[clang-tidy] Move implicit-cast-in-loop check to upstream.

Summary: This is implemented originally by Alex Pilkiewicz (pilki@google.com).

Reviewers: alexfh

Subscribers: cfe-commits

Patch by Haojian Wu!

Differential Revision: http://reviews.llvm.org/D16721

llvm-svn: 259195

1 files changed, 19 insertions, 0 deletions

diff --git a/clang-tools-extra/docs/clang-tidy/checks/performance-implicit-cast-in-loop.rst b/clang-tools-extra/docs/clang-tidy/checks/performance-implicit-cast-in-loop.rst
new file mode 100644
index 00000000000..f7e3cdcc6e7
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/performance-implicit-cast-in-loop.rst
@@ -0,0 +1,19 @@
+performance-implicit-cast-in-loop
+=================================
+
+This warning appears in range-based loop with loop variable of const ref type
+where the type of the variable does not match the one returned by the iterator.
+This means that an implicit cast has been added, which can for example result in
+expensive deep copies.
+
+Example:
+
+.. code:: c++
+
+   map<int, vector<string>> my_map;
+   for (const pair<int, vector<string>>& p : my_map) {}
+   // The iterator type is in fact pair<const int, vector<string>>, which means
+   // that the compiler added a cast, resulting in a copy of the vectors.
+
+The easiest solution is usually to use "const auto&" instead of writing the type
+manually.