Add the configuration for rustfmt which has been used thus far to the package.

This ensures that contributors can run the autoformatter and get results which are compatible with the existing code. Otherwise, the autoformatter will run with its default configuration, heavily modifying existing formatting and obscuring the real changes.

Unfortunately, the current configuration depends on features which are available only in nightly. This fact has been noted both in rustfmt.toml and CONTRIBUTING.md.

See also https://rust-lang.github.io/rustfmt/?version=v1.5.1&search=

PiperOrigin-RevId: 517365782

Author: hovinenb

CONTRIBUTING.md

@@ -26,3 +26,15 @@ information on using pull requests.
 
 This project follows [Google's Open Source Community
 Guidelines](https://opensource.google/conduct/).
+
+## Running the autoformatter rustfmt
+
+This repository uses a custom configuration for rustfmt which currently requires
+that one run the *nightly* version:
+
+```
+cargo +nightly fmt
+```
+
+The stable version will generate error messages and modify a lot of the
+existing formatting, obscuring any real changes you are making.

rustfmt.toml

@@ -0,0 +1,29 @@
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+edition = "2021"
+
+# Because of this, one must use the *nightly* version of rustfmt, e.g.
+#   cargo +nightly fmt
+version = "Two"
+
+use_small_heuristics = "Max"
+
+newline_style = "Unix"
+
+# This is required to keep the autoformatter from clobbering the markdown table
+# in the crate-level documentation in googletest/src/lib.rs.
+wrap_comments = false
+
+format_generated_files = false