Provide better suggestions when encountering a bare trait as a type

Add the following suggestions:

```
error[E0782]: trait objects must include the `dyn` keyword
  --> $DIR/not-on-bare-trait-2021.rs:11:11
   |
LL | fn bar(x: Foo) -> Foo {
   |           ^^^
   |
help: use a generic type parameter, constrained by the trait `Foo`
   |
LL | fn bar<T: Foo>(x: T) -> Foo {
   |       ++++++++    ~
help: you can also use `impl Foo`, but users won't be able to specify the type paramer when calling the `fn`, having to rely exclusively on type inference
   |
LL | fn bar(x: impl Foo) -> Foo {
   |           ++++
help: alternatively, use a trait object to accept any type that implements `Foo`, accessing its methods at runtime using dynamic dispatch
   |
LL | fn bar(x: &dyn Foo) -> Foo {
   |           ++++

error[E0782]: trait objects must include the `dyn` keyword
  --> $DIR/not-on-bare-trait-2021.rs:11:19
   |
LL | fn bar(x: Foo) -> Foo {
   |                   ^^^
   |
help: use `impl Foo` to return an opaque type, as long as you return a single underlying type
   |
LL | fn bar(x: Foo) -> impl Foo {
   |                   ++++
help: alternatively, you can return an owned trait object
   |
LL | fn bar(x: Foo) -> Box<dyn Foo> {
   |                   +++++++    +
```

compiler/rustc_hir_analysis/src/astconv/lint.rs

@@ -2,6 +2,7 @@ use rustc_ast::TraitObjectSyntax;
 use rustc_errors::{Diagnostic, StashKey};
 use rustc_hir as hir;
 use rustc_lint_defs::{builtin::BARE_TRAIT_OBJECTS, Applicability};
+use rustc_span::Span;
 use rustc_trait_selection::traits::error_reporting::suggestions::NextTypeParamName;
 
 use super::AstConv;
@@ -32,32 +33,120 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
             let of_trait_span = of_trait_ref.path.span;
             // make sure that we are not calling unwrap to abort during the compilation
-            let Ok(impl_trait_name) = tcx.sess.source_map().span_to_snippet(self_ty.span) else {
-                return;
-            };
             let Ok(of_trait_name) = tcx.sess.source_map().span_to_snippet(of_trait_span) else {
                 return;
             };
-            // check if the trait has generics, to make a correct suggestion
-            let param_name = generics.params.next_type_param_name(None);
 
-            let add_generic_sugg = if let Some(span) = generics.span_for_param_suggestion() {
-                (span, format!(", {param_name}: {impl_trait_name}"))
-            } else {
-                (generics.span, format!("<{param_name}: {impl_trait_name}>"))
+            let Ok(impl_trait_name) = self.tcx().sess.source_map().span_to_snippet(self_ty.span)
+            else {
+                return;
+            };
+            let Some(sugg) = self.generics_suggestion(generics, self_ty.span, &impl_trait_name)
+            else {
+                return;
             };
             diag.multipart_suggestion(
                 format!(
-                    "alternatively use a blanket \
-                     implementation to implement `{of_trait_name}` for \
+                    "alternatively use a blanket implementation to implement `{of_trait_name}` for \
                      all types that also implement `{impl_trait_name}`"
                 ),
-                vec![(self_ty.span, param_name), add_generic_sugg],
+                sugg,
                 Applicability::MaybeIncorrect,
             );
         }
     }
 
+    fn generics_suggestion(
+        &self,
+        generics: &hir::Generics<'_>,
+        self_ty_span: Span,
+        impl_trait_name: &str,
+    ) -> Option<Vec<(Span, String)>> {
+        // check if the trait has generics, to make a correct suggestion
+        let param_name = generics.params.next_type_param_name(None);
+
+        let add_generic_sugg = if let Some(span) = generics.span_for_param_suggestion() {
+            (span, format!(", {param_name}: {impl_trait_name}"))
+        } else {
+            (generics.span, format!("<{param_name}: {impl_trait_name}>"))
+        };
+        Some(vec![(self_ty_span, param_name), add_generic_sugg])
+    }
+
+    /// Make sure that we are in the condition to suggest `impl Trait`.
+    fn maybe_lint_impl_trait(&self, self_ty: &hir::Ty<'_>, diag: &mut Diagnostic) -> bool {
+        let tcx = self.tcx();
+        let parent_id = tcx.hir().get_parent_item(self_ty.hir_id).def_id;
+        let (hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, generics, _), .. })
+        | hir::Node::TraitItem(hir::TraitItem {
+            kind: hir::TraitItemKind::Fn(sig, _),
+            generics,
+            ..
+        })) = tcx.hir_node_by_def_id(parent_id)
+        else {
+            return false;
+        };
+        let Ok(trait_name) = self.tcx().sess.source_map().span_to_snippet(self_ty.span) else {
+            return false;
+        };
+        let impl_sugg = vec![(self_ty.span.shrink_to_lo(), "impl ".to_string())];
+        if let hir::FnRetTy::Return(ty) = sig.decl.output
+            && ty.hir_id == self_ty.hir_id
+        {
+            diag.multipart_suggestion_verbose(
+                format!("use `impl {trait_name}` to return an opaque type, as long as you return a single underlying type"),
+                impl_sugg,
+                Applicability::MachineApplicable,
+            );
+            diag.multipart_suggestion_verbose(
+                "alternatively, you can return an owned trait object",
+                vec![
+                    (ty.span.shrink_to_lo(), "Box<dyn ".to_string()),
+                    (ty.span.shrink_to_hi(), ">".to_string()),
+                ],
+                Applicability::MachineApplicable,
+            );
+            return true;
+        }
+        for ty in sig.decl.inputs {
+            if ty.hir_id == self_ty.hir_id {
+                if let Some(sugg) = self.generics_suggestion(generics, self_ty.span, &trait_name) {
+                    diag.multipart_suggestion_verbose(
+                        format!("use a new generic type parameter, constrained by `{trait_name}`"),
+                        sugg,
+                        Applicability::MachineApplicable,
+                    );
+                    diag.multipart_suggestion_verbose(
+                        "you can also use an opaque type, but users won't be able to specify the \
+                         type parameter when calling the `fn`, having to rely exclusively on type \
+                         inference",
+                        impl_sugg,
+                        Applicability::MachineApplicable,
+                    );
+                }
+                let sugg = if let hir::TyKind::TraitObject([_, _, ..], _, _) = self_ty.kind {
+                    // There are more than one trait bound, we need surrounding parentheses.
+                    vec![
+                        (self_ty.span.shrink_to_lo(), "&(dyn ".to_string()),
+                        (self_ty.span.shrink_to_hi(), ")".to_string()),
+                    ]
+                } else {
+                    vec![(self_ty.span.shrink_to_lo(), "&dyn ".to_string())]
+                };
+                diag.multipart_suggestion_verbose(
+                    format!(
+                        "alternatively, use a trait object to accept any type that implements \
+                         `{trait_name}`, accessing its methods at runtime using dynamic dispatch",
+                    ),
+                    sugg,
+                    Applicability::MachineApplicable,
+                );
+                return true;
+            }
+        }
+        false
+    }
+
     pub(super) fn maybe_lint_bare_trait(&self, self_ty: &hir::Ty<'_>, in_path: bool) {
         let tcx = self.tcx();
         if let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
@@ -98,7 +187,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 let label = "add `dyn` keyword before this trait";
                 let mut diag =
                     rustc_errors::struct_span_err!(tcx.dcx(), self_ty.span, E0782, "{}", msg);
-                if self_ty.span.can_be_used_for_suggestions() {
+                if self_ty.span.can_be_used_for_suggestions()
+                    && !self.maybe_lint_impl_trait(self_ty, &mut diag)
+                {
                     diag.multipart_suggestion_verbose(
                         label,
                         sugg,
@@ -116,11 +207,15 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     self_ty.span,
                     msg,
                     |lint| {
-                        lint.multipart_suggestion_verbose(
-                            "use `dyn`",
-                            sugg,
-                            Applicability::MachineApplicable,
-                        );
+                        if self_ty.span.can_be_used_for_suggestions()
+                            && !self.maybe_lint_impl_trait(self_ty, lint)
+                        {
+                            lint.multipart_suggestion_verbose(
+                                "use `dyn`",
+                                sugg,
+                                Applicability::MachineApplicable,
+                            );
+                        }
                         self.maybe_lint_blanket_trait_impl(self_ty, lint);
                     },
                 );

tests/ui/traits/bound/not-on-bare-trait-2021.rs

@@ -0,0 +1,17 @@
+// edition:2021
+trait Foo {
+    fn dummy(&self) {}
+}
+
+// This should emit the less confusing error, not the more confusing one.
+
+fn foo(_x: Foo + Send) {
+    //~^ ERROR trait objects must include the `dyn` keyword
+}
+fn bar(x: Foo) -> Foo {
+    //~^ ERROR trait objects must include the `dyn` keyword
+    //~| ERROR trait objects must include the `dyn` keyword
+    x
+}
+
+fn main() {}

tests/ui/traits/bound/not-on-bare-trait-2021.stderr

@@ -0,0 +1,56 @@
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/not-on-bare-trait-2021.rs:8:12
+   |
+LL | fn foo(_x: Foo + Send) {
+   |            ^^^^^^^^^^
+   |
+help: use a new generic type parameter, constrained by `Foo + Send`
+   |
+LL | fn foo<T: Foo + Send>(_x: T) {
+   |       +++++++++++++++     ~
+help: you can also use an opaque type, but users won't be able to specify the type parameter when calling the `fn`, having to rely exclusively on type inference
+   |
+LL | fn foo(_x: impl Foo + Send) {
+   |            ++++
+help: alternatively, use a trait object to accept any type that implements `Foo + Send`, accessing its methods at runtime using dynamic dispatch
+   |
+LL | fn foo(_x: &(dyn Foo + Send)) {
+   |            +++++           +
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/not-on-bare-trait-2021.rs:11:11
+   |
+LL | fn bar(x: Foo) -> Foo {
+   |           ^^^
+   |
+help: use a new generic type parameter, constrained by `Foo`
+   |
+LL | fn bar<T: Foo>(x: T) -> Foo {
+   |       ++++++++    ~
+help: you can also use an opaque type, but users won't be able to specify the type parameter when calling the `fn`, having to rely exclusively on type inference
+   |
+LL | fn bar(x: impl Foo) -> Foo {
+   |           ++++
+help: alternatively, use a trait object to accept any type that implements `Foo`, accessing its methods at runtime using dynamic dispatch
+   |
+LL | fn bar(x: &dyn Foo) -> Foo {
+   |           ++++
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/not-on-bare-trait-2021.rs:11:19
+   |
+LL | fn bar(x: Foo) -> Foo {
+   |                   ^^^
+   |
+help: use `impl Foo` to return an opaque type, as long as you return a single underlying type
+   |
+LL | fn bar(x: Foo) -> impl Foo {
+   |                   ++++
+help: alternatively, you can return an owned trait object
+   |
+LL | fn bar(x: Foo) -> Box<dyn Foo> {
+   |                   +++++++    +
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0782`.

tests/ui/traits/bound/not-on-bare-trait.stderr

@@ -7,10 +7,18 @@ LL | fn foo(_x: Foo + Send) {
    = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
    = note: `#[warn(bare_trait_objects)]` on by default
-help: use `dyn`
+help: use a new generic type parameter, constrained by `Foo + Send`
    |
-LL | fn foo(_x: dyn Foo + Send) {
-   |            +++
+LL | fn foo<T: Foo + Send>(_x: T) {
+   |       +++++++++++++++     ~
+help: you can also use an opaque type, but users won't be able to specify the type parameter when calling the `fn`, having to rely exclusively on type inference
+   |
+LL | fn foo(_x: impl Foo + Send) {
+   |            ++++
+help: alternatively, use a trait object to accept any type that implements `Foo + Send`, accessing its methods at runtime using dynamic dispatch
+   |
+LL | fn foo(_x: &(dyn Foo + Send)) {
+   |            +++++           +
 
 error[E0277]: the size for values of type `(dyn Foo + Send + 'static)` cannot be known at compilation time
   --> $DIR/not-on-bare-trait.rs:7:8