Dyn upcast safety

This document proposes a resolution to the last outstanding question blocking dyn upcast.

Tracking issue: https://github.com/rust-lang/rust/issues/101336

Background

We are trying to enable "upcasts" from a dyn Trait to its supertraits:

#![allow(unused)]
fn main() {
trait Foo {
    fn foo_method(&self);
}
trait Bar: Foo {
    fn bar_method(&self);
}

let x: &dyn Bar = /* ... */;
let y: &dyn Foo = x; // compiles
}

The key factor for these upcasts is that they require adjusting the vtable. The current implementation strategy is that the vtables for the Bar trait embed pointers to a Foo vtable within them:

+----------------------------+
| Bar vtable for some type T |
|----------------------------|
| Foo vtable                 | ----------> +------------------+
| foo_method                 | -----+      | Foo vtable for T |
| bar_method                 | --+  |      |------------------|
+----------------------------+   |  |      | foo_method       | ---+
                                 |  |      +------------------+    |
                                 |  |                              |
                                 |  +---> <T as Foo>::foo_method <-+
                                 v
                     <T as Bar>::bar_method
                 
(this diagram is only meant to convey the general idea of the vtable
 layout, and doesn't represent the exact offsets we would use etc;
 in fact, with the current implementation,
 the first supertrait is stored 'inline' and hence no load is required)

This way, given a &dyn Bar object, we convert its Bar vtable to the appropriate Foo vtable by loading the appropriate field.

Although we don't want to commit to a particular implementation strategy, we do want to leave room for this strategy. One implication is that performing an upcast may require loading from the vtable, which implies that the vtable must be a valid pointer to an actual Rust vtable. Although &dyn Bar references would always contain a valid vtable, the same is not necessarily true for a raw pointer like *const dyn Bar or *mut dyn Bar.

In the language today, we only support "noop upcasts" that don't affect the vtable, and these are safe (e.g., converting from *const dyn Foo + Send to *const dyn Foo). If we extend the set of upcasts to permit vtable-adjusting upcasts, like *const dyn Bar to *const dyn Foo, this implies that, for safe code at least, all *const dyn Trait values must have a valid vtable, so that we know we can safely load the required field and perform the upcast.

On the other hand, we do not generally require raw *mut T pointers to point to valid data. In fact, we explicitly permit them to have any value, including null, and only require that they point to valid data when they are dereferenced. Because dereferencing a raw pointer is an unsafe operation, it has always been considered safe to expose an arbitrary raw pointer to unsafe code -- the unsafety arises when you take a raw pointer from an unknown source and dereference it, since unless you can trace the origin of that pointer you can't possible guarantee that it is valid to dereference.

This brings us to the conflict:

  • It has historically been safe to "release" a raw pointer to safe code, but not safe to receive one (since you cannot know if it is valid).
  • It has historically been safe to upcast *const dyn values (e.g., *const dyn Foo + Send to *const dyn Foo).
    • Unlike the upcasts we are considering now, this upcast does not require changing the vtable at runtime, but the distinction is subtle for end-users.
    • Moreover, there are future extensions (e.g., upcasting *const dyn Foo + Bar to *const dyn Foo) that would require adjusting the vtable much like the upcasts currently being stabilized.

There have been requests to extend traits with the option to include raw pointer methods:

#![allow(unused)]
fn main() {
trait PtrLike {
    fn is_null(v: *const Self) -> bool;
}
}

These methods would be useful when writing unsafe code because having an &self method requires satisfying the validity conditions of an &-reference, which may not be possible. If we did have such methods, however, it raises the question of whether it would be safe to invoke is_null on a *const dyn PtrLike reference. Just as with upcasting, invoking a method from the vtable requires loading from the vtable, and hence requires a valid vtable generated by the compiler.

The solution we propose in this document also resolves this future dilemma.

Definitions: validity vs safety invariant

We adopt the terms validity and safety invariant from the unsafe code guidelines:

The validity invariant is an invariant that all data must uphold any time it is accessed or copied in a typed manner. This invariant is known to the compiler and exploited by optimizations such as improved enum layout or eliding in-bounds checks.

The safety invariant is an invariant that safe code may assume all data to uphold. This invariant is used to justify which operations safe code can perform. The safety invariant can be temporarily violated by unsafe code, but must always be upheld when interfacing with unknown safe code. It is not relevant when arguing whether some program has UB, but it is relevant when arguing whether some code safely encapsulates its unsafety -- in other words, it is relevant when arguing whether some library is sound.

In short, the validity invariant defines a condition that must always be true, even in unsafe code, and the safety invariant defines an invariant that unsafe code must guarantee before a value can be released to be used by arbitrary code.

Contours of the solution space

We can fix this conflict in one of two basic ways:

First, we could make vtable-adjusting upcasts casts (and *Self method calls) unsafe. This is difficult to implement and would require changes to the Coerce trait, which is already excessively complicated. In exchange, it offers at best marginal benefit: raw *dyn pointers can be released to safe code, but safe code can't do anything interesting with them. For this reason, we do not recommend this option.

If vtable-adjusting casts (and *Self method calls) are safe, then the safety invariant for *dyn types must be that their metadata points to a fully valid vtable (i.e., a vtable created by the compiler). This ensures safe code can perform upcasts or dynamic dispatch. This also implies that std::ptr::null (which is safe) cannot be extended to T where T: ?Sized unless further changes are made, since we would need to provide a valid vtable (it would be possible to permit a sentinel value, like null, to be used, but that would imply that upcasting must have a branch, making it less efficient).

There are, however, various options for the validity invariant, ranging from no invariant to requiring a fully valid vtable at all times. The strict invariants offer some benefits, such as the ability to have a niche for *dyn pointers. We survey the options here:

Validity invariant for *dyn metadataSupports nicheCan be initialized with std::mem::zeroedConstant initializer
None✅✅
Word-aligned✅✅✅
Word-aligned, non-null✅✅
Valid vtable✅

Explanations for the column titles:

  • Validity invariant for *dyn metadata -- describes the invariant that applies to the metadata for a *dyn value.
  • Supports niche -- true if there is a niche value so that sizeof(Option<*const dyn Foo>) == sizeof(*const dyn Foo).
  • Can be initialized with std::mem::zeroed -- true if std::mem::zeroed can be used to create a valid value (very convenient). This makes it trivial to innitialize a *const dyn with a dummy value, though the value cannot be released to safe code.
  • Constant initializer -- true if there is some constant value for *const dyn Foo that satisfies the validity invariant, no matter the trait Foo. This makes it easy to initialize a *const dyn with a dummy value, though the value cannot be released to safe code.

Other points:

  • *dyn values currently have a niche.
  • Other pointer-like values (such as fn and &-references) are expected to have a validity invariant of word-aligned, non-null.
  • Whichever option we use, we can backwards-compatibly move "up the table" and adopt a less-strict validity invariant without introducing UB into any extant programs.

Proposal

The proposal is as follows:

  • Vtable-adjusting upcasts are safe operations. The upcast is UB if performed on a value without a valid vtable
  • As such, the "safety invariant" requires a fully valid vtable.
  • The "validity invariant" requires *dyn metadata to be word-aligned and non-null.

Vtable-adjusting upcasts are defined as:

  • Trait upcasts that alter the set of methods that can be invoked on the resulting value at runtime (e.g., dyn Bar to dyn Foo from the introduction). In particular, upcasts that simply add or remove auto-traits are not vtable-adjusting (e.g., dyn Debug + Send to dyn Debug).

This approach...

  • permits safe upcasting (and method calls, in the future);
  • preserves the existing niche for *const dyn;
  • is consistent with other validity requirements for "pointer-like" things such as fn;

The rules also imply that...

  • valid (i.e., compiler-generated) vtables are only required for a *dyn pointer when
    • the *dyn pointer is upcast (or invoke methods);
    • or, when the *dyn pointer is released to arbitrary code, because that code may upcast (or invoke methods).
      • By implication, extending std::ptr::null to permit T: ?Sized would not be safe.

Possible future changes...

  • without introducing UB, we could loosen the validity invariant if desired, for example to permit NULL as a valid value in unsafe code.
  • without introducing UB, we could also loosen the safety invariant to permit a sentinel value (such as NULL), but that would require a branch or other check in the upcast code, which would be less efficient.