Add comments to UnusedPruner

libjulia/optimiser/UnusedPruner.cpp

@@ -55,6 +55,11 @@ void UnusedPruner::operator()(Block& _block)
 		else if (statement.type() == typeid(VariableDeclaration))
 		{
 			VariableDeclaration& varDecl = boost::get<VariableDeclaration>(statement);
+			// Multi-variable declarations are special. We can only remove it
+			// if all vairables are unused and the right-hand-side is either
+			// movable or it return a single value. In the latter case, we
+			// replace `let a := f()` by `pop(f())` (in pure IULIA, this will be
+			// `drop(f())`).
 			if (boost::algorithm::none_of(
 				varDecl.variables,
 				[=](TypedName const& _typedName) { return used(_typedName.name); }
@@ -68,6 +73,8 @@ void UnusedPruner::operator()(Block& _block)
 					statement = Block{std::move(varDecl.location), {}};
 				}
 				else if (varDecl.variables.size() == 1)
+					// In pure IULIA, this should be replaced by a function call to `drop`
+					// instead of `pop`.
 					statement = ExpressionStatement{varDecl.location, FunctionalInstruction{
 						varDecl.location,
 						solidity::Instruction::POP,