Add scope guards to libstd #9842
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This adds a struct which runs a block when it falls out of scope,
similar to
scope
in D ordefer
in Go. Using this module directlyrequires code to assign the ScopeGuard struct to a variable manually
which is unnecessary in many situations.
It'd be nice to add a syntax extension to automatically expand
to something like
but I don't think that syntax extensions can currently expand to statements as opposed to expressions or items and I'm not familiar enough with libsyntax to add it easily. I put this in libstd instead of libextra since it'll need to be there for the syntax extension to work reasonably.
cc #9835