New trait OwnableRefCounted for ARef<->Owned conversion.

This allows to convert between ARef<T> and Owned<T> by
implementing the new trait OwnedRefCounted.

This way we will have a shared/unique reference counting scheme
for types with built-in refcounts in analogy to Arc/UniqueArc.

To: Miguel Ojeda <[email protected]>
To: Alex Gaynor <[email protected]>
To: Boqun Feng <[email protected]>
To: Gary Guo <[email protected]>
To: Björn Roy Baron <[email protected]>
To: Benno Lossin <[email protected]>
To: Andreas Hindborg <[email protected]>
To: Alice Ryhl <[email protected]>
To: Trevor Gross <[email protected]>
To: Asahi Lina <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Oliver Mangold <[email protected]>
---
Changes in v8:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v7: https://lore.kernel.org/r/[email protected]

Changes in v7:
- Squash patch to make Owned::from_raw/into_raw public into parent
- Added Signed-off-by to other people's commits
- Link to v6: https://lore.kernel.org/r/[email protected]

Changes in v6:
- Changed comments/formatting as suggested by Miguel Ojeda
- Included and used new config flag RUSTC_HAS_DO_NOT_RECOMMEND,
  thus no changes to types.rs will be needed when the attribute
  becomes available.
- Fixed commit message for Owned patch.
- Link to v5: https://lore.kernel.org/r/[email protected]

Changes in v5:
- Rebase the whole thing on top of the Ownable/Owned traits by Asahi Lina.
- Rename AlwaysRefCounted to RefCounted and make AlwaysRefCounted a
  marker trait instead to allow to obtain an ARef<T> from an &T,
  which (as Alice pointed out) is unsound when combined with UniqueRef/Owned.
- Change the Trait design and naming to implement this feature,
  UniqueRef/UniqueRefCounted is dropped in favor of Ownable/Owned and
  OwnableRefCounted is used to provide the functions to convert
  between Owned and ARef.
- Link to v4: https://lore.kernel.org/r/[email protected]

Changes in v4:
- Just a minor change in naming by request from Andreas Hindborg,
  try_shared_to_unique() -> try_from_shared(),
  unique_to_shared() -> into_shared(),
  which is more in line with standard Rust naming conventions.
- Link to v3: https://lore.kernel.org/r/Z8Wuud2UQX6Yukyr@mango

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
  "series": {
    "revision": 8,
    "change-id": "20250305-unique-ref-29fcd675f9e9",
    "prefixes": [],
    "history": {
      "v4": [
        "Z8Wuud2UQX6Yukyr@mango",
        "[email protected]"
      ],
      "v5": [
        "[email protected]"
      ],
      "v6": [
        "[email protected]"
      ],
      "v7": [
        "[email protected]"
      ]
    }
  }
}

Author: Oliver Mangold