Auto merge of #95519 - oli-obk:tait_ub2, r=compiler-errors

Enforce well formedness for type alias impl trait's hidden type

fixes #84657

This was not an issue with return-position-impl-trait because the generic bounds of the function are the same as those of the opaque type, and the hidden type must already be well formed within the function.

With type-alias-impl-trait the hidden type could be defined in a function that has *more* lifetime bounds than the type alias. This is fine, but the hidden type must still be well formed without those additional bounds.

compiler/rustc_data_structures/src/vec_map.rs

@@ -144,7 +144,7 @@ fn into_iter(self) -> Self::IntoIter {
     }
 }
 
-impl<'a, K, V> IntoIterator for &'a mut VecMap<K, V> {
+impl<'a, K: 'a, V: 'a> IntoIterator for &'a mut VecMap<K, V> {
     type Item = (&'a K, &'a mut V);
     type IntoIter = impl Iterator<Item = Self::Item>;
 

compiler/rustc_typeck/src/check/check.rs

@@ -12,12 +12,13 @@
 use rustc_hir::{def::Res, ItemKind, Node, PathSegment};
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{RegionVariableOrigin, TyCtxtInferExt};
+use rustc_infer::traits::Obligation;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
-use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt};
+use rustc_middle::ty::{self, ParamEnv, ToPredicate, Ty, TyCtxt};
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
 use rustc_span::symbol::sym;
 use rustc_span::{self, Span};
@@ -674,6 +675,13 @@ fn check_opaque_meets_bounds<'tcx>(
             }
         }
 
+        // Additionally require the hidden type to be well-formed with only the generics of the opaque type.
+        // Defining use functions may have more bounds than the opaque type, which is ok, as long as the
+        // hidden type is well formed even without those bounds.
+        let predicate =
+            ty::Binder::dummy(ty::PredicateKind::WellFormed(hidden_type.into())).to_predicate(tcx);
+        inh.register_predicate(Obligation::new(misc_cause, param_env, predicate));
+
         // Check that all obligations are satisfied by the implementation's
         // version.
         let errors = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx);

compiler/rustc_typeck/src/check/regionck.rs

@@ -171,8 +171,8 @@ pub fn regionck_expr(&self, body: &'tcx hir::Body<'tcx>) {
 
     /// Region checking during the WF phase for items. `wf_tys` are the
     /// types from which we should derive implied bounds, if any.
+    #[instrument(level = "debug", skip(self))]
     pub fn regionck_item(&self, item_id: hir::HirId, span: Span, wf_tys: FxHashSet<Ty<'tcx>>) {
-        debug!("regionck_item(item.id={:?}, wf_tys={:?})", item_id, wf_tys);
         let subject = self.tcx.hir().local_def_id(item_id);
         let mut rcx = RegionCtxt::new(self, item_id, Subject(subject), self.param_env);
         rcx.outlives_environment.add_implied_bounds(self, wf_tys, item_id, span);

src/test/ui/type-alias-impl-trait/underconstrained_generic.rs

+#![feature(type_alias_impl_trait)]
+
+use std::marker::PhantomData;
+
+trait Trait {
+    fn foo<T, U>(t: T) -> U;
+}
+
+trait ProofForConversion<X> {
+    fn convert<T, U>(_: PhantomData<Self>, r: T) -> U;
+}
+
+impl<X: Trait> ProofForConversion<X> for () {
+    fn convert<T, U>(_: PhantomData<Self>, r: T) -> U {
+        X::foo(r)
+    }
+}
+
+type Converter<T> = impl ProofForConversion<T>;
+//~^ ERROR the trait bound `T: Trait` is not satisfied
+
+fn _defining_use<T: Trait>() -> Converter<T> {
+    ()
+}
+
+
+fn main() {
+}

src/test/ui/type-alias-impl-trait/underconstrained_generic.stderr

+error[E0277]: the trait bound `T: Trait` is not satisfied
+  --> $DIR/underconstrained_generic.rs:19:21
+   |
+LL | type Converter<T> = impl ProofForConversion<T>;
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `T`
+   |
+note: required because of the requirements on the impl of `ProofForConversion<T>` for `()`
+  --> $DIR/underconstrained_generic.rs:13:16
+   |
+LL | impl<X: Trait> ProofForConversion<X> for () {
+   |                ^^^^^^^^^^^^^^^^^^^^^     ^^
+help: consider restricting type parameter `T`
+   |
+LL | type Converter<T: Trait> = impl ProofForConversion<T>;
+   |                 +++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/type-alias-impl-trait/underconstrained_lifetime.rs

+#![feature(type_alias_impl_trait)]
+
+use std::marker::PhantomData;
+
+trait ProofForConversion<'a, 'b> {
+    fn convert<T: ?Sized>(_: PhantomData<Self>, r: &'a T) -> &'b T;
+}
+
+impl<'a, 'b> ProofForConversion<'a, 'b> for &'b &'a () {
+    fn convert<T: ?Sized>(_: PhantomData<Self>, r: &'a T) -> &'b T {
+        r
+    }
+}
+
+type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
+//~^ ERROR reference has a longer lifetime than the data it references
+
+// Even _defining_use with an explicit `'a: 'b` compiles fine, too.
+fn _defining_use<'a, 'b>(x: &'b &'a ()) -> Converter<'a, 'b> {
+    x
+}
+
+fn extend_lifetime<'a, 'b, T: ?Sized>(x: &'a T) -> &'b T {
+    Converter::<'a, 'b>::convert(PhantomData, x)
+}
+
+fn main() {
+    let d;
+    {
+        let x = "Hello World".to_string();
+        d = extend_lifetime(&x);
+    }
+    println!("{}", d);
+}

src/test/ui/type-alias-impl-trait/underconstrained_lifetime.stderr

+error[E0491]: in type `&'b &'a ()`, reference has a longer lifetime than the data it references
+  --> $DIR/underconstrained_lifetime.rs:15:26
+   |
+LL | type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the pointer is valid for the lifetime `'b` as defined here
+  --> $DIR/underconstrained_lifetime.rs:15:20
+   |
+LL | type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
+   |                    ^^
+note: but the referenced data is only valid for the lifetime `'a` as defined here
+  --> $DIR/underconstrained_lifetime.rs:15:16
+   |
+LL | type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
+   |                ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0491`.