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2229: Handle capturing a reference into a repr packed struct 

RFC 1240 states that it is unsafe to capture references into a
packed-struct. This PR ensures that when a closure captures a precise
path, we aren't violating this safety constraint.

To acheive so we restrict the capture precision to the struct itself.

An interesting edge case:
```rust
struct Foo(String);

let foo: Foo;
let c = || {
    println!("{}", foo.0);
    let x = foo.0;
}
```

Given how closures get desugared today, foo.0 will be moved into the
closure, making the `println!`, safe. However this can be very subtle
and also will be unsafe if the closure gets inline.

Closes: https://github.com/rust-lang/project-rfc-2229/issues/33
This commit is contained in:
Aman Arora 2021-03-06 21:47:53 -05:00
parent dfe519b344
commit 612a9b2f95
5 changed files with 381 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

61
compiler/rustc_typeck/src/check/upvar.rs
View File

@ -1053,6 +1053,52 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
}
}
/// Truncate the capture so that the place being borrowed is in accordance with RFC 1240,
/// which states that it's unsafe to take a reference into a struct marked `repr(packed)`.
fn restrict_repr_packed_field_ref_capture<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
place: &Place<'tcx>,
) -> Place<'tcx> {
let pos = place.projections.iter().enumerate().position(|(i, p)| {
let ty = place.ty_before_projection(i);
// Return true for fields of packed structs, unless those fields have alignment 1.
match p.kind {
ProjectionKind::Field(..) => match ty.kind() {
ty::Adt(def, _) if def.repr.packed() => {
match tcx.layout_raw(param_env.and(p.ty)) {
Ok(layout) if layout.align.abi.bytes() == 1 => {
// if the alignment is 1, the type can't be further
// disaligned.
debug!(
"restrict_repr_packed_field_ref_capture: ({:?}) - align = 1",
place
);
false
}
_ => {
debug!("restrict_repr_packed_field_ref_capture: ({:?}) - true", place);
true
}
}
}
_ => false,
},
_ => false,
}
});
let mut place = place.clone();
if let Some(pos) = pos {
place.projections.truncate(pos);
}
place
}
struct InferBorrowKind<'a, 'tcx> {
fcx: &'a FnCtxt<'a, 'tcx>,
@ -1391,8 +1437,15 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
place_with_id, diag_expr_id, bk
);
let place = restrict_repr_packed_field_ref_capture(
self.fcx.tcx,
self.fcx.param_env,
&place_with_id.place,
);
let place_with_id = PlaceWithHirId { place, ..*place_with_id };
if !self.capture_information.contains_key(&place_with_id.place) {
self.init_capture_info_for_place(place_with_id, diag_expr_id);
self.init_capture_info_for_place(&place_with_id, diag_expr_id);
}
match bk {
@ -1409,11 +1462,7 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
fn mutate(&mut self, assignee_place: &PlaceWithHirId<'tcx>, diag_expr_id: hir::HirId) {
debug!("mutate(assignee_place={:?}, diag_expr_id={:?})", assignee_place, diag_expr_id);
if !self.capture_information.contains_key(&assignee_place.place) {
self.init_capture_info_for_place(assignee_place, diag_expr_id);
}
self.adjust_upvar_borrow_kind_for_mut(assignee_place, diag_expr_id);
self.borrow(assignee_place, diag_expr_id, ty::BorrowKind::MutBorrow);
}
}

36
src/test/ui/closures/2229_closure_analysis/diagnostics/repr_packed.rs Normal file
View File

@ -0,0 +1,36 @@
// check-pass
#![feature(capture_disjoint_fields)]
//~^ WARNING: the feature `capture_disjoint_fields` is incomplete
// Given how the closure desugaring is implemented (at least at the time of writing this test),
// we don't need to truncate the captured path to a reference into a packed-struct if the field
// being referenced will be moved into the closure, since it's safe to move out a field from a
// packed-struct.
//
// However to avoid surprises for the user, or issues when the closure is
// inlined we will truncate the capture to access just the struct regardless of if the field
// might get moved into the closure.
//
// It is possible for someone to try writing the code that relies on the desugaring to access a ref
// into a packed-struct without explicity using unsafe. Here we test that the compiler warns the
// user that such an access is still unsafe.
fn test_missing_unsafe_warning_on_repr_packed() {
#[repr(packed)]
struct Foo { x: String }
let foo = Foo { x: String::new() };
let c = || {
println!("{}", foo.x);
//~^ WARNING: borrow of packed field is unsafe and requires unsafe function or block
//~| WARNING: this was previously accepted by the compiler but is being phased out
let _z = foo.x;
};
c();
}
fn main() {
test_missing_unsafe_warning_on_repr_packed();
}

22
src/test/ui/closures/2229_closure_analysis/diagnostics/repr_packed.stderr Normal file
View File

@ -0,0 +1,22 @@
warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
--> $DIR/repr_packed.rs:3:12
|
LL | #![feature(capture_disjoint_fields)]
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(incomplete_features)]` on by default
= note: see issue #53488 <https://github.com/rust-lang/rust/issues/53488> for more information
warning: borrow of packed field is unsafe and requires unsafe function or block (error E0133)
--> $DIR/repr_packed.rs:25:24
|
LL | println!("{}", foo.x);
| ^^^^^
|
= note: `#[warn(safe_packed_borrows)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>
= note: fields of packed structs might be misaligned: dereferencing a misaligned pointer or even just creating a misaligned reference is undefined behavior
warning: 2 warnings emitted

103
src/test/ui/closures/2229_closure_analysis/repr_packed.rs Normal file
View File

@ -0,0 +1,103 @@
#![feature(capture_disjoint_fields)]
//~^ WARNING: the feature `capture_disjoint_fields` is incomplete
//~| `#[warn(incomplete_features)]` on by default
//~| see issue #53488 <https://github.com/rust-lang/rust/issues/53488>
#![feature(rustc_attrs)]
// `u8` aligned at a byte and are unaffected by repr(packed).
// Therefore we can precisely (and safely) capture references to both the fields.
fn test_alignment_not_affected() {
#[repr(packed)]
struct Foo { x: u8, y: u8 }
let mut foo = Foo { x: 0, y: 0 };
let mut c = #[rustc_capture_analysis]
//~^ ERROR: attributes on expressions are experimental
//~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
|| {
//~^ ERROR: First Pass analysis includes:
//~| ERROR: Min Capture analysis includes:
let z1: &u8 = &foo.x;
//~^ NOTE: Capturing foo[(0, 0)] -> ImmBorrow
//~| NOTE: Min Capture foo[(0, 0)] -> ImmBorrow
let z2: &mut u8 = &mut foo.y;
//~^ NOTE: Capturing foo[(1, 0)] -> MutBorrow
//~| NOTE: Min Capture foo[(1, 0)] -> MutBorrow
*z2 = 42;
println!("({}, {})", z1, z2);
};
c();
}
// `String`, `u16` are not aligned at a one byte boundry and are thus affected by repr(packed).
//
// Here we test that the closure doesn't capture a reference point to `foo.x` but
// rather capture `foo` entirely.
fn test_alignment_affected() {
#[repr(packed)]
struct Foo { x: String, y: u16 }
let mut foo = Foo { x: String::new(), y: 0 };
let mut c = #[rustc_capture_analysis]
//~^ ERROR: attributes on expressions are experimental
//~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
|| {
//~^ ERROR: First Pass analysis includes:
//~| ERROR: Min Capture analysis includes:
let z1: &String = &foo.x;
let z2: &mut u16 = &mut foo.y;
//~^ NOTE: Capturing foo[] -> MutBorrow
//~| NOTE: Min Capture foo[] -> MutBorrow
*z2 = 42;
println!("({}, {})", z1, z2);
};
c();
}
// Given how the closure desugaring is implemented (at least at the time of writing this test),
// we don't need to truncate the captured path to a reference into a packed-struct if the field
// being referenced will be moved into the closure, since it's safe to move out a field from a
// packed-struct.
//
// However to avoid surprises for the user, or issues when the closure is
// inlined we will truncate the capture to access just the struct regardless of if the field
// might get moved into the closure.
fn test_truncation_when_ref_and_move() {
#[repr(packed)]
struct Foo { x: String }
let mut foo = Foo { x: String::new() };
let c = #[rustc_capture_analysis]
//~^ ERROR: attributes on expressions are experimental
//~| NOTE: see issue #15701 <https://github.com/rust-lang/rust/issues/15701>
|| {
//~^ ERROR: First Pass analysis includes:
//~| ERROR: Min Capture analysis includes:
println!("{}", foo.x);
//~^ NOTE: Capturing foo[] -> ImmBorrow
//~| NOTE: Min Capture foo[] -> ByValue
//~| NOTE: foo[] used here
let _z = foo.x;
//~^ NOTE: Capturing foo[(0, 0)] -> ByValue
//~| NOTE: foo[] captured as ByValue here
};
c();
}
fn main() {
test_truncation_when_ref_and_move();
test_alignment_affected();
test_alignment_not_affected();
}

165
src/test/ui/closures/2229_closure_analysis/repr_packed.stderr Normal file
View File

@ -0,0 +1,165 @@
error[E0658]: attributes on expressions are experimental
--> $DIR/repr_packed.rs:16:17
|
LL | let mut c = #[rustc_capture_analysis]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
= help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
error[E0658]: attributes on expressions are experimental
--> $DIR/repr_packed.rs:47:17
|
LL | let mut c = #[rustc_capture_analysis]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
= help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
error[E0658]: attributes on expressions are experimental
--> $DIR/repr_packed.rs:81:13
|
LL | let c = #[rustc_capture_analysis]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
= help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
--> $DIR/repr_packed.rs:1:12
|
LL | #![feature(capture_disjoint_fields)]
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(incomplete_features)]` on by default
= note: see issue #53488 <https://github.com/rust-lang/rust/issues/53488> for more information
error: First Pass analysis includes:
--> $DIR/repr_packed.rs:19:5
|
LL | / || {
LL | |
LL | |
LL | | let z1: &u8 = &foo.x;
... |
LL | | println!("({}, {})", z1, z2);
LL | | };
| |_____^
|
note: Capturing foo[(0, 0)] -> ImmBorrow
--> $DIR/repr_packed.rs:22:24
|
LL | let z1: &u8 = &foo.x;
| ^^^^^
note: Capturing foo[(1, 0)] -> MutBorrow
--> $DIR/repr_packed.rs:25:32
|
LL | let z2: &mut u8 = &mut foo.y;
| ^^^^^
error: Min Capture analysis includes:
--> $DIR/repr_packed.rs:19:5
|
LL | / || {
LL | |
LL | |
LL | | let z1: &u8 = &foo.x;
... |
LL | | println!("({}, {})", z1, z2);
LL | | };
| |_____^
|
note: Min Capture foo[(0, 0)] -> ImmBorrow
--> $DIR/repr_packed.rs:22:24
|
LL | let z1: &u8 = &foo.x;
| ^^^^^
note: Min Capture foo[(1, 0)] -> MutBorrow
--> $DIR/repr_packed.rs:25:32
|
LL | let z2: &mut u8 = &mut foo.y;
| ^^^^^
error: First Pass analysis includes:
--> $DIR/repr_packed.rs:50:5
|
LL | / || {
LL | |
LL | |
LL | | let z1: &String = &foo.x;
... |
LL | | println!("({}, {})", z1, z2);
LL | | };
| |_____^
|
note: Capturing foo[] -> MutBorrow
--> $DIR/repr_packed.rs:54:33
|
LL | let z2: &mut u16 = &mut foo.y;
| ^^^^^
error: Min Capture analysis includes:
--> $DIR/repr_packed.rs:50:5
|
LL | / || {
LL | |
LL | |
LL | | let z1: &String = &foo.x;
... |
LL | | println!("({}, {})", z1, z2);
LL | | };
| |_____^
|
note: Min Capture foo[] -> MutBorrow
--> $DIR/repr_packed.rs:54:33
|
LL | let z2: &mut u16 = &mut foo.y;
| ^^^^^
error: First Pass analysis includes:
--> $DIR/repr_packed.rs:84:5
|
LL | / || {
LL | |
LL | |
LL | | println!("{}", foo.x);
... |
LL | |
LL | | };
| |_____^
|
note: Capturing foo[] -> ImmBorrow
--> $DIR/repr_packed.rs:87:24
|
LL | println!("{}", foo.x);
| ^^^^^
note: Capturing foo[(0, 0)] -> ByValue
--> $DIR/repr_packed.rs:91:18
|
LL | let _z = foo.x;
| ^^^^^
error: Min Capture analysis includes:
--> $DIR/repr_packed.rs:84:5
|
LL | / || {
LL | |
LL | |
LL | | println!("{}", foo.x);
... |
LL | |
LL | | };
| |_____^
|
note: Min Capture foo[] -> ByValue
--> $DIR/repr_packed.rs:87:24
|
LL | println!("{}", foo.x);
| ^^^^^ foo[] used here
...
LL | let _z = foo.x;
| ^^^^^ foo[] captured as ByValue here
error: aborting due to 9 previous errors; 1 warning emitted
For more information about this error, try `rustc --explain E0658`.