Report nicer lifetime errors for specialization

Add an obligation cause for these error so that the error points to the
implementations that caused the error.

Author: matthewjasper

compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs

@@ -80,7 +80,7 @@ use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
 use rustc_span::Span;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
-use rustc_trait_selection::traits::{self, translate_substs, wf, ObligationCtxt};
+use rustc_trait_selection::traits::{self, translate_substs_with_cause, wf, ObligationCtxt};
 
 pub(super) fn check_min_specialization(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
     if let Some(node) = parent_specialization_node(tcx, impl_def_id) {
@@ -180,8 +180,21 @@ fn get_impl_substs(
         ocx.assumed_wf_types(param_env, tcx.def_span(impl1_def_id), impl1_def_id);
 
     let impl1_substs = InternalSubsts::identity_for_item(tcx, impl1_def_id);
-    let impl2_substs =
-        translate_substs(infcx, param_env, impl1_def_id.to_def_id(), impl1_substs, impl2_node);
+    let impl1_span = tcx.def_span(impl1_def_id);
+    let impl2_substs = translate_substs_with_cause(
+        infcx,
+        param_env,
+        impl1_def_id.to_def_id(),
+        impl1_substs,
+        impl2_node,
+        |_, span| {
+            traits::ObligationCause::new(
+                impl1_span,
+                impl1_def_id,
+                traits::ObligationCauseCode::BindingObligation(impl2_node.def_id(), span),
+            )
+        },
+    );
 
     let errors = ocx.select_all_or_error();
     if !errors.is_empty() {

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -322,7 +322,9 @@ fn negative_impl(tcx: TyCtxt<'_>, impl1_def_id: DefId, impl2_def_id: DefId) -> b
     let selcx = &mut SelectionContext::new(&infcx);
     let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
     let (subject2, obligations) =
-        impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
+        impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs, |_, _| {
+            ObligationCause::dummy()
+        });
 
     !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
 }

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -54,7 +54,9 @@ pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
 pub use self::specialize::specialization_graph::FutureCompatOverlapError;
 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
-pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
+pub use self::specialize::{
+    specialization_graph, translate_substs, translate_substs_with_cause, OverlapError,
+};
 pub use self::structural_match::{
     search_for_adt_const_param_violation, search_for_structural_match_violation,
 };

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -82,6 +82,30 @@ pub fn translate_substs<'tcx>(
     source_impl: DefId,
     source_substs: SubstsRef<'tcx>,
     target_node: specialization_graph::Node,
+) -> SubstsRef<'tcx> {
+    translate_substs_with_cause(
+        infcx,
+        param_env,
+        source_impl,
+        source_substs,
+        target_node,
+        |_, _| ObligationCause::dummy(),
+    )
+}
+
+/// Like [translate_substs], but obligations from the parent implementation
+/// are registered with the provided `ObligationCause`.
+///
+/// This is for reporting *region* errors from those bounds. Type errors should
+/// not happen because the specialization graph already checks for those, and
+/// will result in an ICE.
+pub fn translate_substs_with_cause<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    source_impl: DefId,
+    source_substs: SubstsRef<'tcx>,
+    target_node: specialization_graph::Node,
+    cause: impl Fn(usize, Span) -> ObligationCause<'tcx>,
 ) -> SubstsRef<'tcx> {
     debug!(
         "translate_substs({:?}, {:?}, {:?}, {:?})",
@@ -99,7 +123,7 @@ pub fn translate_substs<'tcx>(
                 return source_substs;
             }
 
-            fulfill_implication(infcx, param_env, source_trait_ref, source_impl, target_impl)
+            fulfill_implication(infcx, param_env, source_trait_ref, source_impl, target_impl, cause)
                 .unwrap_or_else(|()| {
                     bug!(
                         "When translating substitutions from {source_impl:?} to {target_impl:?}, \
@@ -154,7 +178,10 @@ pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId,
     let infcx = tcx.infer_ctxt().build();
 
     // Attempt to prove that impl2 applies, given all of the above.
-    fulfill_implication(&infcx, penv, impl1_trait_ref, impl1_def_id, impl2_def_id).is_ok()
+    fulfill_implication(&infcx, penv, impl1_trait_ref, impl1_def_id, impl2_def_id, |_, _| {
+        ObligationCause::dummy()
+    })
+    .is_ok()
 }
 
 /// Attempt to fulfill all obligations of `target_impl` after unification with
@@ -168,6 +195,7 @@ fn fulfill_implication<'tcx>(
     source_trait_ref: ty::TraitRef<'tcx>,
     source_impl: DefId,
     target_impl: DefId,
+    error_cause: impl Fn(usize, Span) -> ObligationCause<'tcx>,
 ) -> Result<SubstsRef<'tcx>, ()> {
     debug!(
         "fulfill_implication({:?}, trait_ref={:?} |- {:?} applies)",
@@ -195,7 +223,7 @@ fn fulfill_implication<'tcx>(
     let selcx = &mut SelectionContext::new(&infcx);
     let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
     let (target_trait, obligations) =
-        util::impl_subject_and_oblig(selcx, param_env, target_impl, target_substs);
+        util::impl_subject_and_oblig(selcx, param_env, target_impl, target_substs, error_cause);
 
     // do the impls unify? If not, no specialization.
     let Ok(InferOk { obligations: more_obligations, .. }) =

compiler/rustc_trait_selection/src/traits/util.rs

@@ -197,6 +197,7 @@ pub fn impl_subject_and_oblig<'a, 'tcx>(
     param_env: ty::ParamEnv<'tcx>,
     impl_def_id: DefId,
     impl_substs: SubstsRef<'tcx>,
+    cause: impl Fn(usize, Span) -> ObligationCause<'tcx>,
 ) -> (ImplSubject<'tcx>, impl Iterator<Item = PredicateObligation<'tcx>>) {
     let subject = selcx.tcx().impl_subject(impl_def_id);
     let subject = subject.subst(selcx.tcx(), impl_substs);
@@ -208,8 +209,7 @@ pub fn impl_subject_and_oblig<'a, 'tcx>(
     let predicates = predicates.instantiate(selcx.tcx(), impl_substs);
     let InferOk { value: predicates, obligations: normalization_obligations2 } =
         selcx.infcx.at(&ObligationCause::dummy(), param_env).normalize(predicates);
-    let impl_obligations =
-        super::predicates_for_generics(|_, _| ObligationCause::dummy(), param_env, predicates);
+    let impl_obligations = super::predicates_for_generics(cause, param_env, predicates);
 
     let impl_obligations = impl_obligations
         .chain(normalization_obligations1.into_iter())

tests/ui/specialization/min_specialization/specialize_with_generalize_lifetimes.rs

@@ -0,0 +1,50 @@
+// Regression test for #79457.
+
+#![feature(min_specialization)]
+
+use std::any::Any;
+
+pub trait Tr {
+    fn method(self) -> Box<dyn Any + 'static>;
+    fn other(self);
+}
+
+impl<T: Any + 'static> Tr for T {
+    default fn method(self) -> Box<dyn Any + 'static> {
+        Box::new(self)
+    }
+
+    default fn other(self) {}
+}
+
+impl<'a> Tr for &'a i32 {
+    //~^ ERROR does not fulfill the required lifetime
+    fn other(self) {}
+}
+
+fn promote_to_static<'a>(i: &'a i32) -> &'static i32 {
+    *i.method().downcast().unwrap()
+}
+
+struct Wrapper<'a>(&'a i32);
+
+impl<'a> Tr for Wrapper<'a> {
+    //~^ ERROR does not fulfill the required lifetime
+    fn other(self) {}
+}
+
+fn promote_to_static_2<'a>(w: Wrapper<'a>) -> Wrapper<'static> {
+    *w.method().downcast().unwrap()
+}
+
+fn main() {
+    let i = Box::new(100_i32);
+    let static_i: &'static i32 = promote_to_static(&*i);
+    drop(i);
+    println!("{}", *static_i);
+
+    let j = Box::new(200_i32);
+    let static_w: Wrapper<'static> = promote_to_static_2(Wrapper(&*j));
+    drop(j);
+    println!("{}", *static_w.0);
+}

tests/ui/specialization/min_specialization/specialize_with_generalize_lifetimes.stderr

@@ -0,0 +1,27 @@
+error[E0477]: the type `&'a i32` does not fulfill the required lifetime
+  --> $DIR/specialize_with_generalize_lifetimes.rs:20:1
+   |
+LL | impl<'a> Tr for &'a i32 {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type must satisfy the static lifetime as required by this binding
+  --> $DIR/specialize_with_generalize_lifetimes.rs:12:15
+   |
+LL | impl<T: Any + 'static> Tr for T {
+   |               ^^^^^^^
+
+error[E0477]: the type `Wrapper<'a>` does not fulfill the required lifetime
+  --> $DIR/specialize_with_generalize_lifetimes.rs:31:1
+   |
+LL | impl<'a> Tr for Wrapper<'a> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type must satisfy the static lifetime as required by this binding
+  --> $DIR/specialize_with_generalize_lifetimes.rs:12:15
+   |
+LL | impl<T: Any + 'static> Tr for T {
+   |               ^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0477`.