Implement 'note'

[graydon]

When unwinding, we should log things marked with 'note' in the unwinding block. Currently we have no support for this, despite excited claims in the manual.

[brson]

A simple version of this can probably be implemented with a library function and a macro. @nikomatsakis has been pining for the feature lately.

[catamorphism]

Changing this to an enhancement for doing what @brson suggested as a library.

[14427]

I have written a note macro, although I'm not sure if this is what you had in mind: https://gist.github.com/4262524

[thestinger]

D provides this functionality as part of their scope guard support, by allowing you to install them for one of exit, success or failure. Macros for all 3 cases would probably make sense in Rust too.

[pnkfelix]

Can someone point me at the spec for this? I cannot judge whether the note macro proposed in 14427's comment would suffice.

[catamorphism]

(bug triage) Milestone seems right.

[thestinger]

Replaced with #9835, based on recent movement towards RAII idioms. The ability to run code on success or on exit regardless of failure is useful too.