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Auto merge of rust-lang#136764 - traviscross:TC/make-ptr_cast_add_auto_to_object-hard-error, r=<try>
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 Tracking: - rust-lang#127323 - rust-lang#44874 - rust-lang#123430
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‎compiler/rustc_error_codes/src/error_codes/E0802.md

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@@ -0,0 +1,41 @@
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An auto trait cannot be added to the bounds of a `dyn Trait` type via
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a pointer cast.
3+
4+
Erroneous code example:
5+
6+
```rust,edition2021,compile_fail,E0802
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let ptr: *const dyn core::any::Any = &();
8+
_ = ptr as *const (dyn core::any::Any + Send);
9+
```
10+
11+
Adding an auto trait can make the vtable invalid, potentially causing
12+
UB in safe code afterwards. For example:
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14+
```rust,edition2021,no_run
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use core::{mem::transmute, ptr::NonNull};
16+
17+
trait Trait {
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fn f(&self)
19+
where
20+
Self: Send;
21+
}
22+
23+
impl Trait for NonNull<()> {
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fn f(&self) {
25+
unreachable!()
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}
27+
}
28+
29+
fn main() {
30+
let unsend: &dyn Trait = &NonNull::dangling();
31+
let bad: &(dyn Trait + Send) = unsafe { transmute(unsend) };
32+
// This crashes, since the vtable for `NonNull as dyn Trait` does
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// not have an entry for `Trait::f`.
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bad.f();
35+
}
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```
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38+
To fix this error, you can use `transmute` rather than pointer casts,
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but you must ensure that the vtable is right for the pointer's type
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before calling a method on the trait object or allowing other code to
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do so.

‎compiler/rustc_error_codes/src/lib.rs

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@@ -545,6 +545,7 @@ E0798: 0798,
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E0799: 0799,
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E0800: 0800,
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E0801: 0801,
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E0802: 0802,
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);
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)
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}

‎compiler/rustc_hir_typeck/messages.ftl

+6-3
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@@ -154,10 +154,13 @@ hir_typeck_pass_to_variadic_function = can't pass `{$ty}` to variadic function
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.suggestion = cast the value to `{$cast_ty}`
155155
.teach_help = certain types, like `{$ty}`, must be casted before passing them to a variadic function, because of arcane ABI rules dictated by the C standard
156156
157-
hir_typeck_ptr_cast_add_auto_to_object = adding {$traits_len ->
158-
[1] an auto trait {$traits}
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hir_typeck_ptr_cast_add_auto_to_object = cannot add {$traits_len ->
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[1] auto trait {$traits}
159159
*[other] auto traits {$traits}
160-
} to a trait object in a pointer cast may cause UB later on
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} to dyn bound via pointer cast
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.note = this could allow UB elsewhere
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.help = use `transmute` if you're sure this is sound
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.label = unsupported cast
161164
162165
hir_typeck_remove_semi_for_coerce = you might have meant to return the `match` expression
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hir_typeck_remove_semi_for_coerce_expr = this could be implicitly returned but it is a statement, not a tail expression

‎compiler/rustc_hir_typeck/src/cast.rs

+12-16
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@@ -935,23 +935,19 @@ impl<'a, 'tcx> CastCheck<'tcx> {
935935
.collect::<Vec<_>>();
936936

937937
if !added.is_empty() {
938-
tcx.emit_node_span_lint(
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lint::builtin::PTR_CAST_ADD_AUTO_TO_OBJECT,
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self.expr.hir_id,
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self.span,
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errors::PtrCastAddAutoToObject {
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traits_len: added.len(),
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traits: {
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let mut traits: Vec<_> = added
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.into_iter()
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.map(|trait_did| tcx.def_path_str(trait_did))
948-
.collect();
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950-
traits.sort();
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traits.into()
952-
},
938+
tcx.dcx().emit_err(errors::PtrCastAddAutoToObject {
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span: self.span,
940+
traits_len: added.len(),
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traits: {
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let mut traits: Vec<_> = added
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.into_iter()
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.map(|trait_did| tcx.def_path_str(trait_did))
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.collect();
946+
947+
traits.sort();
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traits.into()
953949
},
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)
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});
955951
}
956952

957953
Ok(CastKind::PtrPtrCast)

‎compiler/rustc_hir_typeck/src/errors.rs

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@@ -373,9 +373,14 @@ pub(crate) struct LossyProvenanceInt2Ptr<'tcx> {
373373
pub sugg: LossyProvenanceInt2PtrSuggestion,
374374
}
375375

376-
#[derive(LintDiagnostic)]
377-
#[diag(hir_typeck_ptr_cast_add_auto_to_object)]
376+
#[derive(Diagnostic)]
377+
#[diag(hir_typeck_ptr_cast_add_auto_to_object, code = E0802)]
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#[note]
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#[help]
378380
pub(crate) struct PtrCastAddAutoToObject {
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#[primary_span]
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#[label]
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pub span: Span,
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pub traits_len: usize,
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pub traits: DiagSymbolList<String>,
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}

‎compiler/rustc_lint/src/lib.rs

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@@ -600,6 +600,11 @@ fn register_builtins(store: &mut LintStore) {
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"converted into hard error, \
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see <https://github.com/rust-lang/rust/issues/73333> for more information",
602602
);
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store.register_removed(
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"ptr_cast_add_auto_to_object",
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"converted into hard error, see issue #127323 \
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<https://github.com/rust-lang/rust/issues/127323> for more information",
607+
);
603608
}
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605610
fn register_internals(store: &mut LintStore) {

‎compiler/rustc_lint_defs/src/builtin.rs

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@@ -80,7 +80,6 @@ declare_lint_pass! {
8080
PRIVATE_BOUNDS,
8181
PRIVATE_INTERFACES,
8282
PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
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PTR_CAST_ADD_AUTO_TO_OBJECT,
8483
PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
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PUB_USE_OF_PRIVATE_EXTERN_CRATE,
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REDUNDANT_IMPORTS,
@@ -4825,58 +4824,6 @@ declare_lint! {
48254824
};
48264825
}
48274826

4828-
declare_lint! {
4829-
/// The `ptr_cast_add_auto_to_object` lint detects casts of raw pointers to trait
4830-
/// objects, which add auto traits.
4831-
///
4832-
/// ### Example
4833-
///
4834-
/// ```rust,edition2021,compile_fail
4835-
/// let ptr: *const dyn core::any::Any = &();
4836-
/// _ = ptr as *const dyn core::any::Any + Send;
4837-
/// ```
4838-
///
4839-
/// {{produces}}
4840-
///
4841-
/// ### Explanation
4842-
///
4843-
/// Adding an auto trait can make the vtable invalid, potentially causing
4844-
/// UB in safe code afterwards. For example:
4845-
///
4846-
/// ```ignore (causes a warning)
4847-
/// #![feature(arbitrary_self_types)]
4848-
///
4849-
/// trait Trait {
4850-
/// fn f(self: *const Self)
4851-
/// where
4852-
/// Self: Send;
4853-
/// }
4854-
///
4855-
/// impl Trait for *const () {
4856-
/// fn f(self: *const Self) {
4857-
/// unreachable!()
4858-
/// }
4859-
/// }
4860-
///
4861-
/// fn main() {
4862-
/// let unsend: *const () = &();
4863-
/// let unsend: *const dyn Trait = &unsend;
4864-
/// let send_bad: *const (dyn Trait + Send) = unsend as _;
4865-
/// send_bad.f(); // this crashes, since vtable for `*const ()` does not have an entry for `f`
4866-
/// }
4867-
/// ```
4868-
///
4869-
/// Generally you must ensure that vtable is right for the pointer's type,
4870-
/// before passing the pointer to safe code.
4871-
pub PTR_CAST_ADD_AUTO_TO_OBJECT,
4872-
Warn,
4873-
"detects `as` casts from pointers to `dyn Trait` to pointers to `dyn Trait + Auto`",
4874-
@future_incompatible = FutureIncompatibleInfo {
4875-
reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
4876-
reference: "issue #127323 <https://github.com/rust-lang/rust/issues/127323>",
4877-
};
4878-
}
4879-
48804827
declare_lint! {
48814828
/// The `out_of_scope_macro_calls` lint detects `macro_rules` called when they are not in scope,
48824829
/// above their definition, which may happen in key-value attributes.

‎tests/ui/cast/ptr-to-trait-obj-add-auto.rs

+8-6
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@@ -1,18 +1,20 @@
1-
//@ check-pass
2-
31
trait Trait<'a> {}
42

53
fn add_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send) {
64
x as _
7-
//~^ warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
8-
//~| warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
5+
//~^ ERROR cannot add auto trait `Send` to dyn bound via pointer cast
6+
//~| NOTE unsupported cast
7+
//~| NOTE this could allow UB elsewhere
8+
//~| HELP use `transmute` if you're sure this is sound
99
}
1010

1111
// (to test diagnostic list formatting)
1212
fn add_multiple_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send + Sync + Unpin) {
1313
x as _
14-
//~^ warning: adding auto traits `Send`, `Sync`, and `Unpin` to a trait object in a pointer cast may cause UB later on
15-
//~| warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
14+
//~^ ERROR cannot add auto traits `Send`, `Sync`, and `Unpin` to dyn bound via pointer cast
15+
//~| NOTE unsupported cast
16+
//~| NOTE this could allow UB elsewhere
17+
//~| HELP use `transmute` if you're sure this is sound
1618
}
1719

1820
fn main() {}

‎tests/ui/cast/ptr-to-trait-obj-add-auto.stderr

+11-33
Original file line numberDiff line numberDiff line change
@@ -1,43 +1,21 @@
1-
warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
2-
--> $DIR/ptr-to-trait-obj-add-auto.rs:6:5
1+
error[E0802]: cannot add auto trait `Send` to dyn bound via pointer cast
2+
--> $DIR/ptr-to-trait-obj-add-auto.rs:4:5
33
|
44
LL | x as _
5-
| ^^^^^^
5+
| ^^^^^^ unsupported cast
66
|
7-
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
8-
= note: for more information, see issue #127323 <https://github.com/rust-lang/rust/issues/127323>
9-
= note: `#[warn(ptr_cast_add_auto_to_object)]` on by default
7+
= note: this could allow UB elsewhere
8+
= help: use `transmute` if you're sure this is sound
109

11-
warning: adding auto traits `Send`, `Sync`, and `Unpin` to a trait object in a pointer cast may cause UB later on
10+
error[E0802]: cannot add auto traits `Send`, `Sync`, and `Unpin` to dyn bound via pointer cast
1211
--> $DIR/ptr-to-trait-obj-add-auto.rs:13:5
1312
|
1413
LL | x as _
15-
| ^^^^^^
14+
| ^^^^^^ unsupported cast
1615
|
17-
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
18-
= note: for more information, see issue #127323 <https://github.com/rust-lang/rust/issues/127323>
16+
= note: this could allow UB elsewhere
17+
= help: use `transmute` if you're sure this is sound
1918

20-
warning: 2 warnings emitted
21-
22-
Future incompatibility report: Future breakage diagnostic:
23-
warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
24-
--> $DIR/ptr-to-trait-obj-add-auto.rs:6:5
25-
|
26-
LL | x as _
27-
| ^^^^^^
28-
|
29-
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
30-
= note: for more information, see issue #127323 <https://github.com/rust-lang/rust/issues/127323>
31-
= note: `#[warn(ptr_cast_add_auto_to_object)]` on by default
32-
33-
Future breakage diagnostic:
34-
warning: adding auto traits `Send`, `Sync`, and `Unpin` to a trait object in a pointer cast may cause UB later on
35-
--> $DIR/ptr-to-trait-obj-add-auto.rs:13:5
36-
|
37-
LL | x as _
38-
| ^^^^^^
39-
|
40-
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
41-
= note: for more information, see issue #127323 <https://github.com/rust-lang/rust/issues/127323>
42-
= note: `#[warn(ptr_cast_add_auto_to_object)]` on by default
19+
error: aborting due to 2 previous errors
4320

21+
For more information about this error, try `rustc --explain E0802`.

‎tests/ui/lint/removed-lints/README.md

+4
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@@ -0,0 +1,4 @@
1+
# Removed lints
2+
3+
This directory contains tests to confirm that lints that have been
4+
removed do not cause errors and produce the appropriate warnings.

‎tests/ui/lint/removed-lints/ptr_cast_add_auto_to_object.rs

+5
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
1+
//@ check-pass
2+
3+
#![deny(ptr_cast_add_auto_to_object)]
4+
//~^ WARN lint `ptr_cast_add_auto_to_object` has been removed
5+
fn main() {}

‎tests/ui/lint/removed-lints/ptr_cast_add_auto_to_object.stderr

+10
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,10 @@
1+
warning: lint `ptr_cast_add_auto_to_object` has been removed: converted into hard error, see issue #127323 <https://github.com/rust-lang/rust/issues/127323> for more information
2+
--> $DIR/ptr_cast_add_auto_to_object.rs:3:9
3+
|
4+
LL | #![deny(ptr_cast_add_auto_to_object)]
5+
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
6+
|
7+
= note: `#[warn(renamed_and_removed_lints)]` on by default
8+
9+
warning: 1 warning emitted
10+

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