don't consider regions

Author: Lukas Markeffsky

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2310,7 +2310,7 @@ fn confirm_builtin_candidate<'cx, 'tcx>(
                 );
                 let sized_obligation = obligation.with(tcx, sized_predicate);
                 if self_ty == tail
-                    || selcx.infcx.predicate_must_hold_considering_regions(&sized_obligation)
+                    || selcx.infcx.predicate_must_hold_modulo_regions(&sized_obligation)
                 {
                     // If the `self_ty` is `Sized`, then the metadata is `()`.
                     // We check this before projecting to the metadata of `tail`,

compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs

@@ -41,7 +41,9 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
     /// not entirely accurate if inference variables are involved.
     ///
     /// This version may conservatively fail when outlives obligations
-    /// are required.
+    /// are required. Therefore, this version should only be used for
+    /// optimizations or diagnostics and be treated as if it can always
+    /// return `false`.
     fn predicate_must_hold_considering_regions(
         &self,
         obligation: &PredicateObligation<'tcx>,

tests/ui/traits/pointee-normalize-equate.rs

@@ -38,18 +38,4 @@ fn wrapper_project<T: ?Sized + Project>(ptr: *const T::Assoc) -> *const WrapperP
     cast_same_meta(ptr)
 }
 
-// In this example, `WrapperProject<&'a T>` is `Sized` modulo regions,
-// but `?Sized` considering regions.
-// Normalize `WrapperProject<&'a T>::Metadata` -> `<&'a T as Project>::Assoc::Metadata`
-// and don't require `WrapperProject<&'a T>: Sized`.
-fn sized_modulo_regions<'a, T: ?Sized + 'static>(
-    ptr: *const <&'a T as Project>::Assoc,
-) -> *const WrapperProject<&'a T>
-where
-    for<'b> &'b T: Project,
-    WrapperProject<&'static T>: Sized,
-{
-    cast_same_meta(ptr)
-}
-
 fn main() {}

tests/ui/traits/pointee-normalize-modulo-regions.rs

@@ -0,0 +1,34 @@
+#![feature(ptr_metadata)]
+
+use std::ptr::{self, Pointee};
+
+fn cast_same_meta<T: ?Sized, U: ?Sized>(ptr: *const T) -> *const U
+where
+    T: Pointee<Metadata = <U as Pointee>::Metadata>,
+{
+    let (thin, meta) = ptr.to_raw_parts();
+    ptr::from_raw_parts(thin, meta)
+}
+
+trait Project {
+    type Assoc: ?Sized;
+}
+
+struct WrapperProject<T: ?Sized + Project>(T::Assoc);
+
+// In this example, `WrapperProject<&'a T>` is `Sized` modulo regions, but `?Sized`
+// considering regions.
+// Because normalization does not consider regions, `WrapperProject<&'a T>::Metadata`
+// is normalized to `()` instead of `<&'a T as Project>::Assoc::Metadata` and requires
+// `WrapperProject<&'a T>: Sized` and therefore `'a: 'static`.
+fn sized_modulo_regions<'a, T: ?Sized + 'static>(
+    ptr: *const (),
+) -> *const WrapperProject<&'a T>
+where
+    for<'b> &'b T: Project,
+    WrapperProject<&'static T>: Sized,
+{
+    cast_same_meta(ptr) //~ ERROR lifetime may not live long enough
+}
+
+fn main() {}

tests/ui/traits/pointee-normalize-modulo-regions.stderr

@@ -0,0 +1,15 @@
+error: lifetime may not live long enough
+  --> $DIR/pointee-normalize-modulo-regions.rs:31:5
+   |
+LL | fn sized_modulo_regions<'a, T: ?Sized + 'static>(
+   |                         -- lifetime `'a` defined here
+...
+LL |     cast_same_meta(ptr)
+   |     ^^^^^^^^^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
+   |
+   = note: requirement occurs because of the type `WrapperProject<&T>`, which makes the generic argument `&T` invariant
+   = note: the struct `WrapperProject<T>` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: aborting due to 1 previous error
+