Add a list of allowed and disallowed Abseil things

Bug: webrtc:8821
Change-Id: Ifb3bacd3403bbf823c78fff47571a83159f1da73
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/95880
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24437}

DEPS

@@ -1043,7 +1043,7 @@ include_rules = [
   "+test",
   "+rtc_tools",
 
-  # Abseil whitelist.
+  # Abseil whitelist. Keep this in sync with abseil-in-webrtc-md.
   "+absl/container/inlined_vector.h",
   "+absl/memory/memory.h",
   "+absl/strings/string_view.h",

OWNERS

@@ -18,6 +18,9 @@ per-file pylintrc=phoglund@webrtc.org
 per-file THIRD_PARTY_DEPS=phoglund@webrtc.org
 per-file THIRD_PARTY_DEPS=titovartem@webrtc.org
 per-file WATCHLISTS=*
+per-file abseil-in-webrtc.md=danilchap@webrtc.org
+per-file abseil-in-webrtc.md=kwiberg@webrtc.org
+per-file abseil-in-webrtc.md=mbonadei@webrtc.org
 per-file style-guide.md=danilchap@webrtc.org
 per-file style-guide.md=kwiberg@webrtc.org
 per-file native-api.md=kwiberg@webrtc.org

abseil-in-webrtc.md

@@ -0,0 +1,50 @@
+# Using Abseil in WebRTC
+
+You may use a subset of the utilities provided by the [Abseil][abseil]
+library when writing WebRTC C++ code. Below, we list the explicitly
+*allowed* and the explicitly *disallowed* subsets of Abseil; if you
+find yourself in need of something that isn’t in either subset,
+please add it to the *allowed* subset in this doc in the same CL that
+adds the first use.
+
+[abseil]: https://abseil.io/about/
+
+## **Allowed**
+
+* `absl::InlinedVector`
+* `absl::make_unique` and `absl::WrapUnique`
+* `absl::optional` and related stuff from `absl/types/optional.h`.
+* `absl::string_view`
+* `absl::variant` and related stuff from `absl/types/variant.h`.
+
+## **Disallowed**
+
+### `absl::Mutex`
+
+*Use `rtc::CriticalSection` instead.*
+
+Chromium has a ban on new static initializers, and `absl::Mutex` uses
+one. To make `absl::Mutex` available, we would need to nicely ask the
+Abseil team to remove that initializer (like they already did for a
+spinlock initializer). Additionally, `absl::Mutex` handles time in a
+way that may not be compaible with the rest of WebRTC.
+
+### `absl::Span`
+
+*Use `rtc::ArrayView` instead.*
+
+`absl::Span` differs from `rtc::ArrayView` on several points, and both
+of them differ from the `std::span` that was voted into
+C++20—and `std::span` is likely to undergo further changes
+before C++20 is finalized. We should just keep using `rtc::ArrayView`
+and avoid `absl::Span` until C++20 is finalized and the Abseil team
+has decided if they will change `absl::Span` to match.
+[Bug](https://bugs.webrtc.org/9214).
+
+### `absl::StrCat` and `absl::StrAppend`
+
+*Use `rtc::SimpleStringBuilder` instead.*
+
+These are optimized for speed, not binary size. Even `StrCat` calls
+with a modest number of arguments can easily add several hundred bytes
+to the binary.

style-guide.md

@@ -31,6 +31,13 @@ WebRTC is written in C++11, but with some restrictions:
 
 [chromium-cpp11]: https://chromium-cpp.appspot.com/
 
+### Abseil
+
+You may use a subset of the utilities provided by the [Abseil][abseil]
+library when writing WebRTC C++ code. [Details](abseil-in-webrtc.md).
+
+[abseil]: https://abseil.io/about/
+
 ### <a name="h-cc-pairs"></a>`.h` and `.cc` files come in pairs
 
 `.h` and `.cc` files should come in pairs, with the same name (except