Skip to content

Commit

Permalink
Auto merge of rust-lang#120248 - WaffleLapkin:bonk-ptr-object-casts, …
Browse files Browse the repository at this point in the history
…r=<try>

Make casts of pointers to trait objects stricter

This is an attempt to `fix` rust-lang#120222 and rust-lang#120217.

This is done by adding restrictions on casting pointers to trait objects. Currently the restriction is
> When casting `*const X<dyn A>` -> `*const Y<dyn B>`, if `B` has a principal trait, `dyn A + 'erased: Unsize<dyn B + 'erased>` must hold

This ensures that
1. Principal trait's generic arguments match (no `*const dyn Tr<A>` -> `*const dyn Tr<B>` casts, which are a problem for [rust-lang#120222](rust-lang#120222))
2. Principal trait's lifetime arguments match (no `*const dyn Tr<'a>` -> `*const dyn Tr<'b>` casts, which are a problem for [rust-lang#120217](rust-lang#120217))
3. No auto traits can be _added_ (this is a problem for arbitrary self types, see [this comment](rust-lang#120248 (comment)))

Some notes:
 - We only care about the metadata/last field, so you can still cast `*const dyn T` to `*const WithHeader<dyn T>`, etc
- The lifetime of the trait object itself (`dyn A + 'lt`) is not checked, so you can still cast `*mut FnOnce() + '_` to `*mut FnOnce() + 'static`, etc
  - This feels fishy, but I couldn't come up with a reason it must be checked
- The new checks are only done if `B` has a principal, so you can still do any kinds of cast, if the target only has auto traits
  - This is because auto traits are not enough to get unsoundness issues that this PR fixes
  - ...and so it makes sense to minimize breakage

The plan is to, ~~once the checks are properly implemented~~, run crate to determine how much damage does this do.

The diagnostics are currently not great, to say the least, but as far as I can tell this correctly fixes the issues.

cc `@oli-obk` `@compiler-errors` `@lcnr`
  • Loading branch information
bors committed Feb 13, 2024
2 parents 74c3f5a + 5f27951 commit 13cae60
Show file tree
Hide file tree
Showing 21 changed files with 495 additions and 70 deletions.
49 changes: 48 additions & 1 deletion compiler/rustc_borrowck/src/type_check/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2358,7 +2358,42 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
let cast_ty_from = CastTy::from_ty(ty_from);
let cast_ty_to = CastTy::from_ty(*ty);
match (cast_ty_from, cast_ty_to) {
(Some(CastTy::Ptr(_)), Some(CastTy::Ptr(_))) => (),
(Some(CastTy::Ptr(src)), Some(CastTy::Ptr(dst))) => {
let src_tail = tcx.struct_tail_without_normalization(src.ty);
let dst_tail = tcx.struct_tail_without_normalization(dst.ty);

if let ty::Dynamic(..) = src_tail.kind()
&& let ty::Dynamic(dst_tty, ..) = dst_tail.kind()
&& dst_tty.principal().is_some()
{
// Erase trait object lifetimes, to allow casts like `*mut dyn FnOnce()` -> `*mut dyn FnOnce() + 'static`.
let src_tail =
erase_single_trait_object_lifetime(tcx, src_tail);
let dst_tail =
erase_single_trait_object_lifetime(tcx, dst_tail);

let trait_ref = ty::TraitRef::from_lang_item(
tcx,
LangItem::Unsize,
span,
[src_tail, dst_tail],
);

self.prove_trait_ref(
trait_ref,
location.to_locations(),
ConstraintCategory::Cast {
unsize_to: Some(tcx.fold_regions(dst_tail, |r, _| {
if let ty::ReVar(_) = r.kind() {
tcx.lifetimes.re_erased
} else {
r
}
})),
},
);
}
}
_ => {
span_mirbug!(
self,
Expand Down Expand Up @@ -2881,3 +2916,15 @@ impl<'tcx> TypeOp<'tcx> for InstantiateOpaqueType<'tcx> {
Ok(output)
}
}

fn erase_single_trait_object_lifetime<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Ty<'tcx> {
let &ty::Dynamic(tty, region, dyn_kind @ ty::Dyn) = ty.kind() else {
bug!("expected trait object")
};

if region.is_erased() {
return ty;
}

tcx.mk_ty_from_kind(ty::Dynamic(tty, tcx.lifetimes.re_erased, dyn_kind))
}
105 changes: 74 additions & 31 deletions compiler/rustc_hir_typeck/src/cast.rs
Original file line number Diff line number Diff line change
Expand Up @@ -32,17 +32,17 @@ use super::FnCtxt;

use crate::errors;
use crate::type_error_struct;
use hir::ExprKind;
use rustc_errors::{codes::*, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
use rustc_hir as hir;
use rustc_hir::{self as hir, ExprKind, LangItem};
use rustc_infer::traits::Obligation;
use rustc_macros::{TypeFoldable, TypeVisitable};
use rustc_middle::mir::Mutability;
use rustc_middle::ty::adjustment::AllowTwoPhase;
use rustc_middle::ty::cast::{CastKind, CastTy};
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::{self, Ty, TypeAndMut, TypeVisitableExt, VariantDef};
use rustc_session::lint;
use rustc_span::def_id::{DefId, LOCAL_CRATE};
use rustc_span::def_id::LOCAL_CRATE;
use rustc_span::symbol::sym;
use rustc_span::Span;
use rustc_trait_selection::infer::InferCtxtExt;
Expand Down Expand Up @@ -72,7 +72,7 @@ enum PointerKind<'tcx> {
/// No metadata attached, ie pointer to sized type or foreign type
Thin,
/// A trait object
VTable(Option<DefId>),
VTable(&'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>),
/// Slice
Length,
/// The unsize info of this projection or opaque type
Expand Down Expand Up @@ -100,7 +100,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {

Ok(match *t.kind() {
ty::Slice(_) | ty::Str => Some(PointerKind::Length),
ty::Dynamic(tty, _, ty::Dyn) => Some(PointerKind::VTable(tty.principal_def_id())),
ty::Dynamic(tty, _, ty::Dyn) => Some(PointerKind::VTable(tty)),
ty::Adt(def, args) if def.is_struct() => match def.non_enum_variant().tail_opt() {
None => Some(PointerKind::Thin),
Some(f) => {
Expand Down Expand Up @@ -747,7 +747,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
Err(CastError::IllegalCast)
}

// ptr -> *
// ptr -> ptr
(Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast

// ptr-addr-cast
Expand Down Expand Up @@ -791,40 +791,83 @@ impl<'a, 'tcx> CastCheck<'tcx> {
fn check_ptr_ptr_cast(
&self,
fcx: &FnCtxt<'a, 'tcx>,
m_expr: ty::TypeAndMut<'tcx>,
m_cast: ty::TypeAndMut<'tcx>,
m_src: ty::TypeAndMut<'tcx>,
m_dst: ty::TypeAndMut<'tcx>,
) -> Result<CastKind, CastError> {
debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_src, m_dst);
// ptr-ptr cast. vtables must match.

let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
let src_kind = fcx.tcx.erase_regions(fcx.pointer_kind(m_src.ty, self.span)?);
let dst_kind = fcx.tcx.erase_regions(fcx.pointer_kind(m_dst.ty, self.span)?);

let Some(cast_kind) = cast_kind else {
match (src_kind, dst_kind) {
// We can't cast if target pointer kind is unknown
return Err(CastError::UnknownCastPtrKind);
};

// Cast to thin pointer is OK
if cast_kind == PointerKind::Thin {
return Ok(CastKind::PtrPtrCast);
}
(_, None) => Err(CastError::UnknownCastPtrKind),
// Cast to thin pointer is OK
(_, Some(PointerKind::Thin)) => Ok(CastKind::PtrPtrCast),

let Some(expr_kind) = expr_kind else {
// We can't cast to fat pointer if source pointer kind is unknown
return Err(CastError::UnknownExprPtrKind);
};
(None, _) => Err(CastError::UnknownExprPtrKind),

// thin -> fat? report invalid cast (don't complain about vtable kinds)
(Some(PointerKind::Thin), _) => Err(CastError::SizedUnsizedCast),

// trait object -> trait object? need to do additional checks
(Some(PointerKind::VTable(src_tty)), Some(PointerKind::VTable(dst_tty))) => {
match (src_tty.principal(), dst_tty.principal()) {
// A<dyn Trait + Auto> -> B<dyn Trait' + Auto'>. need to make sure
// - traits are the same & have the same generic arguments
// - Auto' is a subset of Auto
//
// This is checked by checking `dyn Trait + Auto + 'erased: Unsize<dyn Trait' + Auto' + 'erased>`.
(Some(_), Some(_)) => {
let tcx = fcx.tcx;

// We need to reconstruct trait object types.
// `m_src` and `m_dst` won't work for us here because they will potentially
// contain wrappers, which we do not care about.
//
// e.g. we want to allow `dyn T -> (dyn T,)`, etc.
let src_obj = tcx.mk_ty_from_kind(ty::Dynamic(src_tty, tcx.lifetimes.re_erased, ty::Dyn));
let dst_obj = tcx.mk_ty_from_kind(ty::Dynamic(dst_tty, tcx.lifetimes.re_erased, ty::Dyn));

// `dyn Src: Unsize<dyn Dst>`
let cause = fcx.misc(self.span);
let obligation = Obligation::new(
tcx,
cause,
fcx.param_env,
ty::TraitRef::from_lang_item(
tcx,
LangItem::Unsize,
self.span,
[src_obj, dst_obj]
)
);

// thin -> fat? report invalid cast (don't complain about vtable kinds)
if expr_kind == PointerKind::Thin {
return Err(CastError::SizedUnsizedCast);
}
fcx.register_predicate(obligation);

// vtable kinds must match
if fcx.tcx.erase_regions(cast_kind) == fcx.tcx.erase_regions(expr_kind) {
Ok(CastKind::PtrPtrCast)
} else {
Err(CastError::DifferingKinds)
// FIXME: ideally we'd maybe add a flag here, so that borrowck knows that
// it needs to borrowck this ptr cast. this is made annoying by the
// fact that `thir` does not have `CastKind` and mir restores it
// from types.
Ok(CastKind::PtrPtrCast)
}

// dyn Auto -> dyn Auto'? ok.
(None, None)
// dyn Trait -> dyn Auto? ok.
| (Some(_), None)=> Ok(CastKind::PtrPtrCast),

// dyn Auto -> dyn Trait? not ok.
(None, Some(_)) => Err(CastError::DifferingKinds),
}
}

// fat -> fat? metadata kinds must match
(Some(src_kind), Some(dst_kind)) if src_kind == dst_kind => Ok(CastKind::PtrPtrCast),

(_, _) => Err(CastError::DifferingKinds),
}
}

Expand Down
6 changes: 3 additions & 3 deletions library/alloc/src/boxed.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2220,7 +2220,7 @@ impl dyn Error + Send {
let err: Box<dyn Error> = self;
<dyn Error>::downcast(err).map_err(|s| unsafe {
// Reapply the `Send` marker.
Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send))
mem::transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
})
}
}
Expand All @@ -2233,8 +2233,8 @@ impl dyn Error + Send + Sync {
pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
let err: Box<dyn Error> = self;
<dyn Error>::downcast(err).map_err(|s| unsafe {
// Reapply the `Send + Sync` marker.
Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send + Sync))
// Reapply the `Send + Sync` markers.
mem::transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
})
}
}
Expand Down
18 changes: 0 additions & 18 deletions tests/ui/cast/cast-rfc0401-vtable-kinds.rs
Original file line number Diff line number Diff line change
Expand Up @@ -4,25 +4,12 @@

#![feature(unsized_tuple_coercion)]

trait Foo<T> {
fn foo(&self, _: T) -> u32 { 42 }
}

trait Bar { //~ WARN trait `Bar` is never used
fn bar(&self) { println!("Bar!"); }
}

impl<T> Foo<T> for () {}
impl Foo<u32> for u32 { fn foo(&self, _: u32) -> u32 { self+43 } }
impl Bar for () {}

unsafe fn round_trip_and_call<'a>(t: *const (dyn Foo<u32>+'a)) -> u32 {
let foo_e : *const dyn Foo<u16> = t as *const _;
let r_1 = foo_e as *mut dyn Foo<u32>;

(&*r_1).foo(0)
}

#[repr(C)]
struct FooS<T:?Sized>(T);
#[repr(C)]
Expand All @@ -38,11 +25,6 @@ fn tuple_i32_to_u32<T:?Sized>(u: *const (i32, T)) -> *const (u32, T) {


fn main() {
let x = 4u32;
let y : &dyn Foo<u32> = &x;
let fl = unsafe { round_trip_and_call(y as *const dyn Foo<u32>) };
assert_eq!(fl, (43+4));

let s = FooS([0,1,2]);
let u: &FooS<[u32]> = &s;
let u: *const FooS<[u32]> = u;
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/cast/cast-rfc0401-vtable-kinds.stderr
Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
warning: trait `Bar` is never used
--> $DIR/cast-rfc0401-vtable-kinds.rs:11:7
--> $DIR/cast-rfc0401-vtable-kinds.rs:7:7
|
LL | trait Bar {
| ^^^
Expand Down
9 changes: 9 additions & 0 deletions tests/ui/cast/ptr-to-trait-obj-add-auto.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
// check-fail

trait Trait<'a> {}

fn add_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send) {
x as _ //~ error: the trait bound `dyn Trait<'_>: Unsize<dyn Trait<'_> + Send>` is not satisfied
}

fn main() {}
11 changes: 11 additions & 0 deletions tests/ui/cast/ptr-to-trait-obj-add-auto.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
error[E0277]: the trait bound `dyn Trait<'_>: Unsize<dyn Trait<'_> + Send>` is not satisfied
--> $DIR/ptr-to-trait-obj-add-auto.rs:6:5
|
LL | x as _
| ^^^^^^ the trait `Unsize<dyn Trait<'_> + Send>` is not implemented for `dyn Trait<'_>`
|
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.
38 changes: 38 additions & 0 deletions tests/ui/cast/ptr-to-trait-obj-different-args.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,38 @@
// check-fail
//
// issue: <https://github.com/rust-lang/rust/issues/120222>


trait A {}
impl<T> A for T {}
trait B {}
impl<T> B for T {}

trait Trait<G> {}
struct X;
impl<T> Trait<X> for T {}
struct Y;
impl<T> Trait<Y> for T {}

fn main() {
let a: *const dyn A = &();
let b: *const dyn B = a as _; //~ error: the trait bound `dyn A: Unsize<dyn B>` is not satisfied

let x: *const dyn Trait<X> = &();
let y: *const dyn Trait<Y> = x as _; //~ error: the trait bound `dyn Trait<X>: Unsize<dyn Trait<Y>>` is not satisfied

_ = (b, y);
}

fn generic<T>(x: *const dyn Trait<X>, t: *const dyn Trait<T>) {
let _: *const dyn Trait<T> = x as _; //~ error: the trait bound `dyn Trait<X>: Unsize<dyn Trait<T>>` is not satisfied
let _: *const dyn Trait<X> = t as _; //~ error: the trait bound `dyn Trait<T>: Unsize<dyn Trait<X>>` is not satisfied
}

trait Assocked {
type Assoc: ?Sized;
}

fn change_assoc(x: *mut dyn Assocked<Assoc = u8>) -> *mut dyn Assocked<Assoc = u32> {
x as _ //~ error: the trait bound `dyn Assocked<Assoc = u8>: Unsize<dyn Assocked<Assoc = u32>>` is not satisfied
}
51 changes: 51 additions & 0 deletions tests/ui/cast/ptr-to-trait-obj-different-args.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,51 @@
error[E0277]: the trait bound `dyn A: Unsize<dyn B>` is not satisfied
--> $DIR/ptr-to-trait-obj-different-args.rs:19:27
|
LL | let b: *const dyn B = a as _;
| ^^^^^^ the trait `Unsize<dyn B>` is not implemented for `dyn A`
|
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information

error[E0277]: the trait bound `dyn Trait<X>: Unsize<dyn Trait<Y>>` is not satisfied
--> $DIR/ptr-to-trait-obj-different-args.rs:22:34
|
LL | let y: *const dyn Trait<Y> = x as _;
| ^^^^^^ the trait `Unsize<dyn Trait<Y>>` is not implemented for `dyn Trait<X>`
|
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information

error[E0277]: the trait bound `dyn Trait<X>: Unsize<dyn Trait<T>>` is not satisfied
--> $DIR/ptr-to-trait-obj-different-args.rs:28:34
|
LL | let _: *const dyn Trait<T> = x as _;
| ^^^^^^ the trait `Unsize<dyn Trait<T>>` is not implemented for `dyn Trait<X>`
|
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
|
LL | fn generic<T>(x: *const dyn Trait<X>, t: *const dyn Trait<T>) where dyn Trait<X>: Unsize<dyn Trait<T>> {
| ++++++++++++++++++++++++++++++++++++++++

error[E0277]: the trait bound `dyn Trait<T>: Unsize<dyn Trait<X>>` is not satisfied
--> $DIR/ptr-to-trait-obj-different-args.rs:29:34
|
LL | let _: *const dyn Trait<X> = t as _;
| ^^^^^^ the trait `Unsize<dyn Trait<X>>` is not implemented for `dyn Trait<T>`
|
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
|
LL | fn generic<T>(x: *const dyn Trait<X>, t: *const dyn Trait<T>) where dyn Trait<T>: Unsize<dyn Trait<X>> {
| ++++++++++++++++++++++++++++++++++++++++

error[E0277]: the trait bound `dyn Assocked<Assoc = u8>: Unsize<dyn Assocked<Assoc = u32>>` is not satisfied
--> $DIR/ptr-to-trait-obj-different-args.rs:37:5
|
LL | x as _
| ^^^^^^ the trait `Unsize<dyn Assocked<Assoc = u32>>` is not implemented for `dyn Assocked<Assoc = u8>`
|
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information

error: aborting due to 5 previous errors

For more information about this error, try `rustc --explain E0277`.
Loading

0 comments on commit 13cae60

Please sign in to comment.