Non-parametric dropck; instead trust an unsafe attribute (RFC 1238).

Implement cannot-assume-parametricity (CAP) from RFC 1238, and add the
UGEH attribute.

----

Note that we check for the attribute attached to the dtor method, not
the Drop impl.

(This is just to match the specification of RFC and the tests; I am
not wedded to this approach.)

Author: pnkfelix

src/doc/reference.md

@@ -1929,6 +1929,20 @@ macro scope.
 - `simd` - on certain tuple structs, derive the arithmetic operators, which
   lower to the target's SIMD instructions, if any; the `simd` feature gate
   is necessary to use this attribute.
+- `unsafe_destructor_blind_to_params` - on `Drop::drop` method, asserts that the
+  destructor code (and all potential specializations of that code) will
+  never attempt to read from nor write to any references with lifetimes
+  that come in via generic parameters. This is a constraint we cannot
+  currently express via the type system, and therefore we rely on the
+  programmer to assert that it holds. Adding this to a Drop impl causes
+  the associated destructor to be considered "uninteresting" by the
+  Drop-Check rule, and thus it can help sidestep data ordering
+  constraints that would otherwise be introduced by the Drop-Check
+  rule. Such sidestepping of the constraints, if done incorrectly, can
+  lead to undefined behavior (in the form of reading or writing to data
+  outside of its dynamic extent), and thus this attribute has the word
+  "unsafe" in its name. To use this, the
+  `unsafe_destructor_blind_to_params` feature gate must be enabled.
 - `unsafe_no_drop_flag` - on structs, remove the flag that prevents
   destructors from being run twice. Destructors might be run multiple times on
   the same object with this attribute. To use this, the `unsafe_no_drop_flag` feature

src/librustc/middle/ty/util.rs

@@ -578,6 +578,16 @@ impl<'tcx> ty::ctxt<'tcx> {
         });
         let generics = adt.type_scheme(self).generics;
 
+        // RFC 1238: if the destructor method is tagged with the
+        // attribute `unsafe_destructor_blind_to_params`, then the
+        // compiler is being instructed to *assume* that the
+        // destructor will not access borrowed data via a type
+        // parameter, even if such data is otherwise reachable.
+        if self.has_attr(dtor_method, "unsafe_destructor_blind_to_params") {
+            debug!("typ: {:?} assumed blind and thus is dtorck-safe", adt);
+            return false;
+        }
+
         // In `impl<'a> Drop ...`, we automatically assume
         // `'a` is meaningful and thus represents a bound
         // through which we could reach borrowed data.
@@ -592,6 +602,14 @@ impl<'tcx> ty::ctxt<'tcx> {
             return true;
         }
 
+        // RFC 1238: *any* type parameter at all makes this a dtor of
+        // interest (i.e. cannot-assume-parametricity from RFC 1238.)
+        if generics.has_type_params(subst::TypeSpace) {
+            debug!("typ: {:?} has interesting dtor due to type params",
+                   adt);
+            return true;
+        }
+
         let mut seen_items = Vec::new();
         let mut items_to_inspect = vec![impl_did];
         while let Some(item_def_id) = items_to_inspect.pop() {

src/librustc_typeck/check/dropck.rs

@@ -217,26 +217,52 @@ fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
 ///
 /// ----
 ///
-/// The Drop Check Rule is the following:
+/// The simplified (*) Drop Check Rule is the following:
 ///
 /// Let `v` be some value (either temporary or named) and 'a be some
 /// lifetime (scope). If the type of `v` owns data of type `D`, where
 ///
-/// * (1.) `D` has a lifetime- or type-parametric Drop implementation, and
-/// * (2.) the structure of `D` can reach a reference of type `&'a _`, and
-/// * (3.) either:
-///   * (A.) the Drop impl for `D` instantiates `D` at 'a directly,
-///          i.e. `D<'a>`, or,
-///   * (B.) the Drop impl for `D` has some type parameter with a
-///          trait bound `T` where `T` is a trait that has at least
-///          one method,
+/// * (1.) `D` has a lifetime- or type-parametric Drop implementation,
+///        (where that `Drop` implementation does not opt-out of
+///         this check via the `unsafe_destructor_blind_to_params`
+///         attribute), and
+/// * (2.) the structure of `D` can reach a reference of type `&'a _`,
 ///
 /// then 'a must strictly outlive the scope of v.
 ///
 /// ----
 ///
 /// This function is meant to by applied to the type for every
 /// expression in the program.
+///
+/// ----
+///
+/// (*) The qualifier "simplified" is attached to the above
+/// definition of the Drop Check Rule, because it is a simplification
+/// of the original Drop Check rule, which attempted to prove that
+/// some `Drop` implementations could not possibly access data even if
+/// it was technically reachable, due to parametricity.
+///
+/// However, (1.) parametricity on its own turned out to be a
+/// necessary but insufficient condition, and (2.)  future changes to
+/// the language are expected to make it impossible to ensure that a
+/// `Drop` implementation is actually parametric with respect to any
+/// particular type parameter. (In particular, impl specialization is
+/// expected to break the needed parametricity property beyond
+/// repair.)
+///
+/// Therefore we have scaled back Drop-Check to a more conservative
+/// rule that does not attempt to deduce whether a `Drop`
+/// implementation could not possible access data of a given lifetime;
+/// instead Drop-Check now simply assumes that if a destructor has
+/// access (direct or indirect) to a lifetime parameter, then that
+/// lifetime must be forced to outlive that destructor's dynamic
+/// extent. We then provide the `unsafe_destructor_blind_to_params`
+/// attribute as a way for destructor implementations to opt-out of
+/// this conservative assumption (and thus assume the obligation of
+/// ensuring that they do not access data nor invoke methods of
+/// values that have been previously dropped).
+///
 pub fn check_safety_of_destructor_if_necessary<'a, 'tcx>(rcx: &mut Rcx<'a, 'tcx>,
                                                          typ: ty::Ty<'tcx>,
                                                          span: Span,
@@ -356,30 +382,25 @@ fn iterate_over_potentially_unsafe_regions_in_type<'a, 'b, 'tcx>(
     // borrowed data reachable via `typ` must outlive the parent
     // of `scope`. This is handled below.
     //
-    // However, there is an important special case: by
-    // parametricity, any generic type parameters have *no* trait
-    // bounds in the Drop impl can not be used in any way (apart
-    // from being dropped), and thus we can treat data borrowed
-    // via such type parameters remains unreachable.
+    // However, there is an important special case: for any Drop
+    // impl that is tagged as "blind" to their parameters,
+    // we assume that data borrowed via such type parameters
+    // remains unreachable via that Drop impl.
+    //
+    // For example, consider:
+    //
+    // ```rust
+    // #[unsafe_destructor_blind_to_params]
+    // impl<T> Drop for Vec<T> { ... }
+    // ```
     //
-    // For example, consider `impl<T> Drop for Vec<T> { ... }`,
     // which does have to be able to drop instances of `T`, but
     // otherwise cannot read data from `T`.
     //
     // Of course, for the type expression passed in for any such
     // unbounded type parameter `T`, we must resume the recursive
     // analysis on `T` (since it would be ignored by
     // type_must_outlive).
-    //
-    // FIXME (pnkfelix): Long term, we could be smart and actually
-    // feed which generic parameters can be ignored *into* `fn
-    // type_must_outlive` (or some generalization thereof). But
-    // for the short term, it probably covers most cases of
-    // interest to just special case Drop impls where: (1.) there
-    // are no generic lifetime parameters and (2.)  *all* generic
-    // type parameters are unbounded.  If both conditions hold, we
-    // simply skip the `type_must_outlive` call entirely (but
-    // resume the recursive checking of the type-substructure).
     if has_dtor_of_interest(tcx, ty) {
         debug!("iterate_over_potentially_unsafe_regions_in_type \
                 {}ty: {} - is a dtorck type!",

src/libsyntax/feature_gate.rs

@@ -136,6 +136,10 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
     // switch to Accepted; see RFC 320)
     ("unsafe_no_drop_flag", "1.0.0", None, Active),
 
+    // Allows using the unsafe_destructor_blind_to_params attribute
+    // (Needs an RFC link)
+    ("unsafe_destructor_blind_to_params", "1.3.0", Some(28498), Active),
+
     // Allows the use of custom attributes; RFC 572
     ("custom_attribute", "1.0.0", None, Active),
 
@@ -339,6 +343,11 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGat
     ("unsafe_no_drop_flag", Whitelisted, Gated("unsafe_no_drop_flag",
                                                "unsafe_no_drop_flag has unstable semantics \
                                                 and may be removed in the future")),
+    ("unsafe_destructor_blind_to_params",
+     Normal,
+     Gated("unsafe_destructor_blind_to_params",
+           "unsafe_destructor_blind_to_params has unstable semantics \
+            and may be removed in the future")),
     ("unwind", Whitelisted, Gated("unwind_attributes", "#[unwind] is experimental")),
 
     // used in resolve