wf: ignore nested obligations with placeholders

Author: lcnr

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -177,9 +177,12 @@ pub fn predicate_obligations<'tcx>(
 /// `for<'a> where { T: 'a } fn(&'a T)`, at which point the `T: 'a` bound is
 /// trivially implied from the `param_env`.
 ///
-/// This means that we WF bounds aren't always sufficiently checked for now. 
+/// This means that we WF bounds aren't always sufficiently checked for now.
 fn emit_obligations_for<'tcx, T: TypeVisitable<'tcx>>(value: T) -> bool {
-    !value.has_escaping_bound_vars()
+    // We also don't emit obligations for placeholders. This will be incorrect
+    // if we ever decide to also replace `ty::Param` with placeholders during
+    // canonicalization or whatever. Hopefully we will have implications by then.
+    !value.has_escaping_bound_vars() && !value.has_placeholders()
 }
 
 struct WfPredicates<'tcx> {

src/test/ui/wf/higher-ranked/on-impl-not-implied.rs

@@ -0,0 +1,17 @@
+// Checks that manual WF bounds are not use for implied bounds.
+//
+// With the current implementation for WF check, this can easily cause
+// unsoundness, as wf does not emit any obligations containing
+// placeholders or bound variables.
+struct MyStruct<T, U>(T, U);
+
+trait Foo<'a, 'b> {}
+
+// IF THIS TEST STOPS EMITTING ERRORS, PLEASE NOTIFY T-TYPES TO CHECK WHETHER THE CHANGE IS SOUND.
+impl<'a, 'b> Foo<'a, 'b> for () //~ ERROR cannot infer an appropriate lifetime
+where
+    &'a &'b ():,
+    //~^ ERROR in type `&'a &'b ()`, reference has a longer lifetime than the data it references
+{}
+
+fn main() {}

src/test/ui/wf/higher-ranked/on-impl-not-implied.stderr

@@ -0,0 +1,45 @@
+error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'b` due to conflicting requirements
+  --> $DIR/on-impl-not-implied.rs:11:14
+   |
+LL | impl<'a, 'b> Foo<'a, 'b> for ()
+   |              ^^^^^^^^^^^
+   |
+note: first, the lifetime cannot outlive the lifetime `'b` as defined here...
+  --> $DIR/on-impl-not-implied.rs:11:10
+   |
+LL | impl<'a, 'b> Foo<'a, 'b> for ()
+   |          ^^
+note: ...but the lifetime must also be valid for the lifetime `'a` as defined here...
+  --> $DIR/on-impl-not-implied.rs:11:6
+   |
+LL | impl<'a, 'b> Foo<'a, 'b> for ()
+   |      ^^
+note: ...so that the types are compatible
+  --> $DIR/on-impl-not-implied.rs:11:14
+   |
+LL | impl<'a, 'b> Foo<'a, 'b> for ()
+   |              ^^^^^^^^^^^
+   = note: expected `Foo<'a, 'b>`
+              found `Foo<'_, '_>`
+
+error[E0491]: in type `&'a &'b ()`, reference has a longer lifetime than the data it references
+  --> $DIR/on-impl-not-implied.rs:13:5
+   |
+LL |     &'a &'b ():,
+   |     ^^^^^^^^^^
+   |
+note: the pointer is valid for the lifetime `'a` as defined here
+  --> $DIR/on-impl-not-implied.rs:11:6
+   |
+LL | impl<'a, 'b> Foo<'a, 'b> for ()
+   |      ^^
+note: but the referenced data is only valid for the lifetime `'b` as defined here
+  --> $DIR/on-impl-not-implied.rs:11:10
+   |
+LL | impl<'a, 'b> Foo<'a, 'b> for ()
+   |          ^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0491, E0495.
+For more information about an error, try `rustc --explain E0491`.

src/test/ui/wf/higher-ranked/on-impl.rs

@@ -0,0 +1,21 @@
+// check-pass
+trait Foo<A> {}
+impl<'a, T: Foo<A>, A> Foo<A> for &'a mut T
+where
+    // Needed to use `(A,)` because `A:` by itself doesn't emit a WF bound
+    // as of writing this comment.
+    //
+    // This happens in `fn explicit_predicates_of`.
+    (A,):,
+{}
+
+fn tragic<T, F: for<'a> Foo<&'a T>>(_: F) {}
+fn oh_no<T, F: for<'a> Foo<&'a T>>(mut f: F) {
+    // This results in a `for<'a> WF(&'a T)` bound where `'a` is replaced
+    // with a placeholder before we compute the wf requirements.
+    //
+    // This bound would otherwise result in a `T: 'static` bound.
+    tragic::<T, _>(&mut f);
+}
+
+fn main() {}