[clang-tidy] Add readability-make-member-function-const

Summary:
Finds non-static member functions that can be made ``const``
because the functions don't use ``this`` in a non-const way.

The check conservatively tries to preserve logical costness in favor of
physical costness. See readability-make-member-function-const.rst for more
details.

Reviewers: aaron.ballman, gribozavr, hokein, alexfh

Subscribers: mgorny, xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68074

3 files changed, 74 insertions, 0 deletions

diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 4a7fef5bb03..40b7aed4178 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -147,6 +147,12 @@ Improvements to clang-tidy
   Finds classes, structs, and unions that contain redundant member
   access specifiers.
 
+- New :doc:`readability-make-member-function-const
+  <clang-tidy/checks/readability-make-member-function-const>` check.
+
+  Finds non-static member functions that can be made ``const``
+  because the functions don't use ``this`` in a non-const way.
+
 Improvements to include-fixer
 -----------------------------
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index 2801168d632..b499a84fdcf 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -361,6 +361,7 @@ Clang-Tidy Checks
    readability-inconsistent-declaration-parameter-name
    readability-isolate-declaration
    readability-magic-numbers
+   readability-make-member-function-const
    readability-misleading-indentation
    readability-misplaced-array-index
    readability-named-parameter
diff --git a/clang-tools-extra/docs/clang-tidy/checks/readability-make-member-function-const.rst b/clang-tools-extra/docs/clang-tidy/checks/readability-make-member-function-const.rst
new file mode 100644
index 00000000000..2d1de5fdeb7
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/readability-make-member-function-const.rst
@@ -0,0 +1,67 @@
+.. title:: clang-tidy - readability-make-member-function-const
+
+readability-make-member-function-const
+======================================
+
+Finds non-static member functions that can be made ``const``
+because the functions don't use ``this`` in a non-const way.
+
+This check tries to annotate methods according to
+`logical constness <https://isocpp.org/wiki/faq/const-correctness#logical-vs-physical-state>`_
+(not physical constness).
+Therefore, it will suggest to add a ``const`` qualifier to a non-const
+method only if this method does something that is already possible though the
+public interface on a ``const`` pointer to the object:
+
+* reading a public member variable
+* calling a public const-qualified member function
+* returning const-qualified ``this``
+* passing const-qualified ``this`` as a parameter.
+
+This check will also suggest to add a ``const`` qualifier to a non-const
+method if this method uses private data and functions in a limited number of
+ways where logical constness and physical constness coincide:
+
+* reading a member variable of builtin type
+
+Specifically, this check will not suggest to add a ``const`` to a non-const
+method if the method reads a private member variable of pointer type because
+that allows to modify the pointee which might not preserve logical constness.
+For the same reason, it does not allow to call private member functions
+or member functions on private member variables.
+
+In addition, this check ignores functions that
+
+* are declared ``virtual``
+* contain a ``const_cast``
+* are templated or part of a class template
+* have an empty body
+* do not (implicitly) use ``this`` at all
+  (see `readability-convert-member-functions-to-static <readability-convert-member-functions-to-static.html>`_).
+
+The following real-world examples will be preserved by the check:
+
+.. code-block:: c++
+
+  class E1 {
+    Pimpl &getPimpl() const;
+  public:
+    int &get() {
+      // Calling a private member function disables this check.
+      return getPimpl()->i;
+    }
+    ...
+  };
+
+  class E2 {
+  public:
+    const int *get() const;
+    // const_cast disables this check.
+    S *get() {
+      return const_cast<int*>(const_cast<const C*>(this)->get());
+    }
+    ...
+  };
+
+After applying modifications as suggested by the check, runnnig the check again
+might find more opportunities to mark member functions ``const``.