Aligned trait RFC

text/3319-aligned-trait.md

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+- Feature Name: `aligned`
+- Start Date: 2022-09-24
+- RFC PR: [rust-lang/rfcs#3319](https://github.com/rust-lang/rfcs/pull/3319)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+
+Add an `Aligned` marker trait to `core::marker`, as a supertrait of the `Sized` trait. `Aligned` is implemented for all types with an alignment determined at compile time. This includes all `Sized` types, as well as slices and records containing them. Relax `core::mem::align_of<T>()`'s trait bound from `T: Sized` to `T: ?Sized + Aligned`.
+
+# Motivation
+
+Data structures and containers that wish to store unsized types are easier to implement if they can produce a dangling, well-aligned pointer to the unsized type. Being able to determine a type's alignment without a value of the type allows doing this.
+
+In addition, this RFC allows implementing certain object-safe traits for slices, where this was not possible before.
+
+# Guide-level explanation
+
+`Aligned` is a marker trait defined in `core::marker`. It's automatically implemented for all types with an alignment determined at compile time. This includes all `Sized` types (`Aligned` is a supertrait of `Sized`), as well as slices and records containing them. Trait objects are not `Aligned`.
+
+You can't implement `Aligned` yourself.
+
+To get the alignment of a type that implements `Aligned`, call `core::mem::align_of<T>()`.
+
+Implied `Sized` bounds also imply `Aligned`, because `Aligned` is a supertrait of `Sized`. To bound a type parameter by `Aligned` only, write `?Sized + Aligned`.
+
+# Reference-level explanation
+
+`Aligned` is not object-safe. Trait methods bounded by `Self: Aligned` can't be called from a vtable, but don't affect the object safety of the trait as a whole, just like `Self: Sized` currently.
+Relaxing `Self: Sized` bounds to `Self: Aligned` allows implementing certain object-safe traits for slices, where this was not previously possible.
+
+# Drawbacks
+
+- Slightly compicates situation around implied `Sized` bounds.
+- May make certain object safety diagnostics more confusing, as they will now refer to the new, lesser-known `Aligned` trait instead of `Sized.`
+
+# Rationale and alternatives
+
+`core::mem::align_of<T>()` for slices could be implemented with a library. However, a library would be unable to support records that contain a slice as the last field. Also, relaxing the trait dyn safety requirements can only be done with a language feature.
+
+# Prior art
+
+None that I am aware of.
+
+# Unresolved questions
+
+- Should `Aligned` be `#[fundamental]`? `Sized` is.
+
+# Future possibilities
+
+- Relaxing `NonNull::<T>::dangling()`'s trait bound from `T: Sized` to `T: ?Sized + Aligned + Pointee<Metadata: ~const Default>` may be desirable once the necessary library and language features are stabilized.
+- `extern type`s may want to be able to implement `Aligned`.
+- `Aligned` may warrant an addition the next edition's prelude.