Enforce function returns at compile time

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 @@
 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 @@
 	---@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 @@
 							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]
@@ -378,7 +388,7 @@
 
 	---@param data { [1]: Token<string>, [2]: Node, [3]: Node, [4]: Node?, [5]: Node } var start stop step block
 	[NodeVariant.For] = function (self, trace, data)
 		local var, start, stop, step = data[1], self:CompileExpr(data[2]), self:CompileExpr(data[3]), data[4] and self:CompileExpr(data[4]) or data[4]
 
 		local block = self:Scope(function(scope)
 			scope.data.loop = true
@@ -512,14 +522,16 @@
 	---@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 @@
 			}
 		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
@@ -545,9 +566,9 @@
 
 					if state.__break__ then
 						state.__break__ = false
 						goto exit
 					elseif state.__return__ then -- Yes this should only be checked if the switch is inside a function, but I don't care enough about the performance of switch case to add another duplicated 30 lines to the file
 						goto exit
 					else -- Fallthrough, run every case until break found.
 						for j = i + 1, ncases do
 							cases[j][2](state)
@@ -668,7 +689,7 @@
 			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 @@
 
 					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 @@
 
 					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 @@
 
 				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 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)