mark borrowed pointer parameters without a named lifetime as nocapture

[thestinger]

From the LLVM documentation:

nocapture
This indicates that the callee does not make any copies of the pointer that outlive the callee itself. This is not a valid attribute for return values.

@nikomatsakis: is this a sane optimization? it seems so

[nikomatsakis]

Yes, this should be valid.

[Aatch]

We could do this, but it is worth nothing that the LLVM optimizations are able to infer this most of the time.

[pnkfelix]

visiting for triage, email from 2013 sep 09.

Seems like a potentially simple thing to try out, see how it goes.

(one potential caveat is that if we did just rely on LLVM optimizations to infer this, then we wouldn't have to worry about the nocapture marking getting invalidated by unsafe code. Of course unsafe code probably shouldn't be capturing the values of borrowed pointers anyway...)

[thestinger]

It looks like nocapture might end up being how escape analysis will be implemented in LLVM, so this will get more important.