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Unrolled build for rust-lang#131898
Rollup merge of rust-lang#131898 - lukas-code:ptr-cast-cleanup, r=compiler-errors minor `*dyn` cast cleanup Small follow-up to rust-lang#130234 to remove a redundant check and clean up comments. No functional changes. Also, explain why casts cannot drop the principal even though coercions can, and add a test because apparently we didn't have one already. r? `@WaffleLapkin` or `@compiler-errors`
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‎compiler/rustc_hir_typeck/src/cast.rs

+37-16
Original file line numberDiff line numberDiff line change
@@ -876,20 +876,14 @@ impl<'a, 'tcx> CastCheck<'tcx> {
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// A<dyn Src<...> + SrcAuto> -> B<dyn Dst<...> + DstAuto>. need to make sure
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// - `Src` and `Dst` traits are the same
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// - traits have the same generic arguments
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// - `SrcAuto` is a superset of `DstAuto`
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(Some(src_principal), Some(dst_principal)) => {
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// - projections are the same
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// - `SrcAuto` (+auto traits implied by `Src`) is a superset of `DstAuto`
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//
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// Note that trait upcasting goes through a different mechanism (`coerce_unsized`)
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// and is unaffected by this check.
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(Some(src_principal), Some(_)) => {
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let tcx = fcx.tcx;
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// Check that the traits are actually the same.
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// The `dyn Src = dyn Dst` check below would suffice,
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// but this may produce a better diagnostic.
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//
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// Note that trait upcasting goes through a different mechanism (`coerce_unsized`)
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// and is unaffected by this check.
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if src_principal.def_id() != dst_principal.def_id() {
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return Err(CastError::DifferingKinds { src_kind, dst_kind });
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}
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// We need to reconstruct trait object types.
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// `m_src` and `m_dst` won't work for us here because they will potentially
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// contain wrappers, which we do not care about.
@@ -912,8 +906,8 @@ impl<'a, 'tcx> CastCheck<'tcx> {
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ty::Dyn,
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));
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// `dyn Src = dyn Dst`, this checks for matching traits/generics
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// This is `demand_eqtype`, but inlined to give a better error.
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// `dyn Src = dyn Dst`, this checks for matching traits/generics/projections
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// This is `fcx.demand_eqtype`, but inlined to give a better error.
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let cause = fcx.misc(self.span);
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if fcx
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.at(&cause, fcx.param_env)
@@ -965,8 +959,35 @@ impl<'a, 'tcx> CastCheck<'tcx> {
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// dyn Auto -> dyn Auto'? ok.
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(None, None) => Ok(CastKind::PtrPtrCast),
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// dyn Trait -> dyn Auto? should be ok, but we used to not allow it.
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// FIXME: allow this
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// dyn Trait -> dyn Auto? not ok (for now).
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//
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// Although dropping the principal is already allowed for unsizing coercions
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// (e.g. `*const (dyn Trait + Auto)` to `*const dyn Auto`), dropping it is
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// currently **NOT** allowed for (non-coercion) ptr-to-ptr casts (e.g
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// `*const Foo` to `*const Bar` where `Foo` has a `dyn Trait + Auto` tail
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// and `Bar` has a `dyn Auto` tail), because the underlying MIR operations
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// currently work very differently:
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//
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// * A MIR unsizing coercion on raw pointers to trait objects (`*const dyn Src`
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// to `*const dyn Dst`) is currently equivalent to downcasting the source to
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// the concrete sized type that it was originally unsized from first (via a
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// ptr-to-ptr cast from `*const Src` to `*const T` with `T: Sized`) and then
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// unsizing this thin pointer to the target type (unsizing `*const T` to
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// `*const Dst`). In particular, this means that the pointer's metadata
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// (vtable) will semantically change, e.g. for const eval and miri, even
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// though the vtables will always be merged for codegen.
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//
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// * A MIR ptr-to-ptr cast is currently equivalent to a transmute and does not
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// change the pointer metadata (vtable) at all.
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//
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// In addition to this potentially surprising difference between coercion and
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// non-coercion casts, casting away the principal with a MIR ptr-to-ptr cast
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// is currently considered undefined behavior:
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//
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// As a validity invariant of pointers to trait objects, we currently require
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// that the principal of the vtable in the pointer metadata exactly matches
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// the principal of the pointee type, where "no principal" is also considered
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// a kind of principal.
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(Some(_), None) => Err(CastError::DifferingKinds { src_kind, dst_kind }),
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// dyn Auto -> dyn Trait? not ok.

‎tests/ui/cast/ptr-to-trait-obj-drop-principal.rs

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

‎tests/ui/cast/ptr-to-trait-obj-drop-principal.stderr

+11
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@@ -0,0 +1,11 @@
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error[E0606]: casting `*const (dyn Trait + Send + 'static)` as `*const Wrapper<dyn Send>` is invalid
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--> $DIR/ptr-to-trait-obj-drop-principal.rs:18:37
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|
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LL | const _: *const Wrapper<dyn Send> = OBJECT as _;
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| ^^^^^^^^^^^
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|
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= note: the trait objects may have different vtables
8+
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error: aborting due to 1 previous error
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For more information about this error, try `rustc --explain E0606`.

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