diff options
| -rw-r--r-- | compiler/rustc_hir_typeck/src/cast.rs | 31 | ||||
| -rw-r--r-- | tests/ui/cast/ptr-to-trait-obj-drop-principal.rs | 21 | ||||
| -rw-r--r-- | tests/ui/cast/ptr-to-trait-obj-drop-principal.stderr | 11 |
3 files changed, 61 insertions, 2 deletions
diff --git a/compiler/rustc_hir_typeck/src/cast.rs b/compiler/rustc_hir_typeck/src/cast.rs index 7341d3fa3c4..86af6c97af5 100644 --- a/compiler/rustc_hir_typeck/src/cast.rs +++ b/compiler/rustc_hir_typeck/src/cast.rs @@ -959,8 +959,35 @@ impl<'a, 'tcx> CastCheck<'tcx> { // 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. diff --git a/tests/ui/cast/ptr-to-trait-obj-drop-principal.rs b/tests/ui/cast/ptr-to-trait-obj-drop-principal.rs new file mode 100644 index 00000000000..01dd91fc77d --- /dev/null +++ b/tests/ui/cast/ptr-to-trait-obj-drop-principal.rs @@ -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() {} diff --git a/tests/ui/cast/ptr-to-trait-obj-drop-principal.stderr b/tests/ui/cast/ptr-to-trait-obj-drop-principal.stderr new file mode 100644 index 00000000000..719e0711f5b --- /dev/null +++ b/tests/ui/cast/ptr-to-trait-obj-drop-principal.stderr @@ -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`. |