[clang-tools-extra] [clang-tidy] Add readability-use-span-first-last check (PR #118074)

[Denis Mikhailov via cfe-commits]

================
@@ -0,0 +1,149 @@
+// RUN: %check_clang_tidy -std=c++20 %s readability-use-span-first-last %t
+
+namespace std {
+template <typename T>
+class span {
+  T* ptr;
+  __SIZE_TYPE__ len;
+
+public:
+  span(T* p, __SIZE_TYPE__ l) : ptr(p), len(l) {}
+  
+  span<T> subspan(__SIZE_TYPE__ offset) const {
+    return span(ptr + offset, len - offset);
+  }
+  
+  span<T> subspan(__SIZE_TYPE__ offset, __SIZE_TYPE__ count) const {
+    return span(ptr + offset, count);
+  }
+
+  span<T> first(__SIZE_TYPE__ count) const {
+    return span(ptr, count);
+  }
+
+  span<T> last(__SIZE_TYPE__ count) const {
+    return span(ptr + (len - count), count);
+  }
+
+  __SIZE_TYPE__ size() const { return len; }
+};
+} // namespace std
+
+// Add here, right after the std namespace closes:
+namespace std::ranges {
+  template<typename T>
+  __SIZE_TYPE__ size(const span<T>& s) { return s.size(); }
+}
+
+void test() {
+  int arr[] = {1, 2, 3, 4, 5};
+  std::span<int> s(arr, 5);
+
+  auto sub1 = s.subspan(0, 3);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::first()' over 'subspan()'
+  // CHECK-FIXES: auto sub1 = s.first(3);
+
+  auto sub2 = s.subspan(s.size() - 2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto sub2 = s.last(2);
+
+  __SIZE_TYPE__ n = 2;
+  auto sub3 = s.subspan(0, n);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::first()' over 'subspan()'
+  // CHECK-FIXES: auto sub3 = s.first(n);
+
+  auto sub4 = s.subspan(1, 2);  // No warning
+  auto sub5 = s.subspan(2);     // No warning
+
+
+#define ZERO 0
+#define TWO 2
+#define SIZE_MINUS(s, n) s.size() - n
+#define MAKE_SUBSPAN(obj, n) obj.subspan(0, n)
+#define MAKE_LAST_N(obj, n) obj.subspan(obj.size() - n)
+
+  auto sub6 = s.subspan(SIZE_MINUS(s, 2));
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto sub6 = s.last(2);
+
+  auto sub7 = MAKE_SUBSPAN(s, 3);
+  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: prefer 'span::first()' over 'subspan()'
+  // CHECK-FIXES: auto sub7 = s.first(3);
+
+  auto sub8 = MAKE_LAST_N(s, 2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto sub8 = s.last(2);
+
+}
+
+template <typename T>
+void testTemplate() {
+  T arr[] = {1, 2, 3, 4, 5};
+  std::span<T> s(arr, 5);
+
+  auto sub1 = s.subspan(0, 3);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::first()' over 'subspan()'
+  // CHECK-FIXES: auto sub1 = s.first(3);
+
+  auto sub2 = s.subspan(s.size() - 2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto sub2 = s.last(2);
+
+  __SIZE_TYPE__ n = 2;
+  auto sub3 = s.subspan(0, n);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::first()' over 'subspan()'
+  // CHECK-FIXES: auto sub3 = s.first(n);
+
+  auto sub4 = s.subspan(1, 2);  // No warning
+  auto sub5 = s.subspan(2);     // No warning
+
+  auto complex = s.subspan(0 + (s.size() - 2), 3);  // No warning
+
+  auto complex2 = s.subspan(100 + (s.size() - 2));  // No warning
+}
+
+// Test instantiation
+void testInt() {
+  testTemplate<int>();
+}
+
+void test_ranges() {
+  int arr[] = {1, 2, 3, 4, 5};
+  std::span<int> s(arr, 5);
+
+  auto sub1 = s.subspan(std::ranges::size(s) - 2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto sub1 = s.last(2);
+
+  __SIZE_TYPE__ n = 2;
+  auto sub2 = s.subspan(std::ranges::size(s) - n);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto sub2 = s.last(n);
+}
+
+void test_different_spans() {
+  int arr1[] = {1, 2, 3, 4, 5};
+  int arr2[] = {6, 7, 8, 9, 10};
+  std::span<int> s1(arr1, 5);
+  std::span<int> s2(arr2, 5);
+
+  // These should NOT trigger warnings as they use size() from a different span
+  auto sub1 = s1.subspan(s2.size() - 2);     // No warning
+  auto sub2 = s2.subspan(s1.size() - 3);     // No warning
+  
+  // Also check with std::ranges::size
+  auto sub3 = s1.subspan(std::ranges::size(s2) - 2);  // No warning
+  auto sub4 = s2.subspan(std::ranges::size(s1) - 3);  // No warning
+
+  // Mixed usage should also not trigger
+  auto sub5 = s1.subspan(s2.size() - s1.size());      // No warning
+  
+  // Verify that correct usage still triggers warnings
+  auto good1 = s1.subspan(s1.size() - 2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto good1 = s1.last(2);
+  
+  auto good2 = s2.subspan(std::ranges::size(s2) - 3);
+  // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: prefer 'span::last()' over 'subspan()'
+  // CHECK-FIXES: auto good2 = s2.last(3);
+}
----------------
denzor200 wrote:

please add `test_span_of_bytes`:
```
  std::byte arr[] = {0x1, 0x2, 0x3, 0x4, 0x5};
  std::span<std::byte> s(arr, 5);

  // ...

   auto sub2 = s.subspan(s.size_bytes() - 2);
  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer 'span::last()' over 'subspan()'
  // CHECK-FIXES: auto sub2 = s.last(2);

  // ...
```

https://github.com/llvm/llvm-project/pull/118074