RFC: If statement initializers

docs/syntax-if-statements-initializers.md

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+# Initializers in if statements
+
+# Summary
+
+Introduce an initializer expression that declares and initializes variables in if statements.
+
+# Motivation
+
+Initializers can improve code clarity and reduce scope pollution by allowing developers to declare variables in the conditions of if statements. ⁤⁤ Declaring variables at the point of use in if statements for the scope of the if statement's blocks simplifies code, leading to better readability and understanding of program logic. By limiting the scope of variables to the if statement's blocks, the risk of unintended variable reuse and naming conflicts is reduced. ⁤
+
+The reduced scope pollution improves register space in extreme cases (or auto-generated code) where developers have many variables defined and have to work around register limits by reducing the register size. In some scenarios, especially on Roblox, initializing variables within if statements can lead to improved performance and reduced complexity by avoiding unnecessary calls by developers. ⁤ A common paradigm used by Roblox developers is to use `Instance:FindFirstChild` in their condition and then access it afterwards instead of keeping an existing variable around in the new scope, polluting the existing scope. 
+
+Another benefit provided by initializers is being able to compact verbose guard clauses into a singular if statement.   
+
+Example:
+
+```lua
+function PlayerControl:GetEntities()
+    if self.RemovedSelf then
+        return self.Instances
+    end
+
+    local entity = self:TryGetEntity()
+    if entity == nil then
+        return self.Instances
+    end
+
+    local index = table.find(self.Instances, entity.Root)
+    if index == nil then
+        return self.Instances
+    end
+
+    table.remove(self.Instances, index)
+    self.RemovedSelf = true
+
+    return self.Instances
+end
+```
+
+```lua
+function PlayerControl:GetEntities()
+    if self.RemovedSelf then
+    elseif local entity = self:TryGetEntity() in entity == nil then
+    elseif local index = table.find(self.Instances, entity.Root) then
+        table.remove(self.Instances, index)
+        self.RemovedSelf = true
+    end
+
+    return self.Instances
+end
+```
+
+# Design
+
+If statements with initializers must match the below grammar. The variables declared by an initializer are only available to the if statement's blocks; any code after the if statement won't have the variables defined.
+
+```diff
+  stat = varlist '=' explist |
+      ...
+      'repeat' block 'until' exp |
+-     'if' exp 'then' block {'elseif' exp 'then' block} ['else' block] 'end' |
++     'if' cond 'then' block {'elseif' cond 'then' block} ['else' block] 'end' |
+      'for' binding '=' exp ',' exp [',' exp] 'do' block 'end' |
+      ...
++ cond = 'local' binding '=' exp ['in' exp] |
++     'local' bindinglist '=' explist 'in' exp |
++     exp
+```
+
+In the former case, the value of the first declared variable will be checked.
+
+Example:
+
+```lua
+local function foo()
+    return true
+end
+
+if local b = foo() then
+   print(b, "truthy block")
+else
+   print(b, "falsy block")
+end
+```
+
+`Output: true	truthy block`
+
+In the latter case, the `exp` condition is checked rather than the initializer.
+
+Example:
+
+```lua
+local function foo()
+    return true
+end
+
+if local b = foo() in b == false then
+   print(b, "truthy block")
+else
+   print(b, "falsy block")
+end
+```
+
+`Output: true	falsy block`
+
+When declaring multiple values inside of an initializer, the `in` clause is required.
+
+Example:
+
+```lua
+local function foo()
+    return true, false
+end
+
+if local a, b = foo() in a and b then
+else
+    print'Hello World, from Luau!'
+end
+```
+
+`Output: Hello World, from Luau!`
+
+If statement initializers are also allowed in `elseif` conditions.
+
+Example:
+
+```lua
+local a = false
+local function foo()
+    local b = a
+    a = true
+    return b
+end
+
+if local a = foo() then
+elseif local b = foo() then
+    print(b)
+end
+```
+
+`Output: true`
+
+# Drawbacks
+
+Parser recovery may be more fragile due to the `local` keyword.
+
+Initializers increase the complexity of the language syntax and may obscure control flow in complex conditions.
+
+# Alternatives
+
+A different keyword or token can be used in place of `in`.
+
+Rather than use `in`, we could introduce a new contextual keyword, or use a different existing keyword like `do`.
+
+```lua
+if local a, b = foo() do b > a then
+  print'Hello World, from Luau!'
+end
+```
+
+While Luau is a verbose language that uses keywords for the majority of its syntax, another approach is using semicolons as a separator. This can work well because statements can use semicolons as a separator, which will retain consistency with the language. The same can be said for the comma, which would be consistent with for loop syntax.
+
+```lua
+if local a, b = foo(); b > a then
+  print'Hello World, from Luau!'
+end
+```