QVarLengthArray: deprecate prepend()

All Qt 6 containers have "fast" prepend these days. Except
QVLA. Instead of enabling "fast" prepend for QVLA, slowing down
idiomatic QVLA use, simply deprecate prepend().

There appear to be no users of this function in qtbase outside tests.

[ChangeLog][QtCore][Deprecation Notices][QVarLengthArray] Deprecated
prepend() because QVarLengthArray is the only Qt container without a
"fast" prepend. If you require that functionality, even though it's a
linear operation, then use insert(cbegin(), ~~~) instead.

Change-Id: I39ff1dd7d4de7fc08d5380a5a7450dd8c8996fe2
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

4 files changed, 26 insertions, 3 deletions

diff --git a/src/corelib/tools/qvarlengtharray.h b/src/corelib/tools/qvarlengtharray.h
index 4d25287887..79ecdaee7d 100644
--- a/src/corelib/tools/qvarlengtharray.h
+++ b/src/corelib/tools/qvarlengtharray.h
@@ -271,8 +271,12 @@ public:
     inline QVarLengthArray<T, Prealloc> &operator+=(T &&t)
     { append(std::move(t)); return *this; }
 
+#if QT_DEPRECATED_SINCE(6, 3)
+    QT_DEPRECATED_VERSION_X_6_3("This is slow. If you must, use insert(cbegin(), ~~~) instead.")
     void prepend(T &&t);
+    QT_DEPRECATED_VERSION_X_6_3("This is slow. If you must, use insert(cbegin(), ~~~) instead.")
     void prepend(const T &t);
+#endif
     void insert(qsizetype i, T &&t);
     void insert(qsizetype i, const T &t);
     void insert(qsizetype i, qsizetype n, const T &t);
@@ -596,12 +600,14 @@ template <class T, qsizetype Prealloc>
 template <typename Predicate>
 inline qsizetype QVarLengthArray<T, Prealloc>::removeIf(Predicate pred)
 { return QtPrivate::sequential_erase_if(*this, pred); }
+#if QT_DEPRECATED_SINCE(6, 3)
 template <class T, qsizetype Prealloc>
 inline void QVarLengthArray<T, Prealloc>::prepend(T &&t)
 { insert(cbegin(), std::move(t)); }
 template <class T, qsizetype Prealloc>
 inline void QVarLengthArray<T, Prealloc>::prepend(const T &t)
 { insert(begin(), 1, t); }
+#endif
 
 template <class T, qsizetype Prealloc>
 inline void QVarLengthArray<T, Prealloc>::replace(qsizetype i, const T &t)
diff --git a/src/corelib/tools/qvarlengtharray.qdoc b/src/corelib/tools/qvarlengtharray.qdoc
index b7e6cd6d6e..de57daf4e4 100644
--- a/src/corelib/tools/qvarlengtharray.qdoc
+++ b/src/corelib/tools/qvarlengtharray.qdoc
@@ -545,6 +545,8 @@
     \fn template<class T, qsizetype Prealloc> void QVarLengthArray<T, Prealloc>::prepend(T &&value)
 
     \since 4.8
+    \deprecated [6.3] This is slow. If you must, use \c{insert(cbegin(), ~~~)} instead.
+
     Inserts \a value at the beginning of the array.
 
 
diff --git a/tests/auto/corelib/tools/collections/tst_collections.cpp b/tests/auto/corelib/tools/collections/tst_collections.cpp
index fd857aa875..7306e036a9 100644
--- a/tests/auto/corelib/tools/collections/tst_collections.cpp
+++ b/tests/auto/corelib/tools/collections/tst_collections.cpp
@@ -312,6 +312,11 @@ static_assert(!QTypeTraits::has_operator_equal_v<NoCmpRecursiveHashK>);
 static_assert(!QTypeTraits::has_operator_equal_v<NoCmpRecursiveMultiHashV>);
 static_assert(!QTypeTraits::has_operator_equal_v<NoCmpRecursiveMultiHashK>);
 
+template <typename T>
+constexpr inline bool has_prepend_v = true;
+template <typename T, qsizetype N>
+constexpr inline bool has_prepend_v<QVarLengthArray<T,N>> = false; // deprecated in Qt 6.3
+
 void tst_Collections::typeinfo()
 {
     QVERIFY(QTypeInfo<int*>::isPointer);
@@ -3452,7 +3457,10 @@ template<class Container> void insert_remove_loop_impl()
     t.append(T(IntOrString(1)));
     t << (T(IntOrString(2)));
     t += (T(IntOrString(3)));
-    t.prepend(T(IntOrString(4)));
+    if constexpr (has_prepend_v<Container>)
+        t.prepend(T(IntOrString(4)));
+    else
+        t.insert(t.cbegin(), T(IntOrString(4)));
     t.insert(2, 3 , T(IntOrString(5)));
     t.insert(4, T(IntOrString(6)));
     t.insert(t.begin() + 2, T(IntOrString(7)));
diff --git a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
index 8f2871a54f..73e7abeb8c 100644
--- a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
+++ b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
@@ -72,7 +72,9 @@ private slots:
     void defaultConstructor_int() { defaultConstructor<int>(); }
     void defaultConstructor_QString() { defaultConstructor<QString>(); }
     void append();
+#if QT_DEPRECATED_SINCE(6, 3)
     void prepend();
+#endif
     void emplace();
     void move_int_1() { move_int<1>(); }
     void move_int_2() { move_int<2>(); }
@@ -169,6 +171,9 @@ void tst_QVarLengthArray::append()
     v2.append(5);
 }
 
+#if QT_DEPRECATED_SINCE(6, 3)
+QT_WARNING_PUSH
+QT_WARNING_DISABLE_DEPRECATED
 void tst_QVarLengthArray::prepend()
 {
     QVarLengthArray<QString, 2> v;
@@ -187,6 +192,8 @@ void tst_QVarLengthArray::prepend()
     v.prepend(v.back());
     QCOMPARE(v.front(), v.back());
 }
+QT_WARNING_POP
+#endif // QT_DEPRECATED_SINCE(6, 3)
 
 void tst_QVarLengthArray::emplace()
 {
@@ -1339,7 +1346,7 @@ void tst_QVarLengthArray::insertMove()
         QCOMPARE(MyBase::liveCount, 6);
         QCOMPARE(MyBase::movedCount, 2);
 
-        vec.prepend(std::move(m1));
+        vec.insert(vec.cbegin(), std::move(m1));
         QVERIFY(m1.wasConstructedAt(nullptr));
         QVERIFY(vec.at(0).wasConstructedAt(&m1));
         QVERIFY(vec.at(1).wasConstructedAt(&m3));
@@ -1411,7 +1418,7 @@ void tst_QVarLengthArray::nonCopyable()
     QVERIFY(!val4);
     QVERIFY(ptr3 == vec.at(0).get());
     QVERIFY(ptr4 == vec.at(1).get());
-    vec.prepend(std::move(val1));
+    vec.insert(vec.cbegin(), std::move(val1));
     QVERIFY(!val1);
     QVERIFY(ptr1 == vec.at(0).get());
     QVERIFY(ptr3 == vec.at(1).get());