Auto merge of #105285 - compiler-errors:conflicting-param-env-2, r=estebank

Highlight conflicting param-env candidates, again

Un-reverts #98794 (i.e. reverts #99290).

The previous time I attempted to land this PR, it was because of an incremental issue (#99233). The repro instructions in the issue is no longer manifest the ICE -- I think it's because this ambiguity code was refactored (I think by `@lcnr)` to no longer store the ambiguities in the fulfillment error, but instead recompute them on the fly.

The main motivation for trying to re-land this is that it fixes #105131 by highlighting the root-cause of the issue, which is conflicting param-env candidates:

```
error[E0283]: type annotations needed: cannot satisfy `Self: Gen<'source>`
   |
note: multiple `impl`s or `where` clauses satisfying `Self: Gen<'source>` found
  --> $DIR/conflicting-bounds.rs:3:1
   |
LL | pub trait Gen<'source> {
   | ^^^^^^^^^^^^^^^^^^^^^^
...
LL |         Self: for<'s> Gen<'s, Output = T>;
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.
```

Fixes #105131.
Fixes (again) #98786

Author: bors

compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs

@@ -1,52 +1,101 @@
 use rustc_hir::def_id::DefId;
-use rustc_infer::infer::InferCtxt;
+use rustc_infer::infer::{InferCtxt, LateBoundRegionConversionTime};
+use rustc_infer::traits::util::elaborate_predicates_with_span;
 use rustc_infer::traits::{Obligation, ObligationCause, TraitObligation};
-use rustc_span::DUMMY_SP;
+use rustc_middle::ty;
+use rustc_span::{Span, DUMMY_SP};
 
 use crate::traits::ObligationCtxt;
 
+pub enum Ambiguity {
+    DefId(DefId),
+    ParamEnv(Span),
+}
+
 pub fn recompute_applicable_impls<'tcx>(
     infcx: &InferCtxt<'tcx>,
     obligation: &TraitObligation<'tcx>,
-) -> Vec<DefId> {
+) -> Vec<Ambiguity> {
     let tcx = infcx.tcx;
     let param_env = obligation.param_env;
-    let dummy_cause = ObligationCause::dummy();
+
     let impl_may_apply = |impl_def_id| {
         let ocx = ObligationCtxt::new_in_snapshot(infcx);
         let placeholder_obligation =
             infcx.replace_bound_vars_with_placeholders(obligation.predicate);
         let obligation_trait_ref =
-            ocx.normalize(&dummy_cause, param_env, placeholder_obligation.trait_ref);
+            ocx.normalize(&ObligationCause::dummy(), param_env, placeholder_obligation.trait_ref);
 
         let impl_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
         let impl_trait_ref = tcx.bound_impl_trait_ref(impl_def_id).unwrap().subst(tcx, impl_substs);
         let impl_trait_ref = ocx.normalize(&ObligationCause::dummy(), param_env, impl_trait_ref);
 
-        if let Err(_) = ocx.eq(&dummy_cause, param_env, obligation_trait_ref, impl_trait_ref) {
+        if let Err(_) =
+            ocx.eq(&ObligationCause::dummy(), param_env, obligation_trait_ref, impl_trait_ref)
+        {
             return false;
         }
 
         let impl_predicates = tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs);
-        ocx.register_obligations(
-            impl_predicates
-                .predicates
-                .iter()
-                .map(|&predicate| Obligation::new(tcx, dummy_cause.clone(), param_env, predicate)),
+        ocx.register_obligations(impl_predicates.predicates.iter().map(|&predicate| {
+            Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
+        }));
+
+        ocx.select_where_possible().is_empty()
+    };
+
+    let param_env_candidate_may_apply = |poly_trait_predicate: ty::PolyTraitPredicate<'tcx>| {
+        let ocx = ObligationCtxt::new_in_snapshot(infcx);
+        let placeholder_obligation =
+            infcx.replace_bound_vars_with_placeholders(obligation.predicate);
+        let obligation_trait_ref =
+            ocx.normalize(&ObligationCause::dummy(), param_env, placeholder_obligation.trait_ref);
+
+        let param_env_predicate = infcx.replace_bound_vars_with_fresh_vars(
+            DUMMY_SP,
+            LateBoundRegionConversionTime::HigherRankedType,
+            poly_trait_predicate,
         );
+        let param_env_trait_ref =
+            ocx.normalize(&ObligationCause::dummy(), param_env, param_env_predicate.trait_ref);
+
+        if let Err(_) =
+            ocx.eq(&ObligationCause::dummy(), param_env, obligation_trait_ref, param_env_trait_ref)
+        {
+            return false;
+        }
 
         ocx.select_where_possible().is_empty()
     };
 
-    let mut impls = Vec::new();
+    let mut ambiguities = Vec::new();
+
     tcx.for_each_relevant_impl(
         obligation.predicate.def_id(),
         obligation.predicate.skip_binder().trait_ref.self_ty(),
         |impl_def_id| {
-            if infcx.probe(move |_snapshot| impl_may_apply(impl_def_id)) {
-                impls.push(impl_def_id)
+            if infcx.probe(|_| impl_may_apply(impl_def_id)) {
+                ambiguities.push(Ambiguity::DefId(impl_def_id))
             }
         },
     );
-    impls
+
+    let predicates =
+        tcx.predicates_of(obligation.cause.body_id.owner.to_def_id()).instantiate_identity(tcx);
+    for obligation in
+        elaborate_predicates_with_span(tcx, std::iter::zip(predicates.predicates, predicates.spans))
+    {
+        let kind = obligation.predicate.kind();
+        if let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = kind.skip_binder()
+            && param_env_candidate_may_apply(kind.rebind(trait_pred))
+        {
+            if kind.rebind(trait_pred.trait_ref) == ty::TraitRef::identity(tcx, trait_pred.def_id()) {
+                ambiguities.push(Ambiguity::ParamEnv(tcx.def_span(trait_pred.def_id())))
+            } else {
+                ambiguities.push(Ambiguity::ParamEnv(obligation.cause.span))
+            }
+        }
+    }
+
+    ambiguities
 }

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

@@ -1487,7 +1487,7 @@ trait InferCtxtPrivExt<'tcx> {
     fn annotate_source_of_ambiguity(
         &self,
         err: &mut Diagnostic,
-        impls: &[DefId],
+        impls: &[ambiguity::Ambiguity],
         predicate: ty::Predicate<'tcx>,
     );
 
@@ -2180,13 +2180,22 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 let mut selcx = SelectionContext::new(&self);
                 match selcx.select_from_obligation(&obligation) {
                     Ok(None) => {
-                        let impls = ambiguity::recompute_applicable_impls(self.infcx, &obligation);
+                        let ambiguities =
+                            ambiguity::recompute_applicable_impls(self.infcx, &obligation);
                         let has_non_region_infer =
                             trait_ref.skip_binder().substs.types().any(|t| !t.is_ty_infer());
                         // It doesn't make sense to talk about applicable impls if there are more
                         // than a handful of them.
-                        if impls.len() > 1 && impls.len() < 10 && has_non_region_infer {
-                            self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
+                        if ambiguities.len() > 1 && ambiguities.len() < 10 && has_non_region_infer {
+                            if self.tainted_by_errors().is_some() && subst.is_none() {
+                                // If `subst.is_none()`, then this is probably two param-env
+                                // candidates or impl candidates that are equal modulo lifetimes.
+                                // Therefore, if we've already emitted an error, just skip this
+                                // one, since it's not particularly actionable.
+                                err.cancel();
+                                return;
+                            }
+                            self.annotate_source_of_ambiguity(&mut err, &ambiguities, predicate);
                         } else {
                             if self.tainted_by_errors().is_some() {
                                 err.cancel();
@@ -2434,21 +2443,30 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     fn annotate_source_of_ambiguity(
         &self,
         err: &mut Diagnostic,
-        impls: &[DefId],
+        ambiguities: &[ambiguity::Ambiguity],
         predicate: ty::Predicate<'tcx>,
     ) {
         let mut spans = vec![];
         let mut crates = vec![];
         let mut post = vec![];
-        for def_id in impls {
-            match self.tcx.span_of_impl(*def_id) {
-                Ok(span) => spans.push(span),
-                Err(name) => {
-                    crates.push(name);
-                    if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
-                        post.push(header);
+        let mut has_param_env = false;
+        for ambiguity in ambiguities {
+            match ambiguity {
+                ambiguity::Ambiguity::DefId(impl_def_id) => {
+                    match self.tcx.span_of_impl(*impl_def_id) {
+                        Ok(span) => spans.push(span),
+                        Err(name) => {
+                            crates.push(name);
+                            if let Some(header) = to_pretty_impl_header(self.tcx, *impl_def_id) {
+                                post.push(header);
+                            }
+                        }
                     }
                 }
+                ambiguity::Ambiguity::ParamEnv(span) => {
+                    has_param_env = true;
+                    spans.push(*span);
+                }
             }
         }
         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
@@ -2472,7 +2490,11 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             return;
         }
 
-        let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
+        let msg = format!(
+            "multiple `impl`s{} satisfying `{}` found",
+            if has_param_env { " or `where` clauses" } else { "" },
+            predicate
+        );
         let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
         } else if post.len() == 1 {

src/test/ui/const-generics/generic_const_exprs/issue-72787.min.stderr

@@ -40,17 +40,35 @@ error[E0283]: type annotations needed: cannot satisfy `IsLessOrEqual<I, 8>: True
 LL |     IsLessOrEqual<I, 8>: True,
    |                          ^^^^
    |
-   = note: cannot satisfy `IsLessOrEqual<I, 8>: True`
-   = help: the trait `True` is implemented for `IsLessOrEqual<LHS, RHS>`
+note: multiple `impl`s or `where` clauses satisfying `IsLessOrEqual<I, 8>: True` found
+  --> $DIR/issue-72787.rs:10:1
+   |
+LL | impl<const LHS: u32, const RHS: u32> True for IsLessOrEqual<LHS, RHS> where
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     IsLessOrEqual<I, 8>: True,
+   |                          ^^^^
+...
+LL |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
+   |                                          ^^^^
 
 error[E0283]: type annotations needed: cannot satisfy `IsLessOrEqual<I, 8>: True`
   --> $DIR/issue-72787.rs:21:26
    |
 LL |     IsLessOrEqual<I, 8>: True,
    |                          ^^^^
    |
-   = note: cannot satisfy `IsLessOrEqual<I, 8>: True`
-   = help: the trait `True` is implemented for `IsLessOrEqual<LHS, RHS>`
+note: multiple `impl`s or `where` clauses satisfying `IsLessOrEqual<I, 8>: True` found
+  --> $DIR/issue-72787.rs:10:1
+   |
+LL | impl<const LHS: u32, const RHS: u32> True for IsLessOrEqual<LHS, RHS> where
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     IsLessOrEqual<I, 8>: True,
+   |                          ^^^^
+...
+LL |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
+   |                                          ^^^^
 
 error: aborting due to 6 previous errors
 

src/test/ui/issues/issue-21974.stderr

@@ -4,7 +4,13 @@ error[E0283]: type annotations needed: cannot satisfy `&'a T: Foo`
 LL |     where &'a T : Foo,
    |                   ^^^
    |
-   = note: cannot satisfy `&'a T: Foo`
+note: multiple `impl`s or `where` clauses satisfying `&'a T: Foo` found
+  --> $DIR/issue-21974.rs:11:19
+   |
+LL |     where &'a T : Foo,
+   |                   ^^^
+LL |           &'b T : Foo
+   |                   ^^^
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-24424.stderr

@@ -4,7 +4,11 @@ error[E0283]: type annotations needed: cannot satisfy `T0: Trait0<'l0>`
 LL | impl <'l0, 'l1, T0> Trait1<'l0, T0> for bool where T0 : Trait0<'l0>, T0 : Trait0<'l1> {}
    |                                                         ^^^^^^^^^^^
    |
-   = note: cannot satisfy `T0: Trait0<'l0>`
+note: multiple `impl`s or `where` clauses satisfying `T0: Trait0<'l0>` found
+  --> $DIR/issue-24424.rs:4:57
+   |
+LL | impl <'l0, 'l1, T0> Trait1<'l0, T0> for bool where T0 : Trait0<'l0>, T0 : Trait0<'l1> {}
+   |                                                         ^^^^^^^^^^^       ^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/lifetimes/conflicting-bounds.rs

@@ -0,0 +1,11 @@
+//~ type annotations needed: cannot satisfy `Self: Gen<'source>`
+
+pub trait Gen<'source> {
+    type Output;
+
+    fn gen<T>(&self) -> T
+    where
+        Self: for<'s> Gen<'s, Output = T>;
+}
+
+fn main() {}

src/test/ui/lifetimes/conflicting-bounds.stderr

@@ -0,0 +1,14 @@
+error[E0283]: type annotations needed: cannot satisfy `Self: Gen<'source>`
+   |
+note: multiple `impl`s or `where` clauses satisfying `Self: Gen<'source>` found
+  --> $DIR/conflicting-bounds.rs:3:1
+   |
+LL | pub trait Gen<'source> {
+   | ^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |         Self: for<'s> Gen<'s, Output = T>;
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0283`.

src/test/ui/lifetimes/issue-34979.stderr

@@ -4,8 +4,16 @@ error[E0283]: type annotations needed: cannot satisfy `&'a (): Foo`
 LL |     &'a (): Foo,
    |             ^^^
    |
-   = note: cannot satisfy `&'a (): Foo`
-   = help: the trait `Foo` is implemented for `&'a T`
+note: multiple `impl`s or `where` clauses satisfying `&'a (): Foo` found
+  --> $DIR/issue-34979.rs:2:1
+   |
+LL | impl<'a, T> Foo for &'a T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     &'a (): Foo,
+   |             ^^^
+LL |     &'static (): Foo;
+   |                  ^^^
 
 error: aborting due to previous error
 

src/test/ui/traits/issue-85735.stderr

@@ -4,10 +4,14 @@ error[E0283]: type annotations needed: cannot satisfy `T: FnMut<(&'a (),)>`
 LL |     T: FnMut(&'a ()),
    |        ^^^^^^^^^^^^^
    |
-   = note: cannot satisfy `T: FnMut<(&'a (),)>`
-   = help: the following types implement trait `FnMut<Args>`:
-             &F
-             &mut F
+note: multiple `impl`s or `where` clauses satisfying `T: FnMut<(&'a (),)>` found
+  --> $DIR/issue-85735.rs:7:8
+   |
+LL |     T: FnMut(&'a ()),
+   |        ^^^^^^^^^^^^^
+LL |
+LL |     T: FnMut(&'b ()),
+   |        ^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/type/type-check/issue-40294.stderr

@@ -4,7 +4,13 @@ error[E0283]: type annotations needed: cannot satisfy `&'a T: Foo`
 LL |     where &'a T : Foo,
    |                   ^^^
    |
-   = note: cannot satisfy `&'a T: Foo`
+note: multiple `impl`s or `where` clauses satisfying `&'a T: Foo` found
+  --> $DIR/issue-40294.rs:6:19
+   |
+LL |     where &'a T : Foo,
+   |                   ^^^
+LL |           &'b T : Foo
+   |                   ^^^
 
 error: aborting due to previous error