[ADT] Annotate immutable list/set/map update methods with LLVM_NODISCARD.

Because immutable data structures are, well, immutable, methods like "append",
"add", "set" create a copy of the list (set, map) instead of mutating the
existing map. If the updated object is discarded, it clearly indicates a bug.
Such bugs are introduced frequently, hence the warn_unused_result annotation.

Differential Revision: https://reviews.llvm.org/D47496

llvm-svn: 333672

llvm/include/llvm/ADT/ImmutableList.h

@@ -166,7 +166,7 @@ public:
     if (ownsAllocator()) delete &getAllocator();
   }
 
-  ImmutableList<T> concat(const T& Head, ImmutableList<T> Tail) {
+  LLVM_NODISCARD ImmutableList<T> concat(const T &Head, ImmutableList<T> Tail) {
     // Profile the new list to see if it already exists in our cache.
     FoldingSetNodeID ID;
     void* InsertPos;
@@ -188,7 +188,7 @@ public:
     return L;
   }
 
-  ImmutableList<T> add(const T& D, ImmutableList<T> L) {
+  LLVM_NODISCARD ImmutableList<T> add(const T& D, ImmutableList<T> L) {
     return concat(D, L);
   }
 

llvm/include/llvm/ADT/ImmutableMap.h

@@ -114,12 +114,13 @@ public:
 
     ImmutableMap getEmptyMap() { return ImmutableMap(F.getEmptyTree()); }
 
-    ImmutableMap add(ImmutableMap Old, key_type_ref K, data_type_ref D) {
+    LLVM_NODISCARD ImmutableMap add(ImmutableMap Old, key_type_ref K,
+                                    data_type_ref D) {
       TreeTy *T = F.add(Old.Root, std::pair<key_type,data_type>(K,D));
       return ImmutableMap(Canonicalize ? F.getCanonicalTree(T): T);
     }
 
-    ImmutableMap remove(ImmutableMap Old, key_type_ref K) {
+    LLVM_NODISCARD ImmutableMap remove(ImmutableMap Old, key_type_ref K) {
       TreeTy *T = F.remove(Old.Root,K);
       return ImmutableMap(Canonicalize ? F.getCanonicalTree(T): T);
     }

llvm/include/llvm/ADT/ImmutableSet.h

@@ -1017,7 +1017,7 @@ public:
     ///  of this operation is logarithmic in the size of the original set.
     ///  The memory allocated to represent the set is released when the
     ///  factory object that created the set is destroyed.
-    ImmutableSet add(ImmutableSet Old, value_type_ref V) {
+    LLVM_NODISCARD ImmutableSet add(ImmutableSet Old, value_type_ref V) {
       TreeTy *NewT = F.add(Old.Root, V);
       return ImmutableSet(Canonicalize ? F.getCanonicalTree(NewT) : NewT);
     }
@@ -1029,7 +1029,7 @@ public:
     ///  of this operation is logarithmic in the size of the original set.
     ///  The memory allocated to represent the set is released when the
     ///  factory object that created the set is destroyed.
-    ImmutableSet remove(ImmutableSet Old, value_type_ref V) {
+    LLVM_NODISCARD ImmutableSet remove(ImmutableSet Old, value_type_ref V) {
       TreeTy *NewT = F.remove(Old.Root, V);
       return ImmutableSet(Canonicalize ? F.getCanonicalTree(NewT) : NewT);
     }