document that the null pointer has the 0 address

[RalfJung]

Fixes #116895

Will need t-lang FCP, but I think this is fairly uncontroversial -- there's probably already tons of code out there that relies on this.

[rustbot]

r? @thomcc

(rustbot has picked a reviewer for you, use r? to override)

[scottmcm]

Notably these functions only work on T: Thin, so I think this is also an unavoidable consequence of other things that we've previously guaranteed around Option<NonNull<T>>.

@rfcbot fcp merge

[rfcbot]

Team member @scottmcm has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rustbot labels +I-lang-nominated

I'm going to nominate even though FCP has been proposed so that this comes up at least once in the meeting.

[nikomatsakis]

@rfcbot reviewed

[tmandry]

@traviscross Doesn't our agenda generator include proposed FCPs already?

We already have quite a long list of nominated issues; I don't think nominating every newly proposed FCP is helping with that.

(edit: I am specifically pushing back on nominating for the purpose of getting eyes and checkboxes on the FCP. If your goal is to spark a discussion within the lang team meeting, then nominating is good for that.)

[tmandry]

From #116895 (comment) it sounds like you've already considered the implications for provenance. With those aside I think we should go ahead and commit to this.

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[RalfJung]

Yeah, the good thing is that nobody expects to be able to load from a null ptr so provenance basically does not matter.

[scottmcm]

Yes, all p-FCPs are included in the meeting in the section that starts with

Proposed FCPs

Check your boxes!

So I interpreted the nomination here as "please p-fcp merge this", which is why I removed nomination after doing that.

Checking boxes is generally something that should be done asynchronously via the https://rfcbot.rs/ list, so that the synchronous time in triage can be spent on things with specific questions in the nomination.

[joshtriplett]

This is not a blocker for this issue, but could we please get a comment somewhere appropriate that says what code should do in the rare case where it has memory mapped at 0 and needs to write to it? (e.g. low-level hardware programming, or weird userspace programming situations that need the zero page mapped.) We should identify what function will allow such a write without complaint, and point people to that.

[RalfJung]

Currently there is no way to write such code in Rust, you have to use inline asm. rust-lang/unsafe-code-guidelines#29 tracks providing support for this.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[RalfJung]

@thomcc this should be ready to land then :)

[WaffleLapkin]

Since this is only a documentation change and the semantic was approved by T-lang, I'll go ahead and merge this. Feel free to r- if you think that a T-libs review is required.

[WaffleLapkin]

@bors r+ rollup

[bors]

📌 Commit 98d54da has been approved by WaffleLapkin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 98d54da with merge b049093...

[bors]

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing b049093 to master...

[rust-timer]

Finished benchmarking commit (b049093): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.4%, 2.0%]	6
Regressions ❌ (secondary)	4.2%	[4.2%, 4.2%]	1
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[-0.5%, 2.0%]	8

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.6%, 0.6%]	2
Regressions ❌ (secondary)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.6%, 0.6%]	2

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 662.908s -> 662.932s (0.00%)
Artifact size: 308.97 MiB -> 308.95 MiB (-0.01%)