make offset must_use

[steveklabnik]

https://djugei.github.io/bad-at-unsafe/ describes an error a user had when trying to use offset:

At first I just assumed that the .add() and .offset() methods on pointers would mutate the pointer. They do not. Instead they return a new pointer, which gets dropped silently if you don't use it. Unlike for example Result, which is must_use annotated.

This PR only adds offset, because I wanted to float the idea; I'm imagining that there's more than just add and offset that could use this. I am also very open to re-wording the warning.

r? @rust-lang/libs

[CAD97]

Did you mean to annotate the intrinsic rather than the stable API?

[steveklabnik]

Did you mean to annotate the intrinsic rather than the stable API?

My thought process was:

1. oh, this is the intrinsic.
2. you know what, I think that the stable api is just a re-export?
3. I could go check but, if this is accepted, I'm gonna have to go in and update at least add anyway.
4. I'd rather get the conversation started, so just send it in. I'll fix it later if it needs fixing.

[sfackler]

We've added must_use to similar methods like wrapping_add in the past. r=me with it on the public API

[Mark-Simulacrum]

I would suggest expanding to all pointer methods which don't mutate self - that seems consistent to me. And presumably we should put the attribute on both the intrinsic and the stabilized methods? Seems odd to do just one.

[sfackler]

No one should be calling the intrinsics though.

[Mark-Simulacrum]

I don't mind not touching the intrinsics; I agree no one should be calling them. But the cost is tiny so seems like it wouldn't hurt to at least make sure our internal uses are also doing the right thing. I agree there's very little benefit.

[steveklabnik]

So, before I add them, what's the call here? I personally would choose to add them to both, but don't have a super strong preference. @sfackler , do you actively oppose adding them, or do you not mind if it's both?

[sfackler]

Sure, I have no objection to adding them to the intrinsics.

[steveklabnik]

Sigh, I do not know why this updated some submodules. trying to fix it now

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

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---
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== end clock drift check ==

##[error]Bash exited with code '1'.
##[section]Finishing: Run build
##[section]Starting: Checkout rust-lang/rust@refs/pull/72143/merge to s
Task         : Get sources
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[steveklabnik]

Okay, this is ready for a re-review. I did the intrinsics as well as the functions that returned a new pointer. I'd appreciate someone checking that this is all accurate. :)

[sfackler]

@bors r+ rollup

[bors]

📌 Commit aea0186 has been approved by sfackler