[ADT] Add drop_end.

[curdeius]

Summary

This patch adds drop_end that is analogical to drop_begin.
It tries to fill the functional gap where one could drop first elements but not the last ones.
The need for it came in when refactoring clang-format.

[dblaikie]

Sounds good

llvm/include/llvm/ADT/STLExtras.h

 
 /// Return a range covering \p RangeOrContainer with the first N elements
 /// excluded.
 template <typename T> auto drop_begin(T &&RangeOrContainer, size_t N = 1) {
   return make_range(std::next(adl_begin(RangeOrContainer), N),
                     adl_end(RangeOrContainer));
 }
 
+/// Return a range covering \p RangeOrContainer with the last N elements
+/// excluded.
+template <typename T> auto drop_end(T &&RangeOrContainer, size_t N = 1) {
+  return make_range(adl_begin(RangeOrContainer),
+                    std::prev(adl_end(RangeOrContainer), N));
+}
+
 // mapped_iterator - This is a simple iterator adapter that causes a function to
 // be applied whenever operator* is invoked on the iterator.
 
 template <typename ItTy, typename FuncTy,
           typename ReferenceTy =
               decltype(std::declval<FuncTy>()(*std::declval<ItTy>()))>
 class mapped_iterator
     : public iterator_adaptor_base<

llvm/unittests/ADT/STLExtrasTest.cpp

TEST(STLExtrasTest, DropBeginDefaultTest) {
   int i = 1;
   for (auto &v : drop_begin(vec)) {
     EXPECT_EQ(v, i);
     i += 1;
   }
   EXPECT_EQ(i, 5);
 }
 
+TEST(STLExtrasTest, DropEndTest) {
+  SmallVector<int, 5> vec{0, 1, 2, 3, 4};
+
+  for (int n = 0; n < 5; ++n) {
+    int i = 0;
+    for (auto &v : drop_end(vec, n)) {
+      EXPECT_EQ(v, i);
+      i += 1;
+    }
+    EXPECT_EQ(i, 5 - n);
+  }
+}
+
+TEST(STLExtrasTest, DropEndDefaultTest) {
+  SmallVector<int, 5> vec{0, 1, 2, 3, 4};
+
+  int i = 0;
+  for (auto &v : drop_end(vec)) {
+    EXPECT_EQ(v, i);
+    i += 1;
+  }
+  EXPECT_EQ(i, 4);
+}
+
 TEST(STLExtrasTest, EarlyIncrementTest) {
   std::list<int> L = {1, 2, 3, 4};
 
   auto EIR = make_early_inc_range(L);
 
   auto I = EIR.begin();
   auto EI = EIR.end();
   EXPECT_NE(I, EI);