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[WIP] Forbid object lifetime changing pointer casts #136776
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…, r=<try> [WIP] Forbid object lifetime changing pointer casts Fixes rust-lang#136702 r? `@ghost`
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Most of these are on github; in terms of crates.io regressions all we have is:
Overall, 142 regressions are caused by EDIT: Ah, there's also |
We discussed this in the lang triage call today. We wanted to think more about it, so we're leaving it nominated to discuss again. |
@BoxyUwU Do you think it would be possible to implement this as an FCW? We talked about this in lang triage today and would prefer to start with that if we can. If it's not feasible, a hard error can also work (I would say though that we should upstream PRs to any crates we break). Another small thing I noticed is that the error message links to the Nomicon section on variance, but it would be ideal to link to a tracking issue or something describing this issue in particular. |
To add on to what tmandry, said, in our discussions we did feel that the approach taken in this PR is generally the right way forward, and we're happy to see this progress so as to help clear the way for cc @rust-lang/lang |
@tmandry I do expect it to be possible to FCW this. We can likely do something hacky around to fully emulate the fix (but as a lint), but if that doesn't work out all the regression we found were relatively "simple" cases that can probably be taken advantage of (if need be) to lint a subset of the actual cases we'd break with this PR edit: see compiler-errors' comment, I'm not so convinced this will be possible to FCW anymore and will likely investigate improving the diagnostics here. I've already filed PRs to the affected crates to migrate them over to a transmute to avoid the breakage if this lands |
I was thinking earlier that it may be possible to implement a lint to detect, but it seems to me that MIR borrowck is not equipped to implement such a lint. Specifically, it seems near impossible to answer whether a region outlives constraint (like, To fix this would require some significant engineering effort to refactor how NLL processes its region graph to make it easier to clone and reprocess with new constraints. |
…uto_to_object-hard-error, r=oli-obk Make `ptr_cast_add_auto_to_object` lint into hard error In Rust 1.81, we added a FCW lint (including linting in dependencies) against pointer casts that add an auto trait to dyn bounds. This was part of work making casts of pointers involving trait objects stricter, and was part of the work needed to restabilize trait upcasting. We considered just making this a hard error, but opted against it at that time due to breakage found by crater. This breakage was mostly due to the `anymap` crate which has been a persistent problem for us. It's now a year later, and the fact that this is not yet a hard error is giving us pause about stabilizing arbitrary self types and `derive(CoercePointee)`. So let's see about making a hard error of this. r? ghost cc `@adetaylor` `@Darksonn` `@BoxyUwU` `@RalfJung` `@compiler-errors` `@oli-obk` `@WaffleLapkin` Related: - rust-lang#135881 - rust-lang#136702 - rust-lang#136776 Tracking: - rust-lang#127323 - rust-lang#44874 - rust-lang#123430
…uto_to_object-hard-error, r=oli-obk Make `ptr_cast_add_auto_to_object` lint into hard error In Rust 1.81, we added a FCW lint (including linting in dependencies) against pointer casts that add an auto trait to dyn bounds. This was part of work making casts of pointers involving trait objects stricter, and was part of the work needed to restabilize trait upcasting. We considered just making this a hard error, but opted against it at that time due to breakage found by crater. This breakage was mostly due to the `anymap` crate which has been a persistent problem for us. It's now a year later, and the fact that this is not yet a hard error is giving us pause about stabilizing arbitrary self types and `derive(CoercePointee)`. So let's see about making a hard error of this. r? ghost cc ``@adetaylor`` ``@Darksonn`` ``@BoxyUwU`` ``@RalfJung`` ``@compiler-errors`` ``@oli-obk`` ``@WaffleLapkin`` Related: - rust-lang#135881 - rust-lang#136702 - rust-lang#136776 Tracking: - rust-lang#127323 - rust-lang#44874 - rust-lang#123430
…uto_to_object-hard-error, r=oli-obk Make `ptr_cast_add_auto_to_object` lint into hard error In Rust 1.81, we added a FCW lint (including linting in dependencies) against pointer casts that add an auto trait to dyn bounds. This was part of work making casts of pointers involving trait objects stricter, and was part of the work needed to restabilize trait upcasting. We considered just making this a hard error, but opted against it at that time due to breakage found by crater. This breakage was mostly due to the `anymap` crate which has been a persistent problem for us. It's now a year later, and the fact that this is not yet a hard error is giving us pause about stabilizing arbitrary self types and `derive(CoercePointee)`. So let's see about making a hard error of this. r? ghost cc ```@adetaylor``` ```@Darksonn``` ```@BoxyUwU``` ```@RalfJung``` ```@compiler-errors``` ```@oli-obk``` ```@WaffleLapkin``` Related: - rust-lang#135881 - rust-lang#136702 - rust-lang#136776 Tracking: - rust-lang#127323 - rust-lang#44874 - rust-lang#123430
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I've pushed a first attempt at some special cased diagnostics for this breakage, it links to #141402 which could probably do with some work. |
Oh another update: the |
☔ The latest upstream changes (presumably #143036) made this pull request unmergeable. Please resolve the merge conflicts. |
It seems to me that all of the things requested in #136776 (comment) have been added. @rustbot labels +I-lang-nominated -I-lang-radar |
We talked about this in the lang meeting today. Let's do it. Note that we'll want a Reference PR for this. This is a dual FCP with types. @rfcbot fcp merge |
Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:
Concerns:
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. |
cc @ehuss |
@rfcbot reviewed |
1 similar comment
@rfcbot reviewed |
@rfcbot concern needs FCP proposal want to actually write up something for the types folks first before they have to look at this |
I will not have time to make such a PR myself and am not particularly interested in writing one either. it seems like the existing state of the reference does not really talk whatsoever about the restrictions on pointer casts let alone about subtle details of region constraints arising from each MIR operation. I am surprised that this PR would require a reference PR when previous PRs in this area did not, e.g. disallowing casting |
If documentation is required to land this, then I am happy to write it. |
@rfcbot concern can we do a FCW I want to take a second look at whether a FCW would be possible |
Thanks for that. |
The relevant change for that went in ahead of the PR, in rust-lang/reference#1622.
You'll notice I put up #136764 the day after merging the Reference PR. I don't recall the specifics, but it probably wasn't a coincidence. That said, I'm sure you can find cases where we let something through that should have had a Reference PR but did not. We're trying to be better about this. As you may have seen, e.g., in the PR for the release notes of Rust 1.89, in #144097 (comment), I spot-checked how we did about this for that release. |
I don't see that documentation you linked anywhere in the reference. It's not in the pointer casts section and there's nothing under the big table in the type casts sections that talks about this. |
In rust-lang/reference#1732, it was moved to the footnote here: https://doc.rust-lang.org/1.88.0/reference/expressions/operator-expr.html#footnote-meta-compat Admittedly that's probably not the best place for it. (Partly it's a rendering issue; we'd prefer for footnotes on tables to appear directly below the table.) |
Fixes #136702
r? @ghost