Note design constraints on hypothetical DynSized

src/design_notes/dynsized_constraints.md

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+# Exotically sized types (`DynSized` and `extern type`)
+
+## Overview
+
+In current Rust, there's two kinds of types with respect to sizing:
+if a type is `Sized`, its layout (size and alignment) is known statically,
+and if a type is `?Sized`, its layout may not be known until runtime (e.g. via a vtable).
+
+However, more exotically sized types exist; the most common example is opaque `extern type`.
+`extern type`s have an *unknown* layout to Rust, and as such can only be used behind a pointer type.
+Since the most unsized a type can currently be is `?Sized`, though,
+the compiler has to make up a size and alignment to return from `mem::size_of_val`/`align_of_val`.
+Currently the compiler returns a size of 0 and an alignment of 1.
+Lying in this fashion is considered undesirable \[2].
+
+Additionally, some C-header-interface libraries expose an opaque (incomplete) type
+but also provide a function returning the size of the type and expect the caller to allocate space.
+This is useful to allow the library to change the size of the type,
+but still allow the caller to control allocation (e.g. using a custom arena allocator).
+When bridging to Rust, these types should ideally have access to dynamic size/align.
+
+## Proposed Solution
+
+The most obvious and independently reinvented solution is a "`DynSized`" trait that provides dynamic size/align information.
+`extern type` would not implement `DynSized`, and generic code could opt into `?DynSized` types to support such.
+
+At the time of writing, there is weak approval from T-lang to proceed with an internal-only version of `DynSized`
+which is used to prohibit the use of `extern type` in standard `<T: ?Sized>` generic arguments \[2].
+
+This design document is about the restrictions on what `T: ?Sized + DynSized` actually needs to imply.
+
+## Design Constraints
+
+### `Arc` and `Weak`
+
+`Arc` supports "zombie" references, where all strong `Arc` and the pointee have been dropped,
+but `Weak` handles still exist and so the allocation still exists.
+This means that `Weak` needs to be able to determine the layout of the allocation from a dropped pointee,
+as the `T` is dropped with the last `Arc` but the allocation freed with the last `Weak`.
+
+In addition, `Weak` are pointers to the *reference count* part of the `ArcInner` allocation,
+and thus need to *statically* know the alignment of the pointee type to determine the offset
+(it cannot call `align_of_val_raw` without first knowing the offset).
+
+There are three potential resolutions that handle both size and alignment uniformly:
+
+- Store layout information in the `ArcInner` header, or
+- Require that layout be determined solely from pointee metadata, or
+- Require that layout be determinable from a dropped pointee.[^why]
+
+[^why]: This is trivially the case if determining the layout does not read the pointee (i.e. is derivable by just the potentially wide pointer);
+    alternatively, the pointee could ensure that layout information (e.g. vtable pointer) remains valid to read even after it's been dropped.]
+
+Dealing with alignment can be simplified by changing `Arc<T>` from storing `*mut ArcInner<T>` to
+storing `*mut T` and storing the refcount metadata at a fixed negative offset independent of `T`.
+
+T-lang commented on this in \[3] (w.r.t. const `Weak<T>::[into|from]_raw` and `Weak::new`):
+
+> Consensus from meeting:
+> - We approve the option to make `align_of_val_raw` require a once-valid-but-dropped value, in order to better support thin objects
+>   - we believe the sentinel design \[of `Weak::new`] means that `align_of_val_raw` is only ever invoked on once-valid-but-dropped values
+> - We do not want `align_of_val_raw` to be forced to work for metadata + thin pointer
+> - Implement `Weak::from_raw` to check for sentinel and take some special action if it is observed
+>   - potential cost: for unsized types (only), there is an extra branch (but if custom dst doesn’t require \[dynamic] alignment, we can change this later)
+> - It is not really lang team’s call, but we are -1 on adding more fields to `Rc`/`Arc`
+> - For custom dst, the design will have to accommodate getting the size and alignment from “once-valid-but-dropped” values (values that were once valid but have been dropped); this is a non-issue for known use cases like c-string and thin-objects (which store a vtable)
+>   - (but could be relevant for dynamically allocated vtables)
+
+### `Mutex` (and more generally, `UnsafeCell`)
+
+The problem statement here is the combination of `&Mutex<T>` and `&mut T` both being usable concurrently,
+plus the following presumably sound function:
+
+```rust
+fn noop_write<T: ?Sized>(it: &mut T) {
+    let len = std::mem::size_of_val(it);
+    let ptr = it as *mut T as *mut u8;
+    unsafe { std::ptr::copy(ptr, ptr, len); }
+}
+```
+
+To make the conflict abundantly clear, consider the following:
+
+```rust
+let mutex: &Mutex<ThinCStr> = /* elided */;
+
+join(
+    || {
+        let mut lock = mutex.lock();
+        let it: &mut ThinCStr = &mut *lock;
+        noop_write(it);
+    },
+    || {
+        std::mem::size_of_val(mutex);
+    },
+);
+```
+
+In order to determine the size of `Mutex<ThinCStr>`, you have to know the size of `ThinCStr`, which is inline to the `Mutex`.
+To determine the size of `ThinCStr`, you have to read every byte to find the terminating nul byte (equiv. call `strlen`).
+However, in the other fork, we lock the mutex and use the `&mut ThinCStr` to read and write-back every byte of the `ThinCStr`.
+Because the `&mut` side of the operation is surely nonatomic (and `strlen` likely isn't), this is an unsafe data race, thus UB.
+
+This constraint is more difficult to resolve than the previous one coming from `Arc`/`Weak`.
+Fundamentally, types like `ThinCStr` which require reading the pointee to determine layout information break a core property of `UnsafeCell`
+that `&UnsafeCell<T>` cannot (safely) read (or write) any of `T`'s bytes, if `std::mem::size_of_val` works without locking.
+
+Thus (at the time of writing) there are three known potential resolutions to this constraint:
+
+- Require layout to be calculated solely from thin pointer and pointee metadata,
+- Require `size_of_val` to acquire a read lock (for `Mutex`-like types),
+- Declare `noop_write` is only sound for types which determine layout without reading the pointee, or 
+- Prohibit the use of pointee-determined-layout types in `Mutex`-like types.
+
+## Potential Conclusions
+
+This heading is the notes' author's (@CAD97's) opinion only:
+
+From the above, there result *four* classes of sizedness that Rust *could* care about \[1]:
+
+- "`T:  Sized +  MetaSized +  DynSized`", where the size and alignment are known statically;
+- "`T: ?Sized +  MetaSized +  DynSized`", where the size and alignment are known from the data pointer and metadata;
+- "`T: ?Sized + ?MetaSized +  DynSized`", where the size and alignment require reading the pointee; and
+- "`T: ?Sized + ?MetaSized + ?DynSized`", where the size and alignment cannot be determined by (generic) code.
+
+Examples of these are respectively `u8`, `dyn Trait`, `ThinCStr`, and `extern type`.
+
+@CAD97 posits that in the majority of cases,
+`OwningPointer<T>`-like types want "`?Sized + ?MetaSized + DynSized`",
+`Ref<T>`-like types want "`?Sized + ?MetaSized + ?DynSized`", and
+`UnsafeCell<T>`-like types want "`?Sized + MetaSized + DynSized`".
+
+Additionally, it could be useful to restrict `MetaSized` to only know the pointee metadata and not the data pointer;
+this would allow things like `[T] where T: ?Sized + MetaSized` using both slice and `T` metadata for an extra-fat pointer
+(e.g. `[[T]]` for 2D slices doing the obvious thing (without stride)).
+
+## References
+
+- \[1] https://internals.rust-lang.org/t/erfc-minimal-custom-dsts-via-extern-type-dynsized/16591?u=cad97
+- \[2] https://github.com/rust-lang/rust/issues/49708
+- \[3] https://hackmd.io/7r3_is6uTz-163fsOV8Vfg