Avoid closures to improve compile times.

[nnethercote]

HashMap and HashSet are used widely, and often instantiated many
times. As a result, small differences in how the code is written can
have a significant effect on how much LLVM IR is generated, which
affects compile times.

This commit avoids a lot of small closures by replacing calls to
Option::map, Option::ok_or_else, Option::unwrap_or_else and
Result::unwrap_or_elsewithmatch` expressions. Although this makes
the code less concise, it improves compile times. For example, several
of the benchmarks in rustc-perf compile up to 3.5% faster after this
change is incorporated into std.

Every change is accompanied by a short comment to explain why a match
is used. This may seem excessive, but without these comments it would be
easy for a well-meaning person in the future to change some or all of
these back to the original versions without understanding the
consequences.

[nnethercote]

Here are some measurements from my Linux box, measuring full (non-incremental) builds for the "real-world" benchmarks in rustc-perf.

cargo-debug
        avg: -3.5%      min: -3.5%      max: -3.5%
webrender-wrench-debug
        avg: -2.8%      min: -2.8%      max: -2.8%
regex-debug
        avg: -2.7%      min: -2.7%      max: -2.7%
webrender-debug
        avg: -2.7%      min: -2.7%      max: -2.7%
tokio-webpush-simple-debug
        avg: -1.9%      min: -1.9%      max: -1.9%
webrender-opt
        avg: -1.9%      min: -1.9%      max: -1.9%
piston-image-debug
        avg: -1.4%      min: -1.4%      max: -1.4%
cranelift-codegen-debug
        avg: -1.3%      min: -1.3%      max: -1.3%
cargo-opt
        avg: -1.3%      min: -1.3%      max: -1.3%
ripgrep-debug
        avg: -1.3%      min: -1.3%      max: -1.3%
clap-rs-debug
        avg: -1.2%      min: -1.2%      max: -1.2%
clap-rs-opt
        avg: -1.2%      min: -1.2%      max: -1.2%
html5ever-debug
        avg: -1.1%      min: -1.1%      max: -1.1%
html5ever-opt
        avg: -1.0%      min: -1.0%      max: -1.0%
syn-debug
        avg: -1.0%      min: -1.0%      max: -1.0%
regex-opt
        avg: 1.0%       min: 1.0%       max: 1.0%
cranelift-codegen-opt
        avg: -0.9%      min: -0.9%      max: -0.9%
hyper-2-debug
        avg: -0.8%      min: -0.8%      max: -0.8%
serde-check
        avg: -0.8%      min: -0.8%      max: -0.8%
futures-debug
        avg: -0.8%      min: -0.8%      max: -0.8%
serde-debug
        avg: -0.8%      min: -0.8%      max: -0.8%
serde-opt
        avg: -0.7%      min: -0.7%      max: -0.7%
futures-check
        avg: -0.6%      min: -0.6%      max: -0.6%
clap-rs-check
        avg: -0.6%      min: -0.6%      max: -0.6%
ripgrep-opt
        avg: -0.6%      min: -0.6%      max: -0.6%
tokio-webpush-simple-opt
        avg: -0.6%      min: -0.6%      max: -0.6%
piston-image-opt
        avg: -0.5%      min: -0.5%      max: -0.5%
cargo-check
        avg: -0.5%      min: -0.5%      max: -0.5%
syn-opt
        avg: -0.5%?     min: -0.5%?     max: -0.5%?
hyper-2-check
        avg: -0.5%      min: -0.5%      max: -0.5%
webrender-wrench-opt
        avg: -0.4%      min: -0.4%      max: -0.4%
html5ever-check
        avg: -0.4%      min: -0.4%      max: -0.4%
hyper-2-opt
        avg: -0.4%      min: -0.4%      max: -0.4%
piston-image-check
        avg: -0.4%      min: -0.4%      max: -0.4%
futures-opt
        avg: -0.4%      min: -0.4%      max: -0.4%
webrender-check
        avg: -0.4%      min: -0.4%      max: -0.4%
cranelift-codegen-check
        avg: -0.4%      min: -0.4%      max: -0.4%
ripgrep-check
        avg: -0.4%      min: -0.4%      max: -0.4%
regex-check
        avg: -0.4%      min: -0.4%      max: -0.4%
webrender-wrench-check
        avg: -0.4%      min: -0.4%      max: -0.4%
tokio-webpush-simple-check
        avg: -0.3%      min: -0.3%      max: -0.3%
syn-check
        avg: -0.3%      min: -0.3%      max: -0.3%

[Amanieu]

@bors r+

[bors]

📌 Commit e47bec1 has been approved by Amanieu

[bors]

⌛ Testing commit e47bec1 with merge 09e43a8...

[bors]

☀️ Test successful - checks-travis
Approved by: Amanieu
Pushing 09e43a8 to master...

[nnethercote]

@Amanieu: I know that hashbrown was just upgraded in std to 0.8.1, but I think it would be worth doing a 0.8.2 release with this change so that the compile time benefits can be spread widely?

[Amanieu]

Sure, I'll do that after #184 is merged.