[mlir] mark ChangeResult as nodiscard (llvm#76147)

This enum is used by dataflow analyses to indicate whether further propagation is necessary to reach the fix point. Accidentally discarding such a value will likely lead to propagation stopping early, leading to incomplete or incorrect results. The most egregious example is the duality between `join` on the analysis class, which triggers propagation internally, and `join` on the lattice class that does not and expects the caller to trigger it depending on the returned `ChangeResult`.

Author: ftynse

mlir/include/mlir/Analysis/DataFlowFramework.h

@@ -30,8 +30,8 @@ namespace mlir {
 //===----------------------------------------------------------------------===//
 
 /// A result type used to indicate if a change happened. Boolean operations on
-/// ChangeResult behave as though `Change` is truthy.
-enum class ChangeResult {
+/// ChangeResult behave as though `Change` is truth.
+enum class [[nodiscard]] ChangeResult {
  NoChange,
  Change,
 };

mlir/lib/Analysis/DataFlow/LivenessAnalysis.cpp

@@ -191,7 +191,7 @@ void LivenessAnalysis::visitCallOperand(OpOperand &operand) {
 
 void LivenessAnalysis::setToExitState(Liveness *lattice) {
  // This marks values of type (2) liveness as "live".
- lattice->markLive();
+ (void)lattice->markLive();
 }
 
 //===----------------------------------------------------------------------===//

mlir/test/lib/Analysis/TestDataFlowFramework.cpp

@@ -100,7 +100,7 @@ LogicalResult FooAnalysis::initialize(Operation *top) {
  return top->emitError("expected at least one block in the region");
 
  // Initialize the top-level state.
- getOrCreate<FooState>(&top->getRegion(0).front())->join(0);
+ (void)getOrCreate<FooState>(&top->getRegion(0).front())->join(0);
 
  // Visit all nested blocks and operations.
  for (Block &block : top->getRegion(0)) {