style: add clang-format file (#2310)

* style: adding clang-format as manual hook

* docs: adding a Clang-Format section. (#2803)
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

.clang-format

+---
+# See all possible options and defaults with:
+# clang-format --style=llvm --dump-config
+BasedOnStyle: LLVM
+AccessModifierOffset: -4
+AlignConsecutiveAssignments: true
+AlwaysBreakTemplateDeclarations: Yes
+BinPackArguments: false
+BinPackParameters: false
+BreakBeforeBinaryOperators: All
+BreakConstructorInitializers: BeforeColon
+ColumnLimit: 99
+IndentCaseLabels: true
+IndentPPDirectives: AfterHash
+IndentWidth: 4
+Language: Cpp
+SpaceAfterCStyleCast: true
+# SpaceInEmptyBlock: true # too new
+Standard: Cpp11
+TabWidth: 4
+...

.github/CONTRIBUTING.md

@@ -176,10 +176,35 @@ name, pre-commit):
 pre-commit install
 ```
 
+### Clang-Format
+
+As of v2.6.2, pybind11 ships with a [`clang-format`][clang-format]
+configuration file at the top level of the repo (the filename is
+`.clang-format`). Currently, formatting is NOT applied automatically, but
+manually using `clang-format` for newly developed files is highly encouraged.
+To check if a file needs formatting:
+
+```bash
+clang-format -style=file --dry-run some.cpp
+```
+
+The output will show things to be fixed, if any. To actually format the file:
+
+```bash
+clang-format -style=file -i some.cpp
+```
+
+Note that the `-style-file` option searches the parent directories for the
+`.clang-format` file, i.e. the commands above can be run in any subdirectory
+of the pybind11 repo.
+
 ### Clang-Tidy
 
-To run Clang tidy, the following recipe should work. Files will be modified in
-place, so you can use git to monitor the changes.
+[`clang-tidy`][clang-tidy] performs deeper static code analyses and is
+more complex to run, compared to `clang-format`, but support for `clang-tidy`
+is built into the pybind11 CMake configuration. To run `clang-tidy`, the
+following recipe should work. Files will be modified in place, so you can
+use git to monitor the changes.
 
 ```bash
 docker run --rm -v $PWD:/pybind11 -it silkeh/clang:10
@@ -198,7 +223,7 @@ cmake -S . -B build-iwyu -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=$(which include-what-y
 cmake --build build
 ```
 
-The report is sent to stderr; you can pip it into a file if you wish.
+The report is sent to stderr; you can pipe it into a file if you wish.
 
 ### Build recipes
 
@@ -325,6 +350,8 @@ if you really want to.
 
 
 [pre-commit]: https://pre-commit.com
+[clang-format]: https://clang.llvm.org/docs/ClangFormat.html
+[clang-tidy]: https://clang.llvm.org/extra/clang-tidy/
 [pybind11.readthedocs.org]: http://pybind11.readthedocs.org/en/latest
 [issue tracker]: https://github.com/pybind/pybind11/issues
 [gitter]: https://gitter.im/pybind/Lobby

include/pybind11/iostream.h

@@ -104,8 +104,10 @@ PYBIND11_NAMESPACE_END(detail)
     .. code-block:: cpp
 
         {
-            py::scoped_ostream_redirect output{std::cerr, py::module_::import("sys").attr("stderr")};
-            std::cerr << "Hello, World!";
+            py::scoped_ostream_redirect output{
+                std::cerr,
+                py::module::import("sys").attr("stderr")
+            };
         }
  \endrst */
 class scoped_ostream_redirect {

setup.cfg

@@ -43,12 +43,7 @@ ignore =
     docs/**
     tools/**
     include/**
-    .appveyor.yml
-    .cmake-format.yaml
-    .gitmodules
-    .pre-commit-config.yaml
-    .readthedocs.yml
-    .clang-tidy
+    .*
     pybind11/include/**
     pybind11/share/**
     CMakeLists.txt