improve #[may_dangle] for type parameters

text/3417-dropck-eyepatch-v3.md

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+- Feature Name: (`dropck_eyepatch_v3`)
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Cleanup the rules for implicit drops by splitting `#[may_dangle]` into two separate attributes:
+`#[only_dropped]` and `#[fully_ignored]`. Change `PhantomData` to get completely ignored
+by dropck as its current behavior is confusing and inconsistent.
+
+# Motivation
+[motivation]: #motivation
+
+The current rules around dropck and `#[may_dangle]` are confusing and have even resulted in
+unsoundness [multiple](https://github.com/rust-lang/rust/issues/76367)
+[times](https://github.com/rust-lang/rust/issues/99408). Even without `#[may_dangle]`,
+dropping `PhantomData` is currently quite weird as you get "spooky-dropck-at-a-distance":
+
+```rust
+use std::marker::PhantomData;
+struct PrintOnDrop<'a>(&'a str); // requires `'a` to be live on drop.
+impl Drop for PrintOnDrop<'_> {
+    fn drop(&mut self) {
+        println!("{}", self.0)
+    }
+}
+
+fn assign<T>(_: T) -> PhantomData<T> {
+    PhantomData
+}
+
+// The type of `_x` is `AdtNoDrop<'not_live>` which doesn't have drop glue, OK
+struct AdtNoDrop<'a>(PhantomData<PrintOnDrop<'a>>, u32);
+fn phantom_data_adt_no_drop() {
+    let _x;
+    {
+        let temp = String::from("temporary");
+        _x = AdtNoDrop(assign(PrintOnDrop(&temp)), 0);
+    }
+}
+
+// The type of `_x` is `AdtNoDrop<'not_live>` which has drop glue, ERROR
+struct AdtNeedsDrop<'a>(PhantomData<PrintOnDrop<'a>>, String);
+fn phantom_data_adt_needs_drop() {
+    let _x;
+    {
+        let temp = String::from("temporary");
+        _x = AdtNeedsDrop(assign(PrintOnDrop(&temp)), String::new());
+    }
+}
+```
+[playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=9ce9d368d2f13df9ddcbfaf9580721e0)
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+When a value goes out of scope the compiler adds drop glue for that value, recursively dropping it and all its fields.
+Dropping a type containing a lifetime which is no longer live is accepted if that lifetime is never accessed:
+```rust
+struct MyType<'s> {
+    reference: &'s str,
+    needs_drop: String,
+}
+fn can_drop_dead_reference() {
+    let _x;
+    {
+        let temp = String::from("I am only temporary");
+        _x = MyType {
+            reference: &temp,
+            needs_drop: String::from("I have to get dropped"),
+        };
+    }
+    // We drop `_x` here even though `reference` is no longer live.
+    //
+    // This is fine as dropping a reference is a noop and does not
+    // acess the pointee.
+}
+```
+The above example will however fail if we add a manual `Drop` impl as the compiler conservatively
+assumes that all generic parameters of the `Drop` impl are used:
+[playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e604bcaecb7b2b4cf7fd0440faf165ac).
+
+In case a manual `Drop` impl does not access a generic parameter, you can add
+`#[fully_unused]` or `#[only_dropped]` to that parameter. This **unsafely** asserts
+that the parameter is either completely unused when dropping your type or only
+recursively dropped.
+
+```rust
+struct MyType<'s> {
+    reference: &'s str,
+    needs_drop: String,
+}
+// The impl has to be `unsafe` as the compiler does may not check
+// that `'s` is actually unused.
+unsafe impl<#[only_dropped] 's> Drop for MyType<'s> {
+    fn drop(&mut self) {
+        // println!("{}", reference); // this would be unsound
+        println!("{}", needs_drop);
+    }
+}
+fn can_drop_dead_reference() {
+    let _x;
+    {
+        let temp = String::from("I am only temporary");
+        _x = MyType {
+            reference: &temp,
+            needs_drop: String::from("I have to get dropped"),
+        };
+    }
+    // We drop `_x` here even though `reference` is no longer live.
+    //
+    // This is accepted as `'s` is marked as `#[only_dropped]` in the
+    // `Drop` impl of `MyType`.
+}
+```
+
+The ability to differentiate between `#[fully_unused]` and `#[only_dropped]` is significant
+for type parameters:
+
+```rust
+pub struct BTreeMap<K, V> {
+    root: Option<Root<K, V>>,
+    length: usize,
+}
+
+unsafe impl<#[only_dropped] K, #[only_dropped] V> Drop for BTreeMap<K, V> {
+    fn drop(&mut self) {
+        // Recursively drops the key-value pairs but doesn't otherwise
+        // inspect them, so we can use `#[only_dropped]` here.
+        drop(unsafe {ptr::read(self) }.into_iter())
+    }
+}
+```
+
+A type where `#[fully_unused]` would be useful is a `Weak` pointer for a variant of `Rc`
+where the value is dropped when the last `Rc` goes out of scope. Dropping a `Weak` pointer
+would never access `T` in this case.
+
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Whenever we use a value of a given type this type has to be **well-formed**, requiring
+that all lifetimes in this type are live. An exception to this is the implicit drop when
+a variable goes out of scope. While borrowck ensures that dropping the variable is safe,
+this does not necessarily require all lifetimes to be live.
+
+When implicitly dropping a variable of type `T`, liveness requirements are computed as follows:
+- If `T` does not have any drop glue, do not add any requirements.
+- If `T` is a trait object, `T` has to be live.
+- If `T` has an explicit `Drop` impl, require all generic argument to be live, unless
+    - they are marked with `#[fully_unused]`, in which case they are ignored,
+    - or they are marked with `#[only_dropped]`, in which case recurse into the generic argument.
+- Regardless of whether `T` implements `Drop`, recurse into all types *owned* by `T`:
+    - references, raw pointers, function pointers, function items and scalars do not own
+      anything. They can be trivially dropped.
+    - tuples and arrays consider their element types to be owned.
+    - all fields (of all variants) of ADTs are considered owned. We consider all variants
+      for enums. The only exception here is `ManuallyDrop<U>` which is not considered to own `U`. `PhantomData<U>` does not have any fields and therefore also does not consider
+      `U` to be owned.
+    - closures and generators own their captured upvars.
+
+Checking drop impls may error for generic parameters which are known to be incorrectly marked:
+- `#[fully_unused]` parameters which are recursively owned
+- `#[only_dropped]` parameters which are required to be live by a recursively owned type
+
+This cannot catch all misuses, as the parameters can be incorrectly used by the `Drop` impl itself.
+We therefore require the impl to be marked as `unsafe`.
+
+## How this differs from the status quo
+
+Instead of `#[fully_unused]` and `#[only_dropped]`,there is only the `#[may_dangle]` attribute which
+skips the generic parameter. This is equivalent to the behavior of `#[fully_unused]` and relies on the recursion
+into types owned by `T` to figure out the correct constraints.
+
+`PhantomData<U>` currently considers `U` to be owned while not having drop glue itself. This means
+that `(PhantomData<PrintOnDrop<'s>>, String)` requires `'s` to be live while
+`(PhantomData<PrintOnDrop<'s>>, u32)` does not. This is required for get the
+behavior of `#[only_dropped]` for parameters otherwise not owned by adding `PhantomData` as a field.
+One can easily forget this, which caused the [unsound](https://github.com/rust-lang/rust/issues/76367)
+[issues](https://github.com/rust-lang/rust/issues/99408) mentioned above.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It requires an additional attribute when compared with `#[may_dangle]` and also proposes checks that the
+attributes are correctly used. This adds a small amount of implementation complexity to the compiler.
+These new attributes are still not fully checked by the compiler and require `unsafe`.
+
+This RFC does not explicitly exclude stabilizing these two attributes, as they are clearer and far less
+dangerous to use when compared with `#[may_dangle]`. Stabilizing these attributes will make it harder to
+stabilize a more general solution like type state.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The status quo of `#[may_dangle]` and "spooky-dropck-at-a-distance" is far from ideal and has already
+resulted in unsound issues. Documenting the current behavior makes is more difficult to change later
+while not officially documenting it is bound to lead to more issues and confusion going forward.
+It is therefore quite important to improve the status quo.
+
+A more general extension to deal with partially invalid types is far from trivial. We currently
+assume types to always be well-formed and any approach which generalizes `#[may_dangle]` will
+have major consequences for how well-formedness is handled. This impacts many - often implicit -
+interactions and assumptions. It is highly unlikely that we will have the capacity for any such change
+in the near future. The benefits from such are change are likely to be fairly limited while
+adding significant complexity.
+
+# Prior art
+[prior-art]: #prior-art
+
+`#[may_dangle]` is already a refinement of the previous `#[unsafe_destructor_blind_to_params]` attribute
+([RFC 1327](https://github.com/rust-lang/rfcs/pull/1327)).
+
+There is also [RFC 3390](https://github.com/rust-lang/rfcs/pull/3390) which attempts to define a more
+general extension to replace `#[may_dangle]`. As mentioned in the rationale, such an approach is not
+feasable right now.
+
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+Should these attributes remain purely unstable for use in the standard library or do we want
+to provide them to stable users?
+
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Extending or generalizing the dropck eyepatch... something something type state.
+
+[`IndexVec`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_index/vec/struct.IndexVec.html)