Diagnostics for unnecessary assert

Author: ribru17

changelog.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 <!-- Add all new changes here. They will be moved under a version at release -->
+`2025-3-13`
+* `NEW` `unnecessary-assert` diagnostic warns when asserting values that are always truthy
 
 ## 3.13.9
 `2025-3-13`

locale/en-us/script.lua

@@ -36,6 +36,8 @@ DIAG_OVER_MAX_ARGS      =
 'This function expects a maximum of {:d} argument(s) but instead it is receiving {:d}.'
 DIAG_MISS_ARGS          =
 'This function requires {:d} argument(s) but instead it is receiving {:d}.'
+DIAG_UNNECESSARY_ASSERT =
+'Unnecessary assert: this expression is always truthy.'
 DIAG_OVER_MAX_VALUES    =
 'Only has {} variables, but you set {} values.'
 DIAG_AMBIGUITY_1        =

locale/en-us/setting.lua

@@ -406,6 +406,8 @@ config.diagnostics['missing-return-value']  =
 'Enable diagnostics for return statements without values although the containing function declares returns.'
 config.diagnostics['need-check-nil']        =
 'Enable diagnostics for variable usages if `nil` or an optional (potentially `nil`) value was assigned to the variable before.'
+config.diagnostics['unnecessary-assert']    =
+'Enable diagnostics for redundant assertions on truthy values.'
 config.diagnostics['no-unknown']            =
 'Enable diagnostics for cases in which the type cannot be inferred.'
 config.diagnostics['not-yieldable']         =

locale/ja-jp/script.lua

@@ -36,6 +36,8 @@ DIAG_OVER_MAX_ARGS      =
 'この関数は最大で {:d} 個の引数を受け取りますが、{:d} 個の引数が渡されています。'
 DIAG_MISS_ARGS          =
 'この関数は少なくとも {:d} 個の引数を必要としますが、{:d} 個しか渡されていません。'
+DIAG_UNNECESSARY_ASSERT =
+'不要なアサーション: この式は常に真です。'
 DIAG_OVER_MAX_VALUES    =
 '変数は {} 個しかありませんが、{} 個の値が設定されています。'
 DIAG_AMBIGUITY_1        =

locale/ja-jp/setting.lua

@@ -406,6 +406,8 @@ config.diagnostics['missing-return-value']  = -- TODO: need translate!
 'Enable diagnostics for return statements without values although the containing function declares returns.'
 config.diagnostics['need-check-nil']        = -- TODO: need translate!
 'Enable diagnostics for variable usages if `nil` or an optional (potentially `nil`) value was assigned to the variable before.'
+config.diagnostics['unnecessary-assert']    = -- TODO: need translate!
+'Enable diagnostics for redundant assertions on truthy values.'
 config.diagnostics['no-unknown']            = -- TODO: need translate!
 'Enable diagnostics for cases in which the type cannot be inferred.'
 config.diagnostics['not-yieldable']         = -- TODO: need translate!

locale/pt-br/script.lua

@@ -36,6 +36,8 @@ DIAG_OVER_MAX_ARGS      =
 'A função aceita apenas os parâmetros {:d}, mas você passou {:d}.'
 DIAG_MISS_ARGS          =
 'A função recebe pelo menos {:d} argumentos, mas há {:d}.'
+DIAG_UNNECESSARY_ASSERT =
+'Asserção desnecessária: esta expressão é sempre verdadeira.'
 DIAG_OVER_MAX_VALUES    =
 'Apenas há {} variáveis, mas você declarou {} valores.'
 DIAG_AMBIGUITY_1        =

locale/pt-br/setting.lua

@@ -406,6 +406,8 @@ config.diagnostics['missing-return-value']  = -- TODO: need translate!
 'Enable diagnostics for return statements without values although the containing function declares returns.'
 config.diagnostics['need-check-nil']        = -- TODO: need translate!
 'Enable diagnostics for variable usages if `nil` or an optional (potentially `nil`) value was assigned to the variable before.'
+config.diagnostics['unnecessary-assert']    = -- TODO: need translate!
+'Enable diagnostics for redundant assertions on truthy values.'
 config.diagnostics['no-unknown']            = -- TODO: need translate!
 'Enable diagnostics for cases in which the type cannot be inferred.'
 config.diagnostics['not-yieldable']         = -- TODO: need translate!

locale/zh-cn/script.lua

@@ -36,6 +36,8 @@ DIAG_OVER_MAX_ARGS      =
 '函数最多接收 {:d} 个参数，但获得了 {:d} 个。'
 DIAG_MISS_ARGS          =
 '函数最少接收 {:d} 个参数，但获得了 {:d} 个。'
+DIAG_UNNECESSARY_ASSERT =
+'不必要的断言：此表达式始终为真值。'
 DIAG_OVER_MAX_VALUES    =
 '只有 {} 个变量，但你设置了 {} 个值。'
 DIAG_AMBIGUITY_1        =

locale/zh-cn/setting.lua

@@ -404,6 +404,8 @@ config.diagnostics['missing-return-value']  =
 '函数无值返回但函数使用`@return`标记了返回值'
 config.diagnostics['need-check-nil']        =
 '变量之前被赋值为`nil`或可选值(可能为 `nil`)'
+config.diagnostics['unnecessary-assert']    =
+'启用对冗余断言(针对始终为真值的表达式)的诊断'
 config.diagnostics['no-unknown']            =
 '变量的未知类型无法推断'
 config.diagnostics['not-yieldable']         =

locale/zh-tw/script.lua

@@ -36,6 +36,8 @@ DIAG_OVER_MAX_ARGS      =
 '函式最多接收 {:d} 個引數，但獲得了 {:d} 個。'
 DIAG_MISS_ARGS          =
 '函式最少接收 {:d} 個引數，但獲得了 {:d} 個。'
+DIAG_UNNECESSARY_ASSERT =
+'不必要的斷言：此表達式始終為真值。'
 DIAG_OVER_MAX_VALUES    =
 '只有 {} 個變數，但你設定了 {} 個值。'
 DIAG_AMBIGUITY_1        =

locale/zh-tw/setting.lua

@@ -404,6 +404,8 @@ config.diagnostics['missing-return-value']  = -- TODO: need translate!
 'Enable diagnostics for return statements without values although the containing function declares returns.'
 config.diagnostics['need-check-nil']        = -- TODO: need translate!
 'Enable diagnostics for variable usages if `nil` or an optional (potentially `nil`) value was assigned to the variable before.'
+config.diagnostics['unnecessary-assert']    = -- TODO: need translate!
+'Enable diagnostics for redundant assertions on truthy values.'
 config.diagnostics['no-unknown']            = -- TODO: need translate!
 'Enable diagnostics for cases in which the type cannot be inferred.'
 config.diagnostics['not-yieldable']         = -- TODO: need translate!

script/core/diagnostics/unnecessary-assert.lua

@@ -0,0 +1,29 @@
+local files = require 'files'
+local guide = require 'parser.guide'
+local vm    = require 'vm'
+local lang  = require 'language'
+local await = require 'await'
+
+---@async
+return function (uri, callback)
+    local state = files.getState(uri)
+    if not state then
+        return
+    end
+
+    ---@async
+    guide.eachSourceType(state.ast, 'call', function (source)
+        await.delay()
+        local currentFunc = guide.getParentFunction(source)
+        if currentFunc and source.node.special == 'assert' and source.args[1] then
+            local argNode = vm.compileNode(source.args[1])
+            if argNode:alwaysTruthy() then
+                callback {
+                    start   = source.node.start,
+                    finish  = source.node.finish,
+                    message = lang.script('DIAG_UNNECESSARY_ASSERT'),
+                }
+            end
+        end
+    end)
+end

script/proto/diagnostic.lua

@@ -78,6 +78,7 @@ m.register {
     'cast-type-mismatch',
     'return-type-mismatch',
     'inject-field',
+    'unnecessary-assert',
 } {
     group    = 'type-check',
     severity = 'Warning',

script/vm/node.lua

@@ -119,6 +119,32 @@ function mt:hasFalsy()
     return false
 end
 
+---Almost an inverse of hasFalsy, but stricter about "any" and "unknown" types.
+---@return boolean
+function mt:alwaysTruthy()
+    if self.optional then
+        return false
+    end
+    if #self == 0 then
+        return false
+    end
+    for _, c in ipairs(self) do
+        if c.type == 'nil'
+        or (c.type == 'global' and c.cate == 'type' and c.name == 'nil')
+        or (c.type == 'global' and c.cate == 'type' and c.name == 'false')
+        or (c.type == 'global' and c.cate == 'type' and c.name == 'any')
+        or (c.type == 'global' and c.cate == 'type' and c.name == 'boolean')
+        or (c.type == 'global' and c.cate == 'type' and c.name == 'doc.type.boolean')
+        or (c.type == 'global' and c.cate == 'type' and c.name == 'unknown')
+        or not self:hasKnownType()
+        or (c.type == 'boolean' and c[1] == false)
+        or (c.type == 'doc.type.boolean' and c[1] == false) then
+            return false
+        end
+    end
+    return true
+end
+
 ---@return boolean
 function mt:hasKnownType()
     for _, c in ipairs(self) do

test/diagnostics/init.lua

@@ -100,6 +100,7 @@ check 'missing-parameter'
 check 'missing-return-value'
 check 'missing-return'
 check 'need-check-nil'
+check 'unnecessary-assert'
 check 'newfield-call'
 check 'newline-call'
 check 'not-yieldable'

test/diagnostics/unnecessary-assert.lua

@@ -0,0 +1,56 @@
+TEST [[
+local a
+assert(a)
+
+---@type boolean
+local b
+assert(b)
+
+---@type any
+local c
+assert(c)
+
+---@type unknown
+local d
+assert(d)
+
+---@type boolean
+local e
+assert(e)
+
+---@type number?
+local f
+assert(f)
+
+assert(false)
+
+assert(nil and 5)
+
+---@return string?, string?
+local function f() end
+
+assert(f())
+]]
+
+TEST [[
+<!assert!>(true)
+]]
+
+TEST [[
+---@return integer
+local function hi()
+  return 1
+end
+<!assert!>(hi(1))
+]]
+
+TEST [[
+<!assert!>({}, 'hi')
+]]
+
+TEST [[
+---@return string, string?
+local function f() end
+
+<!assert!>(f())
+]]