Use expose_addr() in fmt::Pointer

[thomcc]

Discussion in https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/Should.20.60fmt.3A.3APointer.60.20expose.20the.20argument.3F on whether or not we should do this (still undecided).

CC @RalfJung

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

I don't have any strong opinions either way (and see benefits in either approach), but for now r? @RalfJung I suppose?

[RalfJung]

This is fine for me but the libs team should be involved. r? libs

[RalfJung]

Oh wait this PR is from a libs team member.^^ @bors r+

[bors]

📌 Commit f2d0366 has been approved by RalfJung

It is now in the queue for this repository.

[nikic]

Do I understand correctly that what this allows is to take print output, parse it as a number, cast that number back to a pointer and then access memory using that? What is the motivation for allowing that?

It makes not practical difference right now, but aspirationally, adding some conditional debug output that happens to print pointer addresses should preferably not impact optimization beyond what is necessary.

[RalfJung]

Do I understand correctly that what this allows is to take print output, parse it as a number, cast that number back to a pointer and then access memory using that?

Yes.

What is the motivation for allowing that?

It is linked from the OP: there's a non-zero chance of people doing that, and no measurable cost to allowing it.