Take into account negative impls in "trait item not found" suggestions

compiler/rustc_typeck/src/check/method/suggest.rs

@@ -12,6 +12,7 @@ use rustc_hir::lang_items::LangItem;
 use rustc_hir::{ExprKind, Node, QPath};
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::hir::map as hir_map;
+use rustc_middle::ty::fast_reject::simplify_type;
 use rustc_middle::ty::print::with_crate_prefix;
 use rustc_middle::ty::{
     self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
@@ -1074,19 +1075,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             } else {
                 "items from traits can only be used if the trait is implemented and in scope"
             });
+            let candidates_len = candidates.len();
             let message = |action| {
                 format!(
                     "the following {traits_define} an item `{name}`, perhaps you need to {action} \
                      {one_of_them}:",
                     traits_define =
-                        if candidates.len() == 1 { "trait defines" } else { "traits define" },
+                        if candidates_len == 1 { "trait defines" } else { "traits define" },
                     action = action,
-                    one_of_them = if candidates.len() == 1 { "it" } else { "one of them" },
+                    one_of_them = if candidates_len == 1 { "it" } else { "one of them" },
                     name = item_name,
                 )
             };
             // Obtain the span for `param` and use it for a structured suggestion.
-            let mut suggested = false;
             if let (Some(ref param), Some(ref table)) =
                 (param_type, self.in_progress_typeck_results)
             {
@@ -1147,7 +1148,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                     Applicability::MaybeIncorrect,
                                 );
                             }
-                            suggested = true;
+                            return;
                         }
                         Node::Item(hir::Item {
                             kind: hir::ItemKind::Trait(.., bounds, _),
@@ -1167,45 +1168,96 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 }),
                                 Applicability::MaybeIncorrect,
                             );
-                            suggested = true;
+                            return;
                         }
                         _ => {}
                     }
                 }
             }
 
-            if !suggested {
-                let action = if let Some(param) = param_type {
-                    format!("restrict type parameter `{}` with", param)
-                } else {
-                    // FIXME: it might only need to be imported into scope, not implemented.
-                    "implement".to_string()
-                };
-                let mut use_note = true;
-                if let [trait_info] = &candidates[..] {
-                    if let Some(span) = self.tcx.hir().span_if_local(trait_info.def_id) {
-                        err.span_note(
-                            self.tcx.sess.source_map().guess_head_span(span),
-                            &format!(
-                                "`{}` defines an item `{}`, perhaps you need to {} it",
-                                self.tcx.def_path_str(trait_info.def_id),
-                                item_name,
-                                action
-                            ),
-                        );
-                        use_note = false
+            let (potential_candidates, explicitly_negative) = if param_type.is_some() {
+                // FIXME: Even though negative bounds are not implemented, we could maybe handle
+                // cases where a positive bound implies a negative impl.
+                (candidates, Vec::new())
+            } else if let Some(simp_rcvr_ty) = simplify_type(self.tcx, rcvr_ty, true) {
+                let mut potential_candidates = Vec::new();
+                let mut explicitly_negative = Vec::new();
+                for candidate in candidates {
+                    // Check if there's a negative impl of `candidate` for `rcvr_ty`
+                    if self
+                        .tcx
+                        .all_impls(candidate.def_id)
+                        .filter(|imp_did| {
+                            self.tcx.impl_polarity(*imp_did) == ty::ImplPolarity::Negative
+                        })
+                        .any(|imp_did| {
+                            let imp = self.tcx.impl_trait_ref(imp_did).unwrap();
+                            let imp_simp = simplify_type(self.tcx, imp.self_ty(), true);
+                            imp_simp.map(|s| s == simp_rcvr_ty).unwrap_or(false)
+                        })
+                    {
+                        explicitly_negative.push(candidate);
+                    } else {
+                        potential_candidates.push(candidate);
                     }
                 }
-                if use_note {
+                (potential_candidates, explicitly_negative)
+            } else {
+                // We don't know enough about `recv_ty` to make proper suggestions.
+                (candidates, Vec::new())
+            };
+
+            let action = if let Some(param) = param_type {
+                format!("restrict type parameter `{}` with", param)
+            } else {
+                // FIXME: it might only need to be imported into scope, not implemented.
+                "implement".to_string()
+            };
+            match &potential_candidates[..] {
+                [] => {}
+                [trait_info] if trait_info.def_id.is_local() => {
+                    let span = self.tcx.hir().span_if_local(trait_info.def_id).unwrap();
+                    err.span_note(
+                        self.tcx.sess.source_map().guess_head_span(span),
+                        &format!(
+                            "`{}` defines an item `{}`, perhaps you need to {} it",
+                            self.tcx.def_path_str(trait_info.def_id),
+                            item_name,
+                            action
+                        ),
+                    );
+                }
+                trait_infos => {
                     let mut msg = message(action);
-                    for (i, trait_info) in candidates.iter().enumerate() {
+                    for (i, trait_info) in trait_infos.iter().enumerate() {
                         msg.push_str(&format!(
                             "\ncandidate #{}: `{}`",
                             i + 1,
                             self.tcx.def_path_str(trait_info.def_id),
                         ));
                     }
-                    err.note(&msg[..]);
+                    err.note(&msg);
+                }
+            }
+            match &explicitly_negative[..] {
+                [] => {}
+                [trait_info] => {
+                    let msg = format!(
+                        "the trait `{}` defines an item `{}`, but is explicitely unimplemented",
+                        self.tcx.def_path_str(trait_info.def_id),
+                        item_name
+                    );
+                    err.note(&msg);
+                }
+                trait_infos => {
+                    let mut msg = format!(
+                        "the following traits define an item `{}`, but are explicitely unimplemented:",
+                        item_name
+                    );
+                    for trait_info in trait_infos {
+                        msg.push_str(&format!("\n{}", self.tcx.def_path_str(trait_info.def_id)));
+                    }
+                    err.note(&msg);
                 }
             }
         }

src/test/ui/traits/negative-impls/explicitly-unimplemented-error-message.rs

@@ -0,0 +1,53 @@
+// This tests issue #79683: note in the error message that the trait is
+// explicitely unimplemented instead of suggesting to implement it.
+
+#![feature(negative_impls)]
+
+struct Qux;
+//~^ NOTE method `clone` not found for this
+//~^^ NOTE method `foo` not found for this
+
+impl !Clone for Qux {}
+
+trait Bar {
+    fn bar(&self);
+}
+
+impl !Bar for u32 {}
+
+trait Foo {
+    fn foo(&self);
+}
+//~^^^ NOTE `Foo` defines an item `foo`, perhaps you need to implement it
+
+trait FooBar {
+    fn foo(&self);
+}
+
+impl !Foo for Qux {}
+
+impl !FooBar for Qux {}
+
+impl !FooBar for u32 {}
+
+fn main() {
+    Qux.clone();
+    //~^ ERROR no method named `clone` found for struct `Qux`
+    //~| NOTE method not found in `Qux`
+    //~| NOTE `Clone` defines an item `clone`, but is explicitely unimplemented
+
+    0_u32.bar();
+    //~^ ERROR no method named `bar` found for type `u32`
+    //~| NOTE method not found in `u32`
+    //~| NOTE `Bar` defines an item `bar`, but is explicitely unimplemented
+
+    Qux.foo();
+    //~^ ERROR no method named `foo` found for struct `Qux`
+    //~| NOTE method not found in `Qux`
+    //~| NOTE the following traits define an item `foo`, but are explicitely unimplemented
+
+    0_u32.foo();
+    //~^ ERROR no method named `foo` found for type `u32`
+    //~| NOTE method not found in `u32`
+    //~| NOTE `FooBar` defines an item `foo`, but is explicitely unimplemented
+}

src/test/ui/traits/negative-impls/explicitly-unimplemented-error-message.stderr

@@ -0,0 +1,60 @@
+error[E0599]: no method named `clone` found for struct `Qux` in the current scope
+  --> $DIR/explicitly-unimplemented-error-message.rs:34:9
+   |
+LL | struct Qux;
+   | ----------- method `clone` not found for this
+...
+LL |     Qux.clone();
+   |         ^^^^^ method not found in `Qux`
+   | 
+  ::: $SRC_DIR/core/src/clone.rs:LL:COL
+   |
+LL |     fn clone(&self) -> Self;
+   |        -----
+   |        |
+   |        the method is available for `Arc<Qux>` here
+   |        the method is available for `Rc<Qux>` here
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the trait `Clone` defines an item `clone`, but is explicitely unimplemented
+
+error[E0599]: no method named `bar` found for type `u32` in the current scope
+  --> $DIR/explicitly-unimplemented-error-message.rs:39:11
+   |
+LL |     0_u32.bar();
+   |           ^^^ method not found in `u32`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the trait `Bar` defines an item `bar`, but is explicitely unimplemented
+
+error[E0599]: no method named `foo` found for struct `Qux` in the current scope
+  --> $DIR/explicitly-unimplemented-error-message.rs:44:9
+   |
+LL | struct Qux;
+   | ----------- method `foo` not found for this
+...
+LL |     Qux.foo();
+   |         ^^^ method not found in `Qux`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following traits define an item `foo`, but are explicitely unimplemented:
+           Foo
+           FooBar
+
+error[E0599]: no method named `foo` found for type `u32` in the current scope
+  --> $DIR/explicitly-unimplemented-error-message.rs:49:11
+   |
+LL |     0_u32.foo();
+   |           ^^^ method not found in `u32`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+note: `Foo` defines an item `foo`, perhaps you need to implement it
+  --> $DIR/explicitly-unimplemented-error-message.rs:18:1
+   |
+LL | trait Foo {
+   | ^^^^^^^^^
+   = note: the trait `FooBar` defines an item `foo`, but is explicitely unimplemented
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0599`.