Clarify doc comments around closing variables

internal/luacode/funcstate.go

@@ -38,7 +38,8 @@ type funcState struct {
 	// instructionsSinceLastAbsLineInfo is a counter
 	// of instructions added since the last [absLineInfo].
 	instructionsSinceLastAbsLineInfo uint8
-	// needClose is true if the function needs to close upvalues when returning.
+	// needClose is true if the function needs to close upvalues and/or to-be-closed variables
+	// when returning.
 	needClose bool
 }
 
@@ -53,8 +54,9 @@ type blockControl struct {
 	numActiveVariables uint8
 
 	// upval is true if some variable in the block is an upvalue.
-	upval     bool
-	isLoop    bool
+	upval  bool
+	isLoop bool
+	// insideTBC is true if any to-be-closed locals are in scope.
 	insideTBC bool
 }
 

internal/luacode/instruction.go

@@ -587,11 +587,17 @@ const (
 	OpReturn OpCode = 70 // RETURN
 	// OpReturn0 instructs control flow to return to the function's caller
 	// with zero results.
-	// This instruction cannot be used in a variadic function.
+	// This instruction cannot be used in a function
+	// that is variadic,
+	// has variables referenced by its functions,
+	// or has to-be-closed variables.
 	OpReturn0 OpCode = 71 // RETURN0
 	// OpReturn1 instructs control flow to return to the function's caller
 	// with a single result stored in R[A].
-	// This instruction cannot be used in a variadic function.
+	// This instruction cannot be used in a function
+	// that is variadic,
+	// has variables referenced by its functions,
+	// or has to-be-closed variables.
 	//
 	//	A return R[A]
 	OpReturn1 OpCode = 72 // RETURN1