Rollup merge of #111252 - matthewjasper:min-spec-improvements, r=compiler-errors

Min specialization improvements

- Don't allow specialization impls with no items, such implementations are probably not correct and only occur as mistakes in the compiler and standard library
- Fix a missing normalization call
- Adds spans for lifetime errors from overly general specializations

Closes #79457
Closes #109815

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

@@ -55,7 +55,9 @@ 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({:?}, {:?}, {:?}, {:?})",
@@ -99,14 +123,13 @@ pub fn translate_substs<'tcx>(
                 return source_substs;
             }
 
-            fulfill_implication(infcx, param_env, source_trait_ref, target_impl).unwrap_or_else(
-                |()| {
+            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:?}, \
                         the expected specialization failed to hold"
                     )
-                },
-            )
+                })
         }
         specialization_graph::Node::Trait(..) => source_trait_ref.substs,
     };
@@ -153,20 +176,12 @@ pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId,
 
     // Create an infcx, taking the predicates of impl1 as assumptions:
     let infcx = tcx.infer_ctxt().build();
-    let impl1_trait_ref =
-        match traits::fully_normalize(&infcx, ObligationCause::dummy(), penv, impl1_trait_ref) {
-            Ok(impl1_trait_ref) => impl1_trait_ref,
-            Err(_errors) => {
-                tcx.sess.delay_span_bug(
-                    tcx.def_span(impl1_def_id),
-                    format!("failed to fully normalize {impl1_trait_ref}"),
-                );
-                impl1_trait_ref
-            }
-        };
 
     // Attempt to prove that impl2 applies, given all of the above.
-    fulfill_implication(&infcx, penv, impl1_trait_ref, 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
@@ -178,23 +193,41 @@ fn fulfill_implication<'tcx>(
     infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'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)",
         param_env, source_trait_ref, target_impl
     );
 
+    let source_trait_ref = match traits::fully_normalize(
+        &infcx,
+        ObligationCause::dummy(),
+        param_env,
+        source_trait_ref,
+    ) {
+        Ok(source_trait_ref) => source_trait_ref,
+        Err(_errors) => {
+            infcx.tcx.sess.delay_span_bug(
+                infcx.tcx.def_span(source_impl),
+                format!("failed to fully normalize {source_trait_ref}"),
+            );
+            source_trait_ref
+        }
+    };
+
     let source_trait = ImplSubject::Trait(source_trait_ref);
 
     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, .. }) =
-        infcx.at(&ObligationCause::dummy(), param_env, ).eq(DefineOpaqueTypes::No,source_trait, target_trait)
+        infcx.at(&ObligationCause::dummy(), param_env).eq(DefineOpaqueTypes::No, source_trait, target_trait)
     else {
         debug!(
             "fulfill_implication: {:?} does not unify with {:?}",

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())