Consider indirect mutation during const qualification dataflow #90214
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How much of MaybeMutBorrowedLocals is duplicated now? Should we try to deduplicate or at least keep them in sync? |
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I would have taken a more minimal approach, although this is fine as well and might be more robust. Thanks for writing those tests by the way! Clearly, our coverage is not great at the moment.
My thought was to check MaybeMutBorrowedLocals whenever an assignment occurs in the Qualif transfer function in addition to checking it when inspecting the qualifs for a type. This is maybe a bit more convoluted, but it allows us to share more code. Did you consider this approach? Do you prefer the one in this PR?
I think giving mutable borrows a bit of field sensitivity is fine BTW. #73255 is another step in that direction (since drop elaboration is field sensitive), and I suspect all of Qualif will be similarly updated at some point for consistency's sake.
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| if !self.state.borrow.contains(local) { | ||
| self.state.qualif.remove(local); | ||
| } |
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This is pretty conservative. IS_CLEARED_ON_MOVE is only used for the Drop qualifs. Wouldn't this prevent code like the following, which I believe retains a frivolous Drop terminator for the argument, from compiling?
fn foo<T>(opt: Option<T>) -> Option<T> {
let _ = &opt; // `T` may have interior mutability
opt
}#73255 would reduce the impact by removing the frivolous drop altogether, although I believe there are still edge cases where a borrowed variable is moved out of in one CFG path and assigned an unqualified value in the other. I don't think we care about those very much, however.
Perhaps you chose to be conservative here because valid raw pointers can point to moved-from variables. I don't think it's guaranteed that the second Box in the following example will be leaked, but that's what we do currently do AFAIK:
let mut x = Box::new(42);
let px = std::ptr::addr_of_mut!(x);
let y = x; // Move `x`
unsafe { *px = Box::new(12); } // This `Box` never gets dropped.There was a problem hiding this comment.
In terms of semantics, I extrapolated the existing logic of MaybeMutBorrowedLocals and consider its effects on overall computation. For this decision it followed that:
- The analysis is field sensitive, because
MaybeMutBorrowedLocalsis field sensitive when looking for the interior mutability. - The qualifs are not cleared after move, because neither are borrows in
MaybeMutBorrowedLocals.
I think it would be fine to clear a qualif after move, under the condition that the local becomes qualified on the assignment if it was borrowed before. Though, this is something I would consider independently of this changes.
The specific example given above is already rejected though, because a local which is borrowed is the same which is dropped, so indirectly_mutable works as intended (note the missing const fn).
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True. No need to change behavior as part of this PR.
Would you mind leaving a note that this isn't fixing a known issue (assuming the raw pointers to moved-from variables doesn't count), but is there to preserve backwards compatibility?
I didn't consider this specific approach. It would indeed allow us to reuse |
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I think I like your approach rather than the "quick fix" I described above. Yours seems easier to extend going forward, and including r=me on the dataflow stuff and overall approach. Maybe @oli-obk wants to weigh in on potential code sharing. Personally I think this version is okay, since your helper functions are nicely encapsulated. Another question for @oli-obk, it looks like const trait and thus const Drop is making progress. Was there any thought given to the implicit mutable borrow that occurs when a |
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r? @oli-obk (see above) |
Previously a local would be qualified if either one of two separate data flow computations indicated so. First determined if a local could contain the qualif, but ignored any forms of indirect mutation. Second determined if a local could be mutably borrowed (and so indirectly mutated), but which in turn ignored the qualif. The end result was incorrect because the effect of indirect mutation was effectivelly ignored in the all but the final stage of computation. In the new implementation the indirect mutation is directly incorporated into the qualif data flow. The local variable becomes immediately qualified once it is mutably borrowed and borrowed place type can contain the qualif. In general we will now reject additional programs, program that were prevously unintentionally accepted. There are also some cases which are now accepted but were previously rejected, because previous implementation didn't consider whether borrowed place could have the qualif under the consideration.
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No. I wasn't aware there was a concern here (due to ignorance, not because anything is "obvious to me"). But I believe your reasoning is correct. Whatever we called drop on may not be accessed again (beyond deallocating its memory). @bors r=ecstatic-morse,oli-obk |
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📌 Commit 93f85f5 has been approved by |
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⌛ Testing commit 93f85f5 with merge 5ceac2fdf966a0f93b678c185849f19caae75063... |
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The job Click to see the possible cause of the failure (guessed by this bot) |
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💔 Test failed - checks-actions |
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@bors retry |
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☀️ Test successful - checks-actions |
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Finished benchmarking commit (37f70a0): comparison url. Summary: This benchmark run did not return any relevant changes. If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf. @rustbot label: -perf-regression |
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| fn address_of_allows_mutation(&self, mt: mir::Mutability, place: mir::Place<'tcx>) -> bool { | ||
| match mt { | ||
| mir::Mutability::Mut => true, | ||
| mir::Mutability::Not => self.shared_borrow_allows_mutation(place), |
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This seems to presuppose a particular resolution of #56604, where casting an &mut to *const will always produce an immutable pointer (except for UnsafeCell). I don't think we have officially committed to this yet.
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Do you think we should change this to be more conservative here? The semantics is inherited from previously used MaybeMutBorrowedLocals.
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Yeah I think it might be good not to assume that addr_of!(x.field) is read-only, for now. I personally think it should be read-only but I am not sure if there is consensus for that.
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Previously a local would be qualified if either one of two separate data
flow computations indicated so. First determined if a local could
contain the qualif, but ignored any forms of indirect mutation. Second
determined if a local could be mutably borrowed (and so indirectly
mutated), but which in turn ignored the qualif.
The end result was incorrect because the effect of indirect mutation was
effectivelly ignored in the all but the final stage of computation.
In the new implementation the indirect mutation is directly incorporated
into the qualif data flow. The local variable becomes immediately
qualified once it is mutably borrowed and borrowed place type can
contain the qualif.
In general we will now reject additional programs, program that were
prevously unintentionally accepted.
There are also some cases which are now accepted but were previously
rejected, because previous implementation didn't consider whether
borrowed place could have the qualif under the consideration.
Fixes #90124.
r? @ecstatic-morse