Karl Wiberg bd0decab82 Ban absl::StrSplit and absl::StrJoin

For binary size reasons; they emit too many instructions at each call
site.

I decided to not point out replacements for them, since rtc::join has
only one caller, and rtc::split isn't the only option for splitting
strings. And both of them have APIs that can cause the creation of
needless temporary strings.

Bug: none
Change-Id: I06161012836a90b09249c2813d2ddc15f167d580
Notry: true
Reviewed-on: https://webrtc-review.googlesource.com/c/124480
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26846}

2019-02-26 00:45:11 +00:00

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Using Abseil in WebRTC

You may use a subset of the utilities provided by the 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.

Allowed

absl::InlinedVector
absl::make_unique and absl::WrapUnique
absl::optional and related stuff from absl/types/optional.h.
absl::string_view
The functions in absl/strings/ascii.h, absl/strings/match.h, and absl/strings/str_replace.h.
absl::is_trivially_copy_constructible, absl::is_trivially_copy_assignable, and absl::is_trivially_destructible from absl/meta/type_traits.h.
absl::variant and related stuff from absl/types/variant.h.
The functions in absl/algorithm/algorithm.h and absl/algorithm/container.h
The macros in absl/base/attributes.h and absl/base/config.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.

`absl::StrCat`, `absl::StrAppend`, `absl::StrJoin`, `absl::StrSplit`

Use rtc::SimpleStringBuilder to build strings.

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.

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