Add safety comments for alloc::string

[yilin0518]

This PR serves as a following PR with #163445 . This PR replace // ignore-tidy-undocumented-unsafe with concrete safety comments. Every // ignore-tidy-undocumented-unsafe in alloc::string is checked. This PR only contains a file for ease of review.

[rustbot]

r? @jhpratt

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[jhpratt]

The error from CI is

error: expected a trait, found type
##[error]    --> /checkout/library/alloc/src/string.rs:2779:6
     |
2779 | impl const Default for String {
     |      ^^^^^^^^^^^^^

This is probably rustfmt being out of date. If you change that around (to const impl Default as it currently is), CI should go green and I'll take a look.

[yilin0518]

This is probably rustfmt being out of date. If you change that around (to const impl Default as it currently is), CI should go green and I'll take a look.

I change that line of code by mistake(by using document formatting in vscode). Now it should be no problem.

[jhpratt]

I figured because I have done the exact same thing in the past.

LGTM with the one change.

View changes since this review

[jhpratt]

Thanks!

@bors r+ rollup

[rust-bors]

📌 Commit 2ccceb0 has been tentatively approved by jhpratt

It will be put into the queue for this repository once PR CI succeeds.