Treat generators as if they have an arbitrary destructor

src/librustc_traits/dropck_outlives.rs

@@ -193,14 +193,38 @@ fn dtorck_constraint_for_ty<'a, 'gcx, 'tcx>(
             .map(|ty| dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty))
             .collect(),
 
-        ty::TyGenerator(def_id, substs, _) => {
-            // Note that the interior types are ignored here.
-            // Any type reachable inside the interior must also be reachable
-            // through the upvars.
-            substs
-                .upvar_tys(def_id, tcx)
-                .map(|ty| dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty))
-                .collect()
+        ty::TyGenerator(def_id, substs, _interior) => {
+            // rust-lang/rust#49918: types can be constructed, stored
+            // in the interior, and sit idle when generator yields
+            // (and is subsequently dropped).
+            //
+            // It would be nice to descend into interior of a
+            // generator to determine what effects dropping it might
+            // have (by looking at any drop effects associated with
+            // its interior).
+            //
+            // However, the interior's representation uses things like
+            // TyGeneratorWitness that explicitly assume they are not
+            // traversed in such a manner. So instead, we will
+            // simplify things for now by treating all generators as
+            // if they were like trait objects, where its upvars must
+            // all be alive for the generator's (potential)
+            // destructor.
+            //
+            // In particular, skipping over `_interior` is safe
+            // because any side-effects from dropping `_interior` can
+            // only take place through references with lifetimes
+            // derived from lifetimes attached to the upvars, and we
+            // *do* incorporate the upvars here.
+
+            let constraint = DtorckConstraint {
+                outlives: substs.upvar_tys(def_id, tcx).map(|t| t.into()).collect(),
+                dtorck_types: vec![],
+                overflows: vec![],
+            };
+            debug!("dtorck_constraint: generator {:?} => {:?}", def_id, constraint);
+
+            Ok(constraint)
         }
 
         ty::TyAdt(def, substs) => {

src/test/ui/generator/borrowing.nll.stderr

@@ -1,14 +1,30 @@
-error: compilation successful
-  --> $DIR/borrowing.rs:15:1
+error[E0597]: `a` does not live long enough
+  --> $DIR/borrowing.rs:18:18
    |
-LL | / fn main() { #![rustc_error] // rust-lang/rust#49855
-LL | |     let _b = {
-LL | |         let a = 3;
-LL | |         unsafe { (|| yield &a).resume() }
-...  |
-LL | |     };
-LL | | }
-   | |_^
+LL |         unsafe { (|| yield &a).resume() }
+   |                  ^^^^^^^^^^^^^
+   |                  |
+   |                  borrowed value does not live long enough
+   |                  borrow may end up in a temporary, created here
+LL |         //~^ ERROR: `a` does not live long enough
+LL |     };
+   |     -- temporary later dropped here, potentially using the reference
+   |     |
+   |     borrowed value only lives until here
 
-error: aborting due to previous error
+error[E0597]: `a` does not live long enough
+  --> $DIR/borrowing.rs:24:9
+   |
+LL | /         || {
+LL | |             yield &a
+LL | |             //~^ ERROR: `a` does not live long enough
+LL | |         }
+   | |_________^ borrowed value does not live long enough
+LL |       };
+   |       - borrowed value only lives until here
+LL |   }
+   |   - borrow later used here, when `_b` is dropped
+
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/generator/borrowing.rs

@@ -8,11 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(generators, generator_trait, rustc_attrs)]
+#![feature(generators, generator_trait)]
 
 use std::ops::Generator;
 
-fn main() { #![rustc_error] // rust-lang/rust#49855
+fn main() {
     let _b = {
         let a = 3;
         unsafe { (|| yield &a).resume() }

src/test/ui/generator/dropck.nll.stderr

@@ -1,14 +1,20 @@
-error: compilation successful
-  --> $DIR/dropck.rs:16:1
+error[E0597]: `ref_` does not live long enough
+  --> $DIR/dropck.rs:22:11
    |
-LL | / fn main() { #![rustc_error] // rust-lang/rust#49855
-LL | |     let (cell, mut gen);
-LL | |     cell = Box::new(RefCell::new(0));
-LL | |     let ref_ = Box::leak(Box::new(Some(cell.borrow_mut())));
-...  |
-LL | |     // drops the RefCell and then the Ref, leading to use-after-free
-LL | | }
-   | |_^
+LL |       gen = || {
+   |  ___________^
+LL | |         // but the generator can use it to drop a `Ref<'a, i32>`.
+LL | |         let _d = ref_.take(); //~ ERROR `ref_` does not live long enough
+LL | |         yield;
+LL | |     };
+   | |_____^ borrowed value does not live long enough
+...
+LL |   }
+   |   -
+   |   |
+   |   borrowed value only lives until here
+   |   borrow later used here, when `gen` is dropped
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/generator/dropck.rs

@@ -8,15 +8,16 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(generators, generator_trait, box_leak, rustc_attrs)]
+#![feature(generators, generator_trait, box_leak)]
 
 use std::cell::RefCell;
 use std::ops::Generator;
 
-fn main() { #![rustc_error] // rust-lang/rust#49855
+fn main() {
     let (cell, mut gen);
     cell = Box::new(RefCell::new(0));
     let ref_ = Box::leak(Box::new(Some(cell.borrow_mut())));
+    //~^ ERROR `*cell` does not live long enough [E0597]
     // the upvar is the non-dropck `&mut Option<Ref<'a, i32>>`.
     gen = || {
         // but the generator can use it to drop a `Ref<'a, i32>`.

src/test/ui/generator/dropck.stderr

@@ -1,5 +1,16 @@
+error[E0597]: `*cell` does not live long enough
+  --> $DIR/dropck.rs:19:40
+   |
+LL |     let ref_ = Box::leak(Box::new(Some(cell.borrow_mut())));
+   |                                        ^^^^ borrowed value does not live long enough
+...
+LL | }
+   | - `*cell` dropped here while still borrowed
+   |
+   = note: values in a scope are dropped in the opposite order they are created
+
 error[E0597]: `ref_` does not live long enough
-  --> $DIR/dropck.rs:23:18
+  --> $DIR/dropck.rs:24:18
    |
 LL |     gen = || {
    |           -- capture occurs here
@@ -12,6 +23,6 @@ LL | }
    |
    = note: values in a scope are dropped in the opposite order they are created
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0597`.

src/test/ui/generator/ref-escapes-but-not-over-yield.nll.stderr

@@ -1,11 +1,19 @@
 error[E0597]: `b` does not live long enough
   --> $DIR/ref-escapes-but-not-over-yield.rs:24:13
    |
-LL |         a = &b;
-   |             ^^ borrowed value does not live long enough
-LL |         //~^ ERROR `b` does not live long enough
-LL |     };
-   |     - borrowed value only lives until here
+LL |       let mut b = move || {
+   |  _________________-
+LL | |         yield();
+LL | |         let b = 5;
+LL | |         a = &b;
+   | |             ^^ borrowed value does not live long enough
+LL | |         //~^ ERROR `b` does not live long enough
+LL | |     };
+   | |     -
+   | |     |
+   | |     borrowed value only lives until here
+   | |_____temporary later dropped here, potentially using the reference
+   |       borrow may end up in a temporary, created here
 
 error: aborting due to previous error