check FnDef return type for WF

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -647,6 +647,8 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
     fn visit_ty(&mut self, t: <TyCtxt<'tcx> as ty::Interner>::Ty) -> Self::Result {
         debug!("wf bounds for t={:?} t.kind={:#?}", t, t.kind());
 
+        let tcx = self.tcx();
+
         match *t.kind() {
             ty::Bool
             | ty::Char
@@ -707,6 +709,16 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
             }
 
             ty::FnDef(did, args) => {
+                // HACK: Check the return type of function definitions for
+                // well-formedness to mostly fix #84533. This is still not
+                // perfect and there may be ways to abuse the fact that we
+                // ignore requirements with escaping bound vars. That's a
+                // more general issue however.
+                //
+                // FIXME(eddyb) add the type to `walker` instead of recursing.
+                let fn_sig = tcx.fn_sig(did).instantiate(tcx, args);
+                fn_sig.output().skip_binder().visit_with(self);
+                
                 let obligations = self.nominal_obligations(did, args);
                 self.out.extend(obligations);
             }
@@ -716,7 +728,7 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                 if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
                     let cause = self.cause(traits::ReferenceOutlivesReferent(t));
                     self.out.push(traits::Obligation::with_depth(
-                        self.tcx(),
+                        tcx,
                         cause,
                         self.recursion_depth,
                         self.param_env,
@@ -805,12 +817,12 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                 // obligations that don't refer to Self and
                 // checking those
 
-                let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
+                let defer_to_coercion = tcx.features().object_safe_for_dispatch;
 
                 if !defer_to_coercion {
                     if let Some(principal) = data.principal_def_id() {
                         self.out.push(traits::Obligation::with_depth(
-                            self.tcx(),
+                            tcx,
                             self.cause(traits::WellFormed(None)),
                             self.recursion_depth,
                             self.param_env,
@@ -835,7 +847,7 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
             ty::Infer(_) => {
                 let cause = self.cause(traits::WellFormed(None));
                 self.out.push(traits::Obligation::with_depth(
-                    self.tcx(),
+                    tcx,
                     cause,
                     self.recursion_depth,
                     self.param_env,
@@ -850,6 +862,8 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
     }
 
     fn visit_const(&mut self, c: <TyCtxt<'tcx> as ty::Interner>::Const) -> Self::Result {
+        let tcx = self.tcx();
+        
         match c.kind() {
             ty::ConstKind::Unevaluated(uv) => {
                 if !c.has_escaping_bound_vars() {
@@ -861,7 +875,7 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                     ));
                     let cause = self.cause(traits::WellFormed(None));
                     self.out.push(traits::Obligation::with_depth(
-                        self.tcx(),
+                        tcx,
                         cause,
                         self.recursion_depth,
                         self.param_env,
@@ -873,7 +887,7 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                 let cause = self.cause(traits::WellFormed(None));
 
                 self.out.push(traits::Obligation::with_depth(
-                    self.tcx(),
+                    tcx,
                     cause,
                     self.recursion_depth,
                     self.param_env,
@@ -895,7 +909,7 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                 ));
                 let cause = self.cause(traits::WellFormed(None));
                 self.out.push(traits::Obligation::with_depth(
-                    self.tcx(),
+                    tcx,
                     cause,
                     self.recursion_depth,
                     self.param_env,

tests/ui/fn/fn-item-lifetime-bounds.rs

@@ -1,37 +0,0 @@
-//@ check-pass
-//@ known-bug: #84533
-
-// Should fail. Lifetimes are checked correctly when `foo` is called, but NOT
-// when only the lifetime parameters are instantiated.
-
-use std::marker::PhantomData;
-
-#[allow(dead_code)]
-fn foo<'b, 'a>() -> PhantomData<&'b &'a ()> {
-    PhantomData
-}
-
-#[allow(dead_code)]
-#[allow(path_statements)]
-fn caller<'b, 'a>() {
-    foo::<'b, 'a>;
-}
-
-// In contrast to above, below code correctly does NOT compile.
-// fn caller<'b, 'a>() {
-//     foo::<'b, 'a>();
-// }
-
-// error: lifetime may not live long enough
-//   --> src/main.rs:22:5
-//   |
-// 21 | fn caller<'b, 'a>() {
-//   |           --  -- lifetime `'a` defined here
-//   |           |
-//   |           lifetime `'b` defined here
-// 22 |     foo::<'b, 'a>();
-//   |     ^^^^^^^^^^^^^^^ requires that `'a` must outlive `'b`
-//   |
-//   = help: consider adding the following bound: `'a: 'b`
-
-fn main() {}

tests/ui/wf/wf-fn-def-check-sig-1.rs

@@ -0,0 +1,44 @@
+// Regression test for #84533.
+
+use std::marker::PhantomData;
+
+fn foo<'b, 'a>() -> PhantomData<&'b &'a ()> {
+    PhantomData
+}
+
+fn extend_lifetime<'a, 'b, T: ?Sized>(x: &'a T) -> &'b T {
+    let f = foo::<'b, 'a>;
+    //~^ ERROR lifetime may not live long enough
+    f.baz(x)
+}
+
+trait Foo<'a, 'b, T: ?Sized> {
+    fn baz(self, s: &'a T) -> &'b T;
+}
+impl<'a, 'b, R, F, T: ?Sized> Foo<'a, 'b, T> for F
+where
+    F: Fn() -> R,
+    R: ProofForConversion<'a, 'b, T>,
+{
+    fn baz(self, s: &'a T) -> &'b T {
+        self().convert(s)
+    }
+}
+
+trait ProofForConversion<'a, 'b, T: ?Sized> {
+    fn convert(self, s: &'a T) -> &'b T;
+}
+impl<'a, 'b, T: ?Sized> ProofForConversion<'a, 'b, T> for PhantomData<&'b &'a ()> {
+    fn convert(self, s: &'a T) -> &'b T {
+        s
+    }
+}
+
+fn main() {
+    let d;
+    {
+        let x = "Hello World".to_string();
+        d = extend_lifetime(&x);
+    }
+    println!("{}", d);
+}

tests/ui/wf/wf-fn-def-check-sig-1.stderr

@@ -0,0 +1,14 @@
+error: lifetime may not live long enough
+  --> $DIR/wf-fn-def-check-sig-1.rs:10:13
+   |
+LL | fn extend_lifetime<'a, 'b, T: ?Sized>(x: &'a T) -> &'b T {
+   |                    --  -- lifetime `'b` defined here
+   |                    |
+   |                    lifetime `'a` defined here
+LL |     let f = foo::<'b, 'a>;
+   |             ^^^^^^^^^^^^^ requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+error: aborting due to previous error
+

tests/ui/wf/wf-fn-def-check-sig-2.rs

@@ -0,0 +1,44 @@
+// Regression test for #84533 involving higher-ranked regions
+// in the return type.
+use std::marker::PhantomData;
+
+fn foo<'c, 'b, 'a>(_: &'c ()) -> (&'c (), PhantomData<&'b &'a ()>) {
+    (&(), PhantomData)
+}
+
+fn extend_lifetime<'a, 'b, T: ?Sized>(x: &'a T) -> &'b T {
+    let f = foo;
+    f.baz(x)
+    //~^ ERROR lifetime may not live long enough
+}
+
+trait Foo<'a, 'b, T: ?Sized> {
+    fn baz(self, s: &'a T) -> &'b T;
+}
+impl<'a, 'b, R, F, T: ?Sized> Foo<'a, 'b, T> for F
+where
+    F: for<'c> Fn(&'c ()) -> (&'c (), R),
+    R: ProofForConversion<'a, 'b, T>,
+{
+    fn baz(self, s: &'a T) -> &'b T {
+        self(&()).1.convert(s)
+    }
+}
+
+trait ProofForConversion<'a, 'b, T: ?Sized> {
+    fn convert(self, s: &'a T) -> &'b T;
+}
+impl<'a, 'b, T: ?Sized> ProofForConversion<'a, 'b, T> for PhantomData<&'b &'a ()> {
+    fn convert(self, s: &'a T) -> &'b T {
+        s
+    }
+}
+
+fn main() {
+    let d;
+    {
+        let x = "Hello World".to_string();
+        d = extend_lifetime(&x);
+    }
+    println!("{}", d);
+}

tests/ui/wf/wf-fn-def-check-sig-2.stderr

@@ -0,0 +1,15 @@
+error: lifetime may not live long enough
+  --> $DIR/wf-fn-def-check-sig-2.rs:11:5
+   |
+LL | fn extend_lifetime<'a, 'b, T: ?Sized>(x: &'a T) -> &'b T {
+   |                    --  -- lifetime `'b` defined here
+   |                    |
+   |                    lifetime `'a` defined here
+LL |     let f = foo;
+LL |     f.baz(x)
+   |     ^^^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+error: aborting due to previous error
+