QString: don't detach in remove(pos, len)

- If this string isn't shared, don't call detach, instead use ->erase() as
  needed
- If this string is shared, create a new string, and copy all elements
  except the ones that would be removed, see task for details

Update unittest to test both code paths.

Task-number: QTBUG-106181
Change-Id: I4c73ff17a6fa89ddcf6966f9c5bf789753f6d39e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qstring.cpp

@@ -3294,9 +3294,22 @@ QString &QString::remove(qsizetype pos, qsizetype len)
         return *this;
 
     len = std::min(len, size() - pos);
-    detach();
-    d->erase(d.begin() + pos, len);
-    d.data()[d.size] = u'\0';
+
+    if (!d->isShared()) {
+        d->erase(d.begin() + pos, len);
+        d.data()[d.size] = u'\0';
+    } else {
+        // TODO: either reserve "size()", which is bigger than needed, or
+        // modify the shrinking-erase docs of this method (since the size
+        // of "copy" won't have any extra capacity any more)
+        const qsizetype sz = size() - len;
+        QString copy{sz, Qt::Uninitialized};
+        auto begin = d.begin();
+        auto toRemove_start = d.begin() + pos;
+        copy.d->copyRanges({{begin, toRemove_start},
+                           {toRemove_start + len, d.end()}});
+        swap(copy);
+    }
     return *this;
 }
 

src/corelib/tools/qarraydataops.h

@@ -213,6 +213,17 @@ public:
         --this->size;
     }
 
+    struct Span { T *begin; T *end; };
+
+    void copyRanges(const std::initializer_list<Span> &ranges)
+    {
+        auto it = this->begin();
+        std::for_each(ranges.begin(), ranges.end(), [&it](const auto &span) {
+            it = std::copy(span.begin, span.end, it);
+        });
+        this->size = std::distance(this->begin(), it);
+    }
+
     void assign(T *b, T *e, parameter_type t) noexcept
     {
         Q_ASSERT(b <= e);

tests/auto/corelib/text/qstring/tst_qstring.cpp

@@ -3318,13 +3318,23 @@ void tst_QString::remove_uint_uint()
     QFETCH( int, index );
     QFETCH( int, len );
     QFETCH( QString, after );
+    QFETCH(QString, result);
 
-    if ( after.size() == 0 ) {
-        QString s1 = string;
-        s1.remove( (uint) index, (uint) len );
-        QTEST( s1, "result" );
-    } else
-        QCOMPARE( 0, 0 ); // shut Qt Test
+    // For the replace() unitests?
+    if ( after.size() != 0 ) {
+        return;
+    }
+
+    // Test when isShared() is true
+    QString s1 = string;
+    s1.remove((qsizetype)index, (qsizetype)len);
+    QCOMPARE(s1, result);
+
+    QString s2 = string;
+    // Test when isShared() is false
+    s2.detach();
+    s2.remove((qsizetype)index, (qsizetype)len);
+    QCOMPARE(s2, result);
 }
 
 void tst_QString::remove_string()