implement scope guards, as a syntax ext or language feature

[thestinger]

http://dlang.org/statement.html#ScopeGuardStatement

[sfackler]

Here's a simple ScopeGuard implementation as a library: https://gist.github.com/sfackler/6965006. Having to manually save the object in a variable isn't great, but a syntax extension to do expand

scope!(Success) { thing }

to something like

let __scope_<FILE>_<LINE>_<COL> = ::std::scope::scope(Success, { thing });

shouldn't be too hard. I don't think it can be done with a macro_rules currently.

[alexcrichton]

The original blocker for this was that closures didn't borrow their contents, so an implementation was unsound. The source of this unsoundness has been fixed, and this was attempted in #11905. The PR was abandoned because the borrowing became too unmanageable (prevented too many legitimate situations).

A new effort may uncover something different, and this may need to be a language feature rather than a library if we do indeed add it.

[japaric]

This issue is a feature request, and needs an approved RFC to be implemented. It should be moved to the rust-lang/rfcs repo. (As per the issues policy)

cc @nick29581

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#394