Introduce `as_slice`/`as_mut_slice` methods on `std::vec::IntoIter` struct.

[frewsxcv]

Tracking issue for: #35447

[SimonSapin]

I’ve wanted to use this today, but I’ll probably have to change my code to work differently.

FCP to stabilize?

[SimonSapin]

For context: this code is using vec::IntoIter<syn::TokenTree> as parser input.

For more context: I’m attempting to replace a compiler plugin (for a procedural expression-macro) by a build script that parses the source file for a Rust module with the syn crate, walks the AST to find invocations of this macro, and replace them with the expansion. Vec<TokenTree> is used to represent the "arguments" of a macro invocation.

[alexcrichton]

@rfcbot fcp merge

Neat methods!

[rfcbot]

Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged teams:

• @BurntSushi
• @Kimundi
• @alexcrichton
• @aturon
• @brson
• @sfackler

No concerns currently listed.

Once these reviewers reach consensus, this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

psst @alexcrichton, I wasn't able to add the final-comment-period label, please do so.

[rfcbot]

The final comment period is now complete.