Enforce function returns at compile time (#2788)

* Enforce function returns at compile time

* Raise cost of dynamic functions

* Revert previous change

Forgot const optimization isn't currently being used. Needs an analyzer step.

Should really not have included that code in the rewrite but it's fine being there for now.

* Add test cases

* Simplify

No need for `returned` field, can just check if scope is dead.

* Add switch case return logic

Adds logic for switch case to be the last statement in a function and be detected for return values.

data/expression2/tests/compiler/compiler/restrictions/fn_return.txt

@@ -0,0 +1,3 @@
+## SHOULD_FAIL:COMPILE
+
+function string nothing() {}

data/expression2/tests/compiler/compiler/restrictions/fn_return_pass.txt

@@ -0,0 +1,22 @@
+## SHOULD_PASS:COMPILE
+
+function string nothing() {
+	return "something"
+}
+
+function number deadcase() {
+	if (1) {
+		return 2158129
+	} else {
+		return 2321515
+	}
+}
+
+function number switchcase() {
+	switch (5) {
+		case 5,
+			return 2
+		default,
+			return 5
+	}
+}

data/expression2/tests/compiler/compiler/restrictions/fn_return_switch.txt

@@ -0,0 +1,12 @@
+## SHOULD_FAIL:COMPILE
+
+function string failure() {
+	switch (5) {
+		case 2,
+			break
+		default,
+			break
+
+		# 'break' does not return a value or cause a runtime error, just early returns switch.
+	}
+}

data/expression2/tests/compiler/compiler/restrictions/fn_return_switch2.txt

@@ -0,0 +1,9 @@
+## SHOULD_FAIL:COMPILE
+
+function string failure() {
+	switch (5) {
+		case 2,
+			return "boowomp"
+		# no default case, compiler can't guarantee that this always runs, fails to compile.
+	}
+}

lua/entities/gmod_wire_expression2/base/compiler.lua

@@ -17,7 +17,7 @@ cvars.AddChangeCallback("wire_expression2_quotatick", function(_, old, new)
 end, "compiler_quota_check")
 
 ---@class ScopeData
----@field dead boolean?
+---@field dead "ret"|true?
 ---@field loop boolean?
 ---@field switch_case boolean?
 ---@field function { [1]: string, [2]: EnvFunction}?
@@ -281,8 +281,10 @@ local CompileVisitors = {
 	---@param data { [1]: Node?, [2]: Node }[]
 	[NodeVariant.If] = function (self, trace, data)
 		local chain = {} ---@type { [1]: RuntimeOperator?, [2]: RuntimeOperator }[]
+		local dead, els = true, false
+
 		for i, ifeif in ipairs(data) do
-			self:Scope(function()
+			self:Scope(function(scope)
 				if ifeif[1] then -- if or elseif
 					local expr, expr_ty = self:CompileExpr(ifeif[1])
 
@@ -301,11 +303,19 @@ local CompileVisitors = {
 							self:CompileStmt(ifeif[2])
 						}
 					end
+
+					dead = dead and scope.data.dead
 				else -- else block
 					chain[i] = { nil, self:CompileStmt(ifeif[2]) }
+					dead, els = dead and scope.data.dead, true
 				end
 			end)
 		end
+
+		if els and dead then -- if (0) { return } else { return } mark any code after as dead
+			self.scope.data.dead = "ret"
+		end
+
 		return function(state) ---@param state RuntimeContext
 			for _, data in ipairs(chain) do
 				local cond, block = data[1], data[2]
@@ -512,14 +522,16 @@ local CompileVisitors = {
 	---@param data { [1]: Node, [2]: {[1]: Node, [2]: Node}[], [3]: Node? }
 	[NodeVariant.Switch] = function (self, trace, data)
 		local expr, expr_ty = self:CompileExpr(data[1])
+		local dead = true
 
 		local cases = {} ---@type { [1]: RuntimeOperator, [2]: RuntimeOperator }[]
 		for i, case in ipairs(data[2]) do
 			local cond, cond_ty = self:CompileExpr(case[1])
-			local block
-			self:Scope(function(scope)
+			local block = self:Scope(function(scope)
 				scope.data.switch_case = true
-				block = self:CompileStmt(case[2])
+				local b = self:CompileStmt(case[2])
+				dead = dead and scope.data.dead == "ret"
+				return b
 			end)
 
 			local eq =  self:GetOperator("eq", { expr_ty, cond_ty }, case[1].trace)
@@ -531,7 +543,16 @@ local CompileVisitors = {
 			}
 		end
 
-		local default = data[3] and self:Scope(function() return self:CompileStmt(data[3]) end)
+		local default = data[3] and self:Scope(function(scope)
+			local b = self:CompileStmt(data[3])
+			dead = dead and scope.data.dead == "ret"
+			return b
+		end)
+
+		if dead and default then -- if all cases dead and has default case, mark scope as dead.
+			self.scope.data.dead = true
+		end
+
 		local ncases = #cases
 
 		return function(state) ---@param state RuntimeContext
@@ -668,7 +689,7 @@ local CompileVisitors = {
 			end
 		end
 
-		local fn = { args = param_types, returns = return_type and { return_type }, meta = meta_type, cost = 20, attrs = {} }
+		local fn = { args = param_types, returns = return_type and { return_type }, meta = meta_type, cost = variadic_ty and 25 or 10, attrs = {} }
 		local sig = table.concat(param_types, "", 1, #param_types - 1) .. ((variadic_ty and ".." or "") .. (param_types[#param_types] or ""))
 
 		if meta_type then
@@ -727,12 +748,8 @@ local CompileVisitors = {
 
 					state.Scopes, state.ScopeID, state.Scope = s_scopes, s_scopeid, s_scope
 
-					if state.__return__ then
-						state.__return__ = false
-						return state.__returnval__
-					elseif return_type then
-						state:forceThrow("Expected function return at runtime of type (" .. return_type .. ")")
-					end
+					state.__return__ = false
+					return state.__returnval__
 				end
 			else -- table
 				function fn.op(state, args, arg_types) ---@param state RuntimeContext
@@ -758,12 +775,8 @@ local CompileVisitors = {
 
 					state.Scopes, state.ScopeID, state.Scope = s_scopes, s_scopeid, s_scope
 
-					if state.__return__ then
-						state.__return__ = false
-						return state.__returnval__
-					elseif return_type then
-						state:forceThrow("Expected function return at runtime of type (" .. return_type .. ")")
-					end
+					state.__return__ = false
+					return state.__returnval__
 				end
 			end
 		else -- Todo: Make this output a different function when it doesn't early return, and/or has no parameters as an optimization.
@@ -784,23 +797,23 @@ local CompileVisitors = {
 
 				state.Scopes, state.ScopeID, state.Scope = s_scopes, s_scopeid, s_scope
 
-				if state.__return__ then
-					state.__return__ = false
-					return state.__returnval__
-				elseif return_type then
-					state:forceThrow("Expected function function at runtime of type (" .. return_type .. ")")
-				end
+				state.__return__ = false
+				return state.__returnval__
 			end
 		end
 
-		block = self:IsolatedScope(function (scope)
+		self:IsolatedScope(function (scope)
 			for i, type in ipairs(param_types) do
 				scope:DeclVar(param_names[i], { type = type, trace_if_unused = data[4][i] and data[4][i].name.trace or trace, initialized = true })
 			end
 
 			scope.data["function"] = { name.value, fn }
 
-			return self:CompileStmt(data[5])
+			block = self:CompileStmt(data[5])
+
+			if return_type then -- Ensure function either returns or errors
+				self:Assert(scope.data.dead, "This function marked to return '" .. data[1].value .. "' must return a value", data[1].trace)
+			end
 		end)
 
 		self:Assert((fn.returns and fn.returns[1]) == return_type, "Function " .. name.value .. " expects to return type (" .. (return_type or "void") .. ") but got type (" .. ((fn.returns and fn.returns[1]) or "void") .. ")", trace)
@@ -888,6 +901,8 @@ local CompileVisitors = {
 		local fn = self.scope:ResolveData("function")
 		self:Assert(fn, "Cannot use `return` outside of a function", trace)
 
+		self.scope.data.dead = "ret"
+
 		local retval, ret_ty
 		if data then
 			retval, ret_ty = self:CompileExpr(data)