Destructure drop structs

text/0000-destructure-drop-structs.md

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+- Feature Name: Destructure structs implementing Drop
+- Start Date: 2017-07-08
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Allow destructuring of structs that implement Drop.
+
+# Motivation
+[motivation]: #motivation
+
+Destructuring allows to reverse the process of creating a struct from individual fields.
+```
+struct Compound<A, B> { a: A, b: B }
+let a: A = ...
+let b: B = ...
+let c = Compound { a: a, b: b } // wraps and into the struct Compound
+let Compound { a: a, b: b } = c; // destructures c into its fields.
+// a and b are now the same they where before c was created
+```
+
+However this is currently not possible if the struct implements `Drop`.
+The current way to destructure these cases anyway, requires to read non-copy fields via 
+`ptr::read(&struct.field)` and then `mem::forget(struct)` to avoid running drop.
+
+This leaves more room for error, than there would have to be: 
+1.  forgetting to read all fields, possibly creating a memory leak,
+2.  acidentally working with `&struct` instead of `struct`, leading to a double free.
+     (The fields are copied, but mem::forget only forgets the reference.)
+
+Allowing to destructure these types would ensure that:
+1.  unused fields are dropped,
+2.  only owned structs can be destructured.
+
+It is not allowed to implicitly move fields out of structs implementing Drop.
+This would make it too easy to accidentally trigger destructuring by accessing a field,
+which then means that drop does not run and possibly cause a memory leak.
+
+# Detailed design
+[design]: #detailed-design
+
+As 
+previously shown destructuring of structs with destructures may create unsoundness
+[[1]](https://github.com/rust-lang/rust/issues/3147)
+[[2]](https://github.com/rust-lang/rust/issues/26940).
+
+One possible solution would be to make this an `unsafe` operation and only allow it inside an unsafe `block`.
+Another possiblity would be to restrict it to modules that are allowed to impement the type in question.
+
+Either restriction would prevent the issue of [[2]](https://github.com/rust-lang/rust/issues/26940).
+
+Say there is the following struct [(taken from here)](https://github.com/rust-lang/rust/issues/26940#issuecomment-120502565):
+```rust
+pub struct MaybeCrashOnDrop { c: bool }
+impl Drop for MaybeCrashOnDrop {
+    fn drop(&mut self) {
+        if self.c {
+            unsafe { *(1 as *mut u8) = 0 }
+        }
+    }
+}
+pub struct InteriorUnsafe { pub m: MaybeCrashOnDrop }
+impl InteriorUnsafe {
+    pub fn new() -> InteriorUnsafe {
+        InteriorUnsafe { m: MaybeCrashOnDrop{ c: true } }
+    }
+}
+impl Drop for InteriorUnsafe {
+    fn drop(&mut self) {
+        self.m.c = false;
+    }
+}
+```
+
+## Approach 1:
+```rust
+let i = InteriorUnsafe::new();
+unsafe {
+     let InteriorUnsafe { m: does_crash } = i;
+}
+```
+Here full responsibility is given to the user, ensuring that the destructuring is sound.
+
+Pros: More flexibility without resorting to ptr::read & mem::forget.
+
+Cons: It is still a special case of destructuring and requires an `unsafe` block.
+
+## Approach 2:
+```rust
+// somewhere in the same module where InteriorUnsafe was declared
+let i = InteriorUnsafe::new();
+let InteriorUnsafe { m: does_crash } = i;
+```
+Destructuring does not require an `unsafe` block, but is limited to the module where the type to be destructured was defined.
+This ensures no third party may create unsound behaviour by incorrectly destructuring a foreign type.
+
+Pros: No unsafe block required
+
+Cons: If a struct implementing Drop needs to be destructured outside the module of definiton, the ptr::read & mem::forget are still required. However this will only affect structs with public fields.
+
+## Approach 1 & 2:
+Inside the module of declaration, no `unsafe` block is required. Otherwhise an `unsafe` block is required.
+
+Pros: does not require an `unsafe` block in most cases.
+Cons: more complexity than required.
+
+## Approach 3:
+Introduce an `unsafe` auto-trait `IndependentDrop`.
+It is automatically derived when all fields implement `IndependentDrop`.
+
+Destructuring of structs is possible if they do not implement `Drop`, or when they implement `IndependentDrop`.
+
+Pros: No `unsafe` block required and destructuring is not limited to the same module.
+Cons: Requires an additional trait that authors have to be aware of.
+
+## Suggested approach: 
+This RFC proposes solution Approch 2.
+
+# How We Teach This
+[how-we-teach-this]: #how-we-teach-this
+
+Probably with a small section in the Nomicon, that explains why destructuring might be dangerous and under what circumstances it is allowed.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Why should we *not* do this?
+
+# Alternatives
+[alternatives]: #alternatives
+
+The trivial alternative is to keep it as is, and keep using `ptr::read` & `mem::forget`.
+This could possibly be automated with a macro.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+Which restrictions are required to make this sound?