2229: Fix issues with move closures and mutability #80092
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rust-highfive
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arora-aman
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Dec 16, 2020
compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs
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| @@ -14,7 +14,6 @@ fn main() { | |||
| let mut c = || { | |||
| //~^ ERROR: cannot borrow `z.0.0.0` as mutable, as it is behind a `&` reference | |||
| z.0.0.0 = format!("X1"); | |||
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I would have expected us to not try read the place z on its own. When the feature isn't enabled we only read z.0.0.0.
Debug logs from the report_mutability_error when the feature is enabled.
DEBUG rustc_mir::borrow_check::diagnostics::mutability_errors report_mutability_error: access_place_desc=Some("z")
....
DEBUG rustc_mir::borrow_check::diagnostics::mutability_errors report_mutability_error: access_place_desc=Some("z.0.0.0")
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This is definitely not the error message we want-- we'd like the error to be reported on z.0.0.0, right? I think this may be related to my comment about the used_mut_upvars but I'm not 100% sure.
arora-aman
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Dec 16, 2020
arora-aman
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Dec 16, 2020
| let mut c = || { | ||
| //~^ ERROR: cannot borrow `z.0.0.0` as mutable, as it is behind a `&` reference | ||
| z.0.0.0 = format!("X1"); | ||
| //~^ ERROR: cannot assign to `z`, as it is not declared as mutable |
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I hack around this error in a later commit
arora-aman
commented
Dec 16, 2020
| // b. Is some path starting from a captured place in our parent closure | ||
| // c. Is a path starting from a local variable in our parent closure. | ||
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| // Handle (a) |
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I think this is maybe missing a case -- if we capture a deref of an &mut. Also, I don't think that (a) is distinct from (b).
| } | ||
| } else { | ||
| this.used_mut.insert(place.local); | ||
| } |
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To handle the deref of &mut` case, we could check here if
- the projection is a deref
- the type of the
place_refis an&mut
| // The place isn't mutable once we dereference a immutable reference. | ||
| ty::Ref(.., hir::Mutability::Not) => return hir::Mutability::Not, | ||
|
|
||
| _ => (), |
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I would add ty::Adt with an assertion that this is a bug, and then make _ => bug!("deref of unexpected pointer type {:?}", pointer_ty.kind()) or something like that
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I'm assuming you meant Adt(boxed)
| @@ -14,7 +14,6 @@ fn main() { | |||
| let mut c = || { | |||
| //~^ ERROR: cannot borrow `z.0.0.0` as mutable, as it is behind a `&` reference | |||
| z.0.0.0 = format!("X1"); | |||
There was a problem hiding this comment.
This is definitely not the error message we want-- we'd like the error to be reported on z.0.0.0, right? I think this may be related to my comment about the used_mut_upvars but I'm not 100% sure.
src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm.rs
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arora-aman
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Dec 26, 2020
src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm.rs
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| // The place isn't mutable once we dereference a immutable reference. | ||
| ty::Ref(.., hir::Mutability::Not) => return hir::Mutability::Not, | ||
|
|
||
| _ => (), |
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I'm assuming you meant Adt(boxed)
| for (i, proj) in place.projections.iter().enumerate() { | ||
| if proj.ty.is_unsafe_ptr() { | ||
| // Don't apply any projections on top of an unsafe ptr | ||
| truncated_length = truncated_length.min(i + 1); |
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I went with this approach here because either we need to do a checked_add or init unsafe_ptr_idx = usize::MAX - 1. compared to the two, this seemed clearer.
arora-aman
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Dec 27, 2020
| let c = || { | ||
| //~^ ERROR: cannot borrow `**ref_mref_x` as mutable, as it is behind a `&` reference | ||
| **ref_mref_x = y; | ||
| //~^ERROR: cannot assign to `ref_mref_x`, as it is not declared as mutable |
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This gets hacked away as well in a later commit.
| let c = || { | ||
| //~^ ERROR: cannot borrow `**mref_ref_x` as mutable, as it is behind a `&` reference | ||
| **mref_ref_x = y; | ||
| //~^ERROR: cannot assign to `mref_ref_x`, as it is not declared as mutable |
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This gets hacked away as well in a later commit.
|
☔ The latest upstream changes (presumably #80503) made this pull request unmergeable. Please resolve the merge conflicts. |
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Conflicts should be resolved now |
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☔ The latest upstream changes (presumably #81113) made this pull request unmergeable. Please resolve the merge conflicts. |
nikomatsakis
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Jan 28, 2021
| #[derive(PartialEq, Clone, Debug, TyEncodable, TyDecodable, TypeFoldable, HashStable)] | ||
| pub struct CapturedPlace<'tcx> { | ||
| /// The `Place` that is captured. |
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I love how you're adding comments here btw 😍
| } | ||
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| impl CapturedPlace<'tcx> { | ||
| pub fn get_root_variable(&self) -> hir::HirId { |
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I'd like to see a comment here, something like
/// Returns the hir-id of the root variable for the captured place. e.g., if a.b.c was captured, would return the hir-id for a.
| #[derive(PartialEq, Clone, Debug, TyEncodable, TyDecodable, TypeFoldable, HashStable)] | ||
| pub struct CapturedPlace<'tcx> { | ||
| /// The `Place` that is captured. |
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So happy to see added comments 😍
| @@ -1374,13 +1358,38 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> { | |||
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| fn propagate_closure_used_mut_upvar(&mut self, operand: &Operand<'tcx>) { | |||
| let propagate_closure_used_mut_place = |this: &mut Self, place: Place<'tcx>| { | |||
| if !place.projection.is_empty() { | |||
| if let Some(field) = this.is_upvar_field_projection(place.as_ref()) { | |||
| // We have three possiblities here: | |||
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Suggested change
| // We have three possiblities here: | |
| // We have three possibilities here: |
| // We have three possiblities here: | ||
| // a. We are modifying something through a mut-ref | ||
| // b. We are modifying something that is local to our parent | ||
| // c. Current body is a nested clsoure, and we are modifying path starting from |
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Suggested change
| // c. Current body is a nested clsoure, and we are modifying path starting from | |
| // c. Current body is a nested closure, and we are modifying a path starting from |
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@bors delegate+ Left a few nits but this looks good. @arora-aman, you wanted to make a final change as well, right? Feel free to r=nikomatsakis afterwards. |
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✌️ @arora-aman can now approve this pull request |
- No Derefs in move closure, this will result in value behind a reference getting moved. - No projections are applied to raw pointers, since these require unsafe blocks. We capture them completely. Motivations for these are recorded here: https://hackmd.io/71qq-IOpTNqzMkPpAI1dVg?view
When `capture_disjoint_fields` is not enabled, checking if the root variable binding is mutable would suffice. However with the feature enabled, the captured place might be mutable because it dereferences a mutable reference. This PR computes the mutability of each capture after capture analysis in rustc_typeck. We store this in `ty::CapturedPlace` and then use `ty::CapturedPlace::mutability` in mir_build and borrow_check.
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@bors r=nikomatsakis |
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📌 Commit 0f4bab2 has been approved by |
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Jan 30, 2021
…ikomatsakis
2229: Fix issues with move closures and mutability
This PR fixes two issues when feature `capture_disjoint_fields` is used.
1. Can't mutate using a mutable reference
2. Move closures try to move value out through a reference.
To do so, we
1. Compute the mutability of the capture and store it as part of the `CapturedPlace` that is written in TypeckResults
2. Restrict capture precision. Note this is temporary for now, to allow the feature to be used with move closures and ByValue captures and might change depending on discussions with the lang team.
- No Derefs are captured for ByValue captures, since that will result in value behind a reference getting moved.
- No projections are applied to raw pointers since these require unsafe blocks. We capture
them completely.
r? `@nikomatsakis`
This was referenced Jan 31, 2021
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…as-schievink Rollup of 11 pull requests Successful merges: - rust-lang#80092 (2229: Fix issues with move closures and mutability) - rust-lang#80404 (Remove const_in_array_repeat) - rust-lang#81255 (Don't link with --export-dynamic on wasm32-wasi) - rust-lang#81480 (Add suggestion for nested fields) - rust-lang#81549 (Misc ip documentation fixes) - rust-lang#81566 (Add a test for rust-lang#71202) - rust-lang#81568 (Fix an old FIXME in redundant paren lint) - rust-lang#81571 (Fix typo in E0759) - rust-lang#81572 (Edit multiple error code Markdown files) - rust-lang#81589 (Fix small typo in string.rs) - rust-lang#81590 (Stabilize int_bits_const) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
This was referenced Feb 1, 2021
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This PR fixes two issues when feature
capture_disjoint_fieldsis used.To do so, we
CapturedPlacethat is written in TypeckResultsthem completely.
r? @nikomatsakis