Optimize fmt::PadAdapter::wrap

[phlopsi]

After adding the first write! usage to my project and printing the result to the console, I noticed, that my binary contains the strings "called Option::unwrap() on a None value`" and more importantly "C:\Users\Patrick Fischer.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\fmt\builders.rs", with my release build being configured as follows:

[profile.release]
panic = "abort"
codegen-units = 1
strip = "symbols" # the important bit
lto = true

I am in a no_std environment and my custom panic handler is a simple loop {}. I did not expect the above information to be preserved. I heavily suspect the edited function to be the culprit. It contains the only direct use of Option::unwrap in the entire file and I tracked the symbols in the assembly to be used from the section _ZN68_$LT$core..fmt..builders..PadAdapter$u20$as$u20$core..fmt..Write$GT$9write_str17ha1d5e5efe167202aE.

Aside from me suspecting this function to be the culprit, the replaced code performs the same operation as Option::insert, but without the unreachable_unchecked optimization Option::insert provides. Therefore, it makes sense to me to use the more optimized version, instead.

As I don't change any semantics, I hope a simple pull request suffices.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @joshtriplett (or someone else) soon.

Please see the contribution instructions for more information.

[jyn514]

@bors r+ rollup

[bors]

📌 Commit 3e82dca has been approved by jyn514