Wrap dyn type with parentheses in suggestion

compiler/rustc_hir/src/hir.rs

@@ -446,6 +446,21 @@ impl GenericBound<'_> {
         }
     }
 
+    pub fn get_inner_ty(&self) -> Option<&Ty<'_>> {
+        match self {
+            GenericBound::Trait(data, _) => {
+                let segment = data.trait_ref.path.segments.first()?;
+                let binding = segment.args().bindings.first()?;
+                if let TypeBindingKind::Equality { term: Term::Ty(ty) } = binding.kind {
+                    Some(ty)
+                } else {
+                    None
+                }
+            }
+            _ => None,
+        }
+    }
+
     pub fn span(&self) -> Span {
         match self {
             GenericBound::Trait(t, ..) => t.span,
@@ -647,6 +662,35 @@ impl<'hir> Generics<'hir> {
         )
     }
 
+    /// Returns bounds span for suggestions.
+    /// If the span including lifetime bound needs parentheses, it returns a tuple of a span to be surrounded by parentheses and true.
+    /// e.g. `dyn Future<Output = ()> + 'static` needs parentheses `(dyn Future<Output = ()>) + 'static`
+    pub fn bounds_span_for_suggestions_with_parentheses(
+        &self,
+        param_def_id: LocalDefId,
+    ) -> Option<(Span, bool)> {
+        self.bounds_for_param(param_def_id).flat_map(|bp| bp.bounds.iter().rev()).find_map(
+            |bound| {
+                let span_for_parentheses = bound.get_inner_ty().and_then(|ty| {
+                    if let TyKind::TraitObject(_, _, TraitObjectSyntax::Dyn) = ty.kind {
+                        let span = ty.span;
+                        span.can_be_used_for_suggestions().then(|| span)
+                    } else {
+                        None
+                    }
+                });
+
+                span_for_parentheses.map_or_else(
+                    || {
+                        let bs = bound.span();
+                        bs.can_be_used_for_suggestions().then(|| (bs.shrink_to_hi(), false))
+                    },
+                    |span| Some((span, true)),
+                )
+            },
+        )
+    }
+
     fn span_for_predicate_removal(&self, pos: usize) -> Span {
         let predicate = &self.predicates[pos];
         let span = predicate.span();

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2386,7 +2386,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 generic_param_scope = self.tcx.local_parent(generic_param_scope);
             }
 
-            // type_param_sugg_span is (span, has_bounds)
+            // type_param_sugg_span is (span, has_bounds, needs_parentheses)
             let (type_scope, type_param_sugg_span) = match bound_kind {
                 GenericKind::Param(ref param) => {
                     let generics = self.tcx.generics_of(generic_param_scope);
@@ -2396,11 +2396,13 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     // We do this to avoid suggesting code that ends up as `T: 'a'b`,
                     // instead we suggest `T: 'a + 'b` in that case.
                     let hir_generics = self.tcx.hir().get_generics(scope).unwrap();
-                    let sugg_span = match hir_generics.bounds_span_for_suggestions(def_id) {
-                        Some(span) => Some((span, true)),
+                    let sugg_span = match hir_generics
+                        .bounds_span_for_suggestions_with_parentheses(def_id)
+                    {
+                        Some((span, needs_parentheses)) => Some((span, true, needs_parentheses)),
                         // If `param` corresponds to `Self`, no usable suggestion span.
                         None if generics.has_self && param.index == 0 => None,
-                        None => Some((self.tcx.def_span(def_id).shrink_to_hi(), false)),
+                        None => Some((self.tcx.def_span(def_id).shrink_to_hi(), false, false)),
                     };
                     (scope, sugg_span)
                 }
@@ -2423,12 +2425,18 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             let mut suggs = vec![];
             let lt_name = self.suggest_name_region(sub, &mut suggs);
 
-            if let Some((sp, has_lifetimes)) = type_param_sugg_span
+            if let Some((sp, has_lifetimes, needs_parentheses)) = type_param_sugg_span
                 && suggestion_scope == type_scope
             {
                 let suggestion =
                     if has_lifetimes { format!(" + {lt_name}") } else { format!(": {lt_name}") };
-                suggs.push((sp, suggestion))
+
+                if needs_parentheses {
+                    suggs.push((sp.shrink_to_lo(), "(".to_string()));
+                    suggs.push((sp.shrink_to_hi(), format!("){suggestion}")));
+                } else {
+                    suggs.push((sp, suggestion))
+                }
             } else if let Some(generics) = self.tcx.hir().get_generics(suggestion_scope) {
                 let pred = format!("{bound_kind}: {lt_name}");
                 let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred);

tests/ui/suggestions/issue-120223.rs

@@ -0,0 +1,9 @@
+use std::{future::Future};
+
+pub fn foo<T>(
+    executor: impl FnOnce(T) -> dyn Future<Output = ()>,
+) -> Box<dyn FnOnce(T) -> dyn Future<Output = ()>> {
+    Box::new(executor) //~ ERROR the parameter type
+}
+
+fn main() {}

tests/ui/suggestions/issue-120223.stderr

@@ -0,0 +1,17 @@
+error[E0310]: the parameter type `impl FnOnce(T) -> dyn Future<Output = ()>` may not live long enough
+  --> $DIR/issue-120223.rs:6:5
+   |
+LL |     Box::new(executor)
+   |     ^^^^^^^^^^^^^^^^^^
+   |     |
+   |     the parameter type `impl FnOnce(T) -> dyn Future<Output = ()>` must be valid for the static lifetime...
+   |     ...so that the type `impl FnOnce(T) -> dyn Future<Output = ()>` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL |     executor: impl FnOnce(T) -> (dyn Future<Output = ()>) + 'static,
+   |                                 +                       +++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0310`.