[mlir] mark ChangeResult as nodiscard (#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)) {