_bytes_written = 0
+  ffi.errno(0)
+  while total_bytes_written < #s do
+    local bytes_written = tonumber(ffi.C.write(
+    wr,
+    ffi.cast('void*', wbuf + total_bytes_written),
+    #s - total_bytes_written))
+    if bytes_written == -1 then
+      local err = ffi.errno(0)
+      if err ~= ffi.C.kPOSIXErrnoEINTR then
+        assert(false, ("write() error: %u: %s ('%s')"):format(
+        err, ffi.string(ffi.C.strerror(err)), s))
+      end
+    elseif bytes_written == 0 then
+      break
+    else
+      total_bytes_written = total_bytes_written + bytes_written
+    end
+  end
+  return total_bytes_written
+end
+
+sc.close = ffi.C.close
+
+--- @param pid integer
+--- @return integer
+function sc.wait(pid)
+  ffi.errno(0)
+  local stat_loc = ffi.new('int[1]', {0})
+  while true do
+    local r = ffi.C.waitpid(pid, stat_loc, ffi.C.kPOSIXWaitWUNTRACED)
+    if r == -1 then
+      local err = ffi.errno(0)
+      if err == ffi.C.kPOSIXErrnoECHILD then
+        break
+      elseif err ~= ffi.C.kPOSIXErrnoEINTR then
+        assert(false, ("waitpid() error: %u: %s"):format(
+        err, ffi.string(ffi.C.strerror(err))))
+      end
+    else
+      assert(r == pid)
+    end
+  end
+  return stat_loc[0]
+end
+
+sc.exit = ffi.C._exit
+
+--- @param lst string[]
+--- @return string
+local function format_list(lst)
+  local ret = {} --- @type string[]
+  for _, v in ipairs(lst) do
+    ret[#ret+1] = assert:format({v, n=1})[1]
+  end
+  return table.concat(ret, ', ')
 end
 
 if os.getenv('NVIM_TEST_PRINT_SYSCALLS') == '1' then
@@ -513,19 +538,26 @@ local tracehelp = dedent([[
 ]])
 
 local function child_sethook(wr)
-  local trace_level = os.getenv('NVIM_TEST_TRACE_LEVEL')
-  if not trace_level or trace_level == '' then
-    trace_level = 0
-  else
-    trace_level = tonumber(trace_level)
+  local trace_level_str = os.getenv('NVIM_TEST_TRACE_LEVEL')
+  local trace_level = 0
+  if trace_level_str and trace_level_str ~= '' then
+    --- @type number
+    trace_level = assert(tonumber(trace_level_str))
   end
+
   if trace_level <= 0 then
     return
   end
+
   local trace_only_c = trace_level <= 1
+  --- @type debuginfo?, string?, integer
   local prev_info, prev_reason, prev_lnum
+
+  --- @param reason string
+  --- @param lnum integer
+  --- @param use_prev boolean
   local function hook(reason, lnum, use_prev)
-    local info = nil
+    local info = nil --- @type debuginfo?
     if use_prev then
       info = prev_info
     elseif reason ~= 'tail return' then  -- tail return
@@ -533,6 +565,7 @@ local function child_sethook(wr)
     end
 
     if trace_only_c and (not info or info.what ~= 'C') and not use_prev then
+      --- @cast info -nil
       if info.source:sub(-9) == '_spec.lua' then
         prev_info = info
         prev_reason = 'saved'
@@ -573,12 +606,8 @@ local function child_sethook(wr)
     end
 
     -- assert(-1 <= lnum and lnum <= 99999)
-    local lnum_s
-    if lnum == -1 then
-      lnum_s = 'nknwn'
-    else
-      lnum_s = ('%u'):format(lnum)
-    end
+    local lnum_s = lnum == -1 and 'nknwn' or ('%u'):format(lnum)
+    --- @type string
     local msg = (  -- lua does not support %*
       ''
       .. msgchar
@@ -600,6 +629,7 @@ end
 
 local trace_end_msg = ('E%s\n'):format((' '):rep(hook_msglen - 2))
 
+--- @type function
 local _debug_log
 
 local debug_log = only_separate(function(...)
@@ -607,6 +637,7 @@ local debug_log = only_separate(function(...)
 end)
 
 local function itp_child(wr, func)
+  --- @param s string
   _debug_log = function(s)
     s = s:sub(1, hook_msglen - 2)
     sc.write(wr, '>' .. s .. (' '):rep(hook_msglen - 2 - #s) .. '\n')
@@ -638,7 +669,7 @@ local function itp_child(wr, func)
 end
 
 local function check_child_err(rd)
-  local trace = {}
+  local trace = {} --- @type string[]
   local did_traceline = false
   local maxtrace = tonumber(os.getenv('NVIM_TEST_MAXTRACE')) or 1024
   while true do
@@ -668,11 +699,14 @@ local function check_child_err(rd)
       local len = tonumber(len_s)
       neq(0, len)
       if os.getenv('NVIM_TEST_TRACE_ON_ERROR') == '1' and #trace ~= 0 then
+        --- @type string
         err = '\nTest failed, trace:\n' .. tracehelp
         for _, traceline in ipairs(trace) do
+          --- @type string
           err = err .. traceline
         end
       end
+      --- @type string
       err = err .. sc.read(rd, len + 1)
     end
     local eres = sc.read(rd, 2)
@@ -686,10 +720,12 @@ local function check_child_err(rd)
         end
       end
       if not did_traceline then
+        --- @type string
         err = err .. '\nNo end of trace occurred'
       end
       local cc_err, cc_emsg = pcall(check_cores, Paths.test_luajit_prg, true)
       if not cc_err then
+        --- @type string
         err = err .. '\ncheck_cores failed: ' .. cc_emsg
       end
     end
@@ -822,9 +858,9 @@ local module = {
   lib = lib,
   cstr = cstr,
   to_cstr = to_cstr,
-  NULL = NULL,
-  OK = OK,
-  FAIL = FAIL,
+  NULL = ffi.cast('void*', 0),
+  OK = 1,
+  FAIL = 0,
   alloc_log_new = alloc_log_new,
   gen_itp = gen_itp,
   only_separate = only_separate,
diff --git a/test/unit/preprocess.lua b/test/unit/preprocess.lua
index 88f7a14d1f..c5fb62dce5 100644
--- a/test/unit/preprocess.lua
+++ b/test/unit/preprocess.lua
@@ -7,6 +7,9 @@ local global_helpers = require('test.helpers')
 local argss_to_cmd = global_helpers.argss_to_cmd
 local repeated_read_cmd = global_helpers.repeated_read_cmd
 
+--- @alias Compiler {path: string[], type: string}
+
+--- @type Compiler[]
 local ccs = {}
 
 local env_cc = os.getenv("CC")
@@ -27,6 +30,8 @@ table.insert(ccs, {path = {"/usr/bin/env", "clang"}, type = "clang"})
 table.insert(ccs, {path = {"/usr/bin/env", "icc"}, type = "gcc"})
 
 -- parse Makefile format dependencies into a Lua table
+--- @param deps string
+--- @return string[]
 local function parse_make_deps(deps)
   -- remove line breaks and line concatenators
   deps = deps:gsub("\n", ""):gsub("\\", "")
@@ -36,7 +41,7 @@ local function parse_make_deps(deps)
   deps = deps:gsub("  +", " ")
 
   -- split according to token (space in this case)
-  local headers = {}
+  local headers = {} --- @type string[]
   for token in deps:gmatch("[^%s]+") do
     -- headers[token] = true
     headers[#headers + 1] = token
@@ -53,57 +58,58 @@ local function parse_make_deps(deps)
   return headers
 end
 
--- will produce a string that represents a meta C header file that includes
--- all the passed in headers. I.e.:
---
--- headerize({"stdio.h", "math.h"}, true)
--- produces:
--- #include <stdio.h>
--- #include <math.h>
---
--- headerize({"vim.h", "memory.h"}, false)
--- produces:
--- #include "vim.h"
--- #include "memory.h"
+--- will produce a string that represents a meta C header file that includes
+--- all the passed in headers. I.e.:
+---
+--- headerize({"stdio.h", "math.h"}, true)
+--- produces:
+--- #include <stdio.h>
+--- #include <math.h>
+---
+--- headerize({"vim.h", "memory.h"}, false)
+--- produces:
+--- #include "vim.h"
+--- #include "memory.h"
+--- @param headers string[]
+--- @param global? boolean
+--- @return string
 local function headerize(headers, global)
-  local pre = '"'
-  local post = pre
-  if global then
-    pre = "<"
-    post = ">"
-  end
-
-  local formatted = {}
+  local fmt = global and '#include <%s>' or '#include "%s"'
+  local formatted = {} --- @type string[]
   for _, hdr in ipairs(headers) do
-    formatted[#formatted + 1] = "#include " ..
-                                tostring(pre) ..
-                                tostring(hdr) ..
-                                tostring(post)
+    formatted[#formatted + 1] = string.format(fmt, hdr)
   end
 
   return table.concat(formatted, "\n")
 end
 
+--- @class Gcc
+--- @field path string
+--- @field preprocessor_extra_flags string[]
+--- @field get_defines_extra_flags string[]
+--- @field get_declarations_extra_flags string[]
 local Gcc = {
   preprocessor_extra_flags = {},
   get_defines_extra_flags = {'-std=c99', '-dM', '-E'},
   get_declarations_extra_flags = {'-std=c99', '-P', '-E'},
 }
 
+--- @param name string
+--- @param args string[]?
+--- @param val string?
 function Gcc:define(name, args, val)
-  local define = '-D' .. name
-  if args ~= nil then
-    define = define .. '(' .. table.concat(args, ',') .. ')'
+  local define = string.format('-D%s', name)
+  if args then
+    define = string.format('%s(%s)', define, table.concat(args, ','))
   end
-  if val ~= nil then
-    define = define .. '=' .. val
+  if val then
+    define = string.format('%s=%s', define, val)
   end
   self.preprocessor_extra_flags[#self.preprocessor_extra_flags + 1] = define
 end
 
 function Gcc:undefine(name)
-  self.preprocessor_extra_flags[#self.preprocessor_extra_flags + 1] = (
-      '-U' .. name)
+  self.preprocessor_extra_flags[#self.preprocessor_extra_flags + 1] = '-U' .. name
 end
 
 function Gcc:init_defines()
@@ -128,6 +134,8 @@ function Gcc:init_defines()
   self:undefine('__BLOCKS__')
 end
 
+--- @param obj? Compiler
+--- @return Gcc
 function Gcc:new(obj)
   obj = obj or {}
   setmetatable(obj, self)
@@ -136,6 +144,7 @@ function Gcc:new(obj)
   return obj
 end
 
+--- @param ... string
 function Gcc:add_to_include_path(...)
   for i = 1, select('#', ...) do
     local path = select(i, ...)
@@ -145,110 +154,115 @@ function Gcc:add_to_include_path(...)
 end
 
 -- returns a list of the headers files upon which this file relies
+--- @param hdr string
+--- @return string[]?
 function Gcc:dependencies(hdr)
+  --- @type string
   local cmd = argss_to_cmd(self.path, {'-M', hdr}) .. ' 2>&1'
-  local out = io.popen(cmd)
+  local out = assert(io.popen(cmd))
   local deps = out:read("*a")
   out:close()
   if deps then
     return parse_make_deps(deps)
-  else
-    return nil
   end
 end
 
+--- @param defines string
+--- @return string
 function Gcc:filter_standard_defines(defines)
   if not self.standard_defines then
     local pseudoheader_fname = 'tmp_empty_pseudoheader.h'
-    local pseudoheader_file = io.open(pseudoheader_fname, 'w')
+    local pseudoheader_file = assert(io.open(pseudoheader_fname, 'w'))
     pseudoheader_file:close()
-    local standard_defines = repeated_read_cmd(self.path,
-                                               self.preprocessor_extra_flags,
-                                               self.get_defines_extra_flags,
-                                               {pseudoheader_fname})
+    local standard_defines = assert(repeated_read_cmd(self.path,
+                                                      self.preprocessor_extra_flags,
+                                                      self.get_defines_extra_flags,
+                                                      {pseudoheader_fname}))
     os.remove(pseudoheader_fname)
-    self.standard_defines = {}
+    self.standard_defines = {} --- @type table<string,true>
     for line in standard_defines:gmatch('[^\n]+') do
       self.standard_defines[line] = true
     end
   end
-  local ret = {}
+
+  local ret = {} --- @type string[]
   for line in defines:gmatch('[^\n]+') do
     if not self.standard_defines[line] then
       ret[#ret + 1] = line
     end
   end
+
   return table.concat(ret, "\n")
 end
 
--- returns a stream representing a preprocessed form of the passed-in headers.
--- Don't forget to close the stream by calling the close() method on it.
+--- returns a stream representing a preprocessed form of the passed-in headers.
+--- Don't forget to close the stream by calling the close() method on it.
+--- @param previous_defines string
+--- @param ... string
+--- @return string, string
 function Gcc:preprocess(previous_defines, ...)
   -- create pseudo-header
   local pseudoheader = headerize({...}, false)
   local pseudoheader_fname = 'tmp_pseudoheader.h'
-  local pseudoheader_file = io.open(pseudoheader_fname, 'w')
+  local pseudoheader_file = assert(io.open(pseudoheader_fname, 'w'))
   pseudoheader_file:write(previous_defines)
   pseudoheader_file:write("\n")
   pseudoheader_file:write(pseudoheader)
   pseudoheader_file:flush()
   pseudoheader_file:close()
 
-  local defines = repeated_read_cmd(self.path, self.preprocessor_extra_flags,
-                                    self.get_defines_extra_flags,
-                                    {pseudoheader_fname})
+  local defines = assert(repeated_read_cmd(self.path, self.preprocessor_extra_flags,
+                                           self.get_defines_extra_flags,
+                                           {pseudoheader_fname}))
   defines = self:filter_standard_defines(defines)
 
-  local declarations = repeated_read_cmd(self.path,
-                                         self.preprocessor_extra_flags,
-                                         self.get_declarations_extra_flags,
-                                         {pseudoheader_fname})
+  local declarations = assert(repeated_read_cmd(self.path,
+                                                self.preprocessor_extra_flags,
+                                                self.get_declarations_extra_flags,
+                                                {pseudoheader_fname}))
 
   os.remove(pseudoheader_fname)
 
-  assert(declarations and defines)
   return declarations, defines
 end
 
-local Clang = Gcc:new()
-local Msvc = Gcc:new()
-
-local type_to_class = {
-  ["gcc"] = Gcc,
-  ["clang"] = Clang,
-  ["msvc"] = Msvc
-}
-
 -- find the best cc. If os.exec causes problems on windows (like popping up
 -- a console window) we might consider using something like this:
 -- http://scite-ru.googlecode.com/svn/trunk/pack/tools/LuaLib/shell.html#exec
+--- @param compilers Compiler[]
+--- @return Gcc?
 local function find_best_cc(compilers)
   for _, meta in pairs(compilers) do
-    local version = io.popen(tostring(meta.path) .. " -v 2>&1")
+    local version = assert(io.popen(tostring(meta.path) .. " -v 2>&1"))
     version:close()
     if version then
-      return type_to_class[meta.type]:new({path = meta.path})
+      return Gcc:new({path = meta.path})
     end
   end
-  return nil
 end
 
 -- find the best cc. If os.exec causes problems on windows (like popping up
 -- a console window) we might consider using something like this:
 -- http://scite-ru.googlecode.com/svn/trunk/pack/tools/LuaLib/shell.html#exec
-local cc = nil
-if cc == nil then
-  cc = find_best_cc(ccs)
+local cc = assert(find_best_cc(ccs))
+
+local M = {}
+
+--- @param hdr string
+--- @return string[]?
+function M.includes(hdr)
+  return cc:dependencies(hdr)
 end
 
-return {
-  includes = function(hdr)
-    return cc:dependencies(hdr)
-  end,
-  preprocess = function(...)
-    return cc:preprocess(...)
-  end,
-  add_to_include_path = function(...)
-    return cc:add_to_include_path(...)
-  end
-}
+--- @param ... string
+--- @return string, string
+function M.preprocess(...)
+  return cc:preprocess(...)
+end
+
+--- @param ... string
+function M.add_to_include_path(...)
+  return cc:add_to_include_path(...)
+end
+
+return M
diff --git a/test/unit/set.lua b/test/unit/set.lua
index f3d68c3042..91b7e2dd70 100644
--- a/test/unit/set.lua
+++ b/test/unit/set.lua
@@ -4,10 +4,15 @@
 -- other:
 -- 1) index => item
 -- 2) item => index
+--- @class Set
+--- @field nelem integer
+--- @field items string[]
+--- @field tbl table
 local Set = {}
 
+--- @param items? string[]
 function Set:new(items)
-  local obj = {}
+  local obj = {} --- @ type Set
   setmetatable(obj, self)
   self.__index = self
 
@@ -26,8 +31,9 @@ function Set:new(items)
   return obj
 end
 
+--- @return Set
 function Set:copy()
-  local obj = {}
+  local obj = {} --- @ type Set
   obj.nelem = self.nelem
   obj.tbl = {}
   obj.items = {}
@@ -43,6 +49,7 @@ function Set:copy()
 end
 
 -- adds the argument Set to this Set
+--- @param other Set
 function Set:union(other)
   for e in other:iterator() do
     self:add(e)
@@ -57,6 +64,7 @@ function Set:union_table(t)
 end
 
 -- subtracts the argument Set from this Set
+--- @param other Set
 function Set:diff(other)
   if other:size() > self:size() then
     -- this set is smaller than the other set
@@ -75,6 +83,7 @@ function Set:diff(other)
   end
 end
 
+--- @param it string
 function Set:add(it)
   if not self:contains(it) then
     local idx = #self.tbl + 1
@@ -84,6 +93,7 @@ function Set:add(it)
   end
 end
 
+--- @param it string
 function Set:remove(it)
   if self:contains(it) then
     local idx = self.items[it]
@@ -93,10 +103,13 @@ function Set:remove(it)
   end
 end
 
+--- @param it string
+--- @return boolean
 function Set:contains(it)
   return self.items[it] or false
 end
 
+--- @return integer
 function Set:size()
   return self.nelem
 end
@@ -113,29 +126,18 @@ function Set:iterator()
   return pairs(self.items)
 end
 
+--- @return string[]
 function Set:to_table()
   -- there might be gaps in @tbl, so we have to be careful and sort first
-  local keys
-  do
-    local _accum_0 = { }
-    local _len_0 = 1
-    for idx, _ in pairs(self.tbl) do
-      _accum_0[_len_0] = idx
-      _len_0 = _len_0 + 1
-    end
-    keys = _accum_0
+  local keys = {} --- @type string[]
+  for idx, _ in pairs(self.tbl) do
+    keys[#keys+1] = idx
   end
+
   table.sort(keys)
-  local copy
-  do
-    local _accum_0 = { }
-    local _len_0 = 1
-    for _index_0 = 1, #keys do
-      local idx = keys[_index_0]
-      _accum_0[_len_0] = self.tbl[idx]
-      _len_0 = _len_0 + 1
-    end
-    copy = _accum_0
+  local copy = {} --- @type string[]
+  for _, idx in ipairs(keys) do
+    copy[#copy+1] = self.tbl[idx]
   end
   return copy
 end