Split may_dangle

text/3414-split-maydangle.md

@@ -0,0 +1,154 @@
+- Feature Name: `split_maydangle`
+- Start Date: 2023-02-13
+- 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
+
+Add `#[needs_drop]`, ignore `PhantomData` for outlives requirements.
+
+# Motivation
+[motivation]: #motivation
+
+This fails to compile:
+
+```rust
+use core::marker::PhantomData;
+
+struct PrintOnDrop<'s>(&'s str);
+impl<'s> Drop for PrintOnDrop<'s> {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+
+fn to_pd<T>(_: T) -> PhantomData<T> {
+    PhantomData
+}
+
+pub fn foo() {
+    let mut x;
+    {
+        let s = String::from("temporary");
+        let p = PrintOnDrop(&s);
+        x = (to_pd(p), String::new());
+    }
+}
+```
+
+And yet, this compiles:
+
+```rust
+use core::marker::PhantomData;
+
+struct PrintOnDrop<'s>(&'s str);
+impl<'s> Drop for PrintOnDrop<'s> {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+
+fn to_pd<T>(_: T) -> PhantomData<T> {
+    PhantomData
+}
+
+pub fn foo() {
+    let mut x;
+    {
+        let s = String::from("temporary");
+        let p = PrintOnDrop(&s);
+        x = (to_pd(p), ());
+    }
+}
+```
+
+Since the values in the tuple are unrelated, they should not affect each other.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+A type marked `#[needs_drop]` gets checked for liveness at drop. This is
+necessary for `Vec`:
+
+```rust
+struct Vec<T> {
+  ...
+}
+
+unsafe impl<#[needs_drop] #[may_dangle] T> Drop for Vec<T> {
+  fn drop(&mut self) {
+    ...
+  }
+}
+```
+
+So that this compiles:
+
+```rust
+fn main() {
+  let mut v = vec![];
+  {
+    v.push(&String::from("temporary"));
+  }
+}
+```
+
+But this cannot compile, as it would be unsound:
+
+```rust
+struct PrintOnDrop<'s>(&'s str);
+impl<'s> Drop for PrintOnDrop<'s> {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+
+fn main() {
+  let mut v = vec![];
+  {
+    v.push(PrintOnDrop(&*String::from("temporary")));
+  }
+}
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+This RFC removes the dropck/outlives constraints from `PhantomData` and moves
+them into the relevant `Drop` impls instead.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Requires mild churn to update things to the new way. Failing to update wouldn't
+break existing code, but would allow unsound code to compile.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+A type which doesn't need drop should never have dropck/outlives contraints,
+but due to the rushed way in which `may_dangle` was implemented, `PhantomData`
+ended up having this unfortunate behaviour. This RFC removes this behaviour and
+allows strictly more code to compile.
+
+# Prior art
+[prior-art]: #prior-art
+
+- Compiler MCP 563: It is the exact same thing as this RFC, but a full RFC
+    seemed appropriate due to observable changes on stable, even if they are
+    fairly obscure.
+- Unsound dropck elaboration for `BTreeMap`: <https://github.com/rust-lang/rust/pull/99413>
+- `may_dangle`: RFC 1238, RFC 1327
+- This is effectively split from RFC PR 3390 and is not intended for
+    stabilization.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+N/A
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+The full RFC 3390, and stabilization.