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elaborate why dropping principal in *dyn casts is non-trivial

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This commit is contained in:
Lukas Markeffsky 2024-10-18 16:47:19 +02:00
parent b89751b8f4
commit 46cc5e9e96
3 changed files with 61 additions and 2 deletions
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31
compiler/rustc_hir_typeck/src/cast.rs
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@ -959,8 +959,35 @@ fn check_ptr_ptr_cast(
// dyn Auto -> dyn Auto'? ok.
(None, None) => Ok(CastKind::PtrPtrCast),
// dyn Trait -> dyn Auto? should be ok, but we used to not allow it.
// FIXME: allow this
// dyn Trait -> dyn Auto? not ok (for now).
//
// Although dropping the principal is already allowed for unsizing coercions
// (e.g. `*const (dyn Trait + Auto)` to `*const dyn Auto`), dropping it is
// currently **NOT** allowed for (non-coercion) ptr-to-ptr casts (e.g
// `*const Foo` to `*const Bar` where `Foo` has a `dyn Trait + Auto` tail
// and `Bar` has a `dyn Auto` tail), because the underlying MIR operations
// currently work very differently:
//
// * A MIR unsizing coercion on raw pointers to trait objects (`*const dyn Src`
// to `*const dyn Dst`) is currently equivalent to downcasting the source to
// the concrete sized type that it was originally unsized from first (via a
// ptr-to-ptr cast from `*const Src` to `*const T` with `T: Sized`) and then
// unsizing this thin pointer to the target type (unsizing `*const T` to
// `*const Dst`). In particular, this means that the pointer's metadata
// (vtable) will semantically change, e.g. for const eval and miri, even
// though the vtables will always be merged for codegen.
//
// * A MIR ptr-to-ptr cast is currently equivalent to a transmute and does not
// change the pointer metadata (vtable) at all.
//
// In addition to this potentially surprising difference between coercion and
// non-coercion casts, casting away the principal with a MIR ptr-to-ptr cast
// is currently considered undefined behavior:
//
// As a validity invariant of pointers to trait objects, we currently require
// that the principal of the vtable in the pointer metadata exactly matches
// the principal of the pointee type, where "no principal" is also considered
// a kind of principal.
(Some(_), None) => Err(CastError::DifferingKinds { src_kind, dst_kind }),
// dyn Auto -> dyn Trait? not ok.

21
tests/ui/cast/ptr-to-trait-obj-drop-principal.rs Normal file
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@ -0,0 +1,21 @@
//! Test that non-coercion casts aren't allowed to drop the principal,
//! because they cannot modify the pointer metadata.
//!
//! We test this in a const context to guard against UB if this is allowed
//! in the future.
trait Trait {}
impl Trait for () {}
struct Wrapper<T: ?Sized>(T);
const OBJECT: *const (dyn Trait + Send) = &();
// coercions are allowed
const _: *const dyn Send = OBJECT as _;
// casts are **not** allowed
const _: *const Wrapper<dyn Send> = OBJECT as _;
//~^ ERROR casting `*const (dyn Trait + Send + 'static)` as `*const Wrapper<dyn Send>` is invalid
fn main() {}

11
tests/ui/cast/ptr-to-trait-obj-drop-principal.stderr Normal file
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@ -0,0 +1,11 @@
error[E0606]: casting `*const (dyn Trait + Send + 'static)` as `*const Wrapper<dyn Send>` is invalid
--> $DIR/ptr-to-trait-obj-drop-principal.rs:18:37
|
LL | const _: *const Wrapper<dyn Send> = OBJECT as _;
| ^^^^^^^^^^^
|
= note: the trait objects may have different vtables
error: aborting due to 1 previous error
For more information about this error, try `rustc --explain E0606`.