RFC - Allow Drop types in statics/const functions

text/0000-drop-types-in-const.md

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+- Feature Name: `drop_types_in_const`
+- Start Date: 2016-01-01
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Allow types with destructors to be used in `static` items and in `const` functions, as long as the destructor never needs to run in const context.
+
+# Motivation
+[motivation]: #motivation
+
+Some of the collection types do not allocate any memory when constructed empty (most notably `Vec`). With the change to make leaking safe, the restriction on `static` items with destructors
+is no longer required to be a hard error (as it is safe and accepted that these destructors may never run).
+
+Allowing types with destructors to be directly used in `const` functions and stored in `static`s will remove the need to have
+runtime-initialisation for global variables.
+
+# Detailed design
+[design]: #detailed-design
+
+- Lift the restriction on types with destructors being used in statics.
+ - `static`s containing Drop-types will not run the destructor upon program/thread exit.
+ - (Optionally adding a lint that warn about the possibility of resource leak)
+- Alloc instantiating structures with destructors in constant expressions,
+- Continue to prevent `const` items from holding types with destructors.
+- Allow `const fn` to return types with destructors.
+- Disallow constant expressions which would result in the destructor being called (if the code were run at runtime).
+
+## Examples
+Assuming that `RwLock` and `Vec` have `const fn new` methods, the following example is possible and avoids runtime validity checks.
+
+```rust
+/// Logging output handler
+trait LogHandler: Send + Sync {
+    // ...
+}
+/// List of registered logging handlers
+static S_LOGGERS: RwLock<Vec< Box<LogHandler> >> = RwLock::new( Vec::new() );
+```
+
+Disallowed code
+```rust
+static VAL: usize = (Vec::<u8>::new(), 0).1;	// The `Vec` would be dropped
+const EMPTY_BYTE_VEC: Vec<u8> = Vec::new();	// `const` items can't have destructors
+
+const fn sample(_v: Vec<u8>) -> usize {
+	0	// Discards the input vector, dropping it
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Destructors do not run on `static` items (by design), so this can lead to unexpected behavior when a type's destructor has effects outside the program (e.g. a RAII temporary folder handle, which deletes the folder on drop). However, this can already happen using the `lazy_static` crate.
+
+# Alternatives
+[alternatives]: #alternatives
+
+- Runtime initialisation of a raw pointer can be used instead (as the `lazy_static` crate currently does on stable)
+- On nightly, a bug related to `static` and `UnsafeCell<Option<T>>` can be used to remove the dynamic allocation.
+ - Both of these alternatives require runtime initialisation, and incur a checking overhead on subsequent accesses.
+- Leaking of objects could be addressed by using C++-style `.dtors` support
+ - This is undesirable, as it introduces confusion around destructor execution order.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- TBD