Add hint for missing lifetime bound on trait object when type alias is used

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -813,17 +813,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             if *outlived_f != ty::ReStatic {
                 return;
             }
+            let suitable_region = self.infcx.tcx.is_suitable_region(f);
+            let Some(suitable_region) = suitable_region else { return; };
 
-            let fn_returns = self
+            let fn_returns = self.infcx.tcx.return_type_impl_or_dyn_traits(suitable_region.def_id);
-                .infcx
-                .tcx
-                .is_suitable_region(f)
-                .map(|r| self.infcx.tcx.return_type_impl_or_dyn_traits(r.def_id))
-                .unwrap_or_default();
-
-            if fn_returns.is_empty() {
-                return;
-            }
 
             let param = if let Some(param) = find_param_with_region(self.infcx.tcx, f, outlived_f) {
                 param
@@ -839,15 +832,43 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             };
             let captures = format!("captures data from {arg}");
 
-            return nice_region_error::suggest_new_region_bound(
+            if !fn_returns.is_empty() {
-                self.infcx.tcx,
+                nice_region_error::suggest_new_region_bound(
-                diag,
+                    self.infcx.tcx,
-                fn_returns,
+                    diag,
-                lifetime.to_string(),
+                    fn_returns,
-                Some(arg),
+                    lifetime.to_string(),
-                captures,
+                    Some(arg),
-                Some((param.param_ty_span, param.param_ty.to_string())),
+                    captures,
-                self.infcx.tcx.is_suitable_region(f).map(|r| r.def_id),
+                    Some((param.param_ty_span, param.param_ty.to_string())),
+                    Some(suitable_region.def_id),
+                );
+                return;
+            }
+
+            let Some((alias_tys, alias_span)) = self
+                .infcx
+                .tcx
+                .return_type_impl_or_dyn_traits_with_type_alias(suitable_region.def_id) else { return; };
+
+            // in case the return type of the method is a type alias
+            let mut spans_suggs: Vec<_> = Vec::new();
+            for alias_ty in alias_tys {
+                if alias_ty.span.desugaring_kind().is_some() {
+                    // Skip `async` desugaring `impl Future`.
+                    ()
+                }
+                if let TyKind::TraitObject(_, lt, _) = alias_ty.kind {
+                    spans_suggs.push((lt.ident.span.shrink_to_hi(), " + 'a".to_string()));
+                }
+            }
+            spans_suggs.push((alias_span.shrink_to_hi(), "<'a>".to_string()));
+            diag.multipart_suggestion_verbose(
+                &format!(
+                    "to declare that the trait object {captures}, you can add a lifetime parameter `'a` in the type alias"
+                ),
+                spans_suggs,
+                Applicability::MaybeIncorrect,
             );
         }
     }

compiler/rustc_hir/src/hir.rs

@@ -3524,6 +3524,13 @@ impl<'hir> Node<'hir> {
         }
     }
 
+    pub fn alias_ty(self) -> Option<&'hir Ty<'hir>> {
+        match self {
+            Node::Item(Item { kind: ItemKind::TyAlias(ty, ..), .. }) => Some(ty),
+            _ => None,
+        }
+    }
+
     pub fn body_id(&self) -> Option<BodyId> {
         match self {
             Node::TraitItem(TraitItem {

compiler/rustc_middle/src/ty/context.rs

@@ -997,6 +997,30 @@ impl<'tcx> TyCtxt<'tcx> {
         v.0
     }
 
+    /// Given a `DefId` for an `fn`, return all the `dyn` and `impl` traits in its return type and associated alias span when type alias is used
+    pub fn return_type_impl_or_dyn_traits_with_type_alias(
+        self,
+        scope_def_id: LocalDefId,
+    ) -> Option<(Vec<&'tcx hir::Ty<'tcx>>, Span)> {
+        let hir_id = self.hir().local_def_id_to_hir_id(scope_def_id);
+        let mut v = TraitObjectVisitor(vec![], self.hir());
+        // when the return type is a type alias
+        if let Some(hir::FnDecl { output: hir::FnRetTy::Return(hir_output), .. }) = self.hir().fn_decl_by_hir_id(hir_id)
+            && let hir::TyKind::Path(hir::QPath::Resolved(
+                None,
+                hir::Path { res: hir::def::Res::Def(DefKind::TyAlias, def_id), .. }, )) = hir_output.kind
+            && let Some(local_id) = def_id.as_local()
+            && let Some(alias_ty) = self.hir().get_by_def_id(local_id).alias_ty() // it is type alias
+            && let Some(alias_generics) = self.hir().get_by_def_id(local_id).generics()
+        {
+            v.visit_ty(alias_ty);
+            if !v.0.is_empty() {
+                return Some((v.0, alias_generics.span));
+            }
+        }
+        return None;
+    }
+
     pub fn return_type_impl_trait(self, scope_def_id: LocalDefId) -> Option<(Ty<'tcx>, Span)> {
         // `type_of()` will fail on these (#55796, #86483), so only allow `fn`s or closures.
         match self.hir().get_by_def_id(scope_def_id) {

tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.fixed

@@ -0,0 +1,45 @@
+// run-rustfix
+
+trait Greeter0 {
+    fn greet(&self);
+}
+
+trait Greeter1 {
+    fn greet(&self);
+}
+
+type BoxedGreeter<'a> = (Box<dyn Greeter0 + 'a>, Box<dyn Greeter1 + 'a>);
+//~^ HELP to declare that the trait object captures data from argument `self`, you can add a lifetime parameter `'a` in the type alias
+
+struct FixedGreeter<'a>(pub &'a str);
+
+impl Greeter0 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("0 {}", self.0)
+    }
+}
+
+impl Greeter1 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("1 {}", self.0)
+    }
+}
+
+struct Greetings(pub Vec<String>);
+
+impl Greetings {
+    pub fn get(&self, i: usize) -> BoxedGreeter {
+        (Box::new(FixedGreeter(&self.0[i])), Box::new(FixedGreeter(&self.0[i])))
+        //~^ ERROR lifetime may not live long enough
+    }
+}
+
+fn main() {
+    let mut g = Greetings {0 : vec!()};
+    g.0.push("a".to_string());
+    g.0.push("b".to_string());
+    g.get(0).0.greet();
+    g.get(0).1.greet();
+    g.get(1).0.greet();
+    g.get(1).1.greet();
+}

tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.rs

@@ -0,0 +1,45 @@
+// run-rustfix
+
+trait Greeter0 {
+    fn greet(&self);
+}
+
+trait Greeter1 {
+    fn greet(&self);
+}
+
+type BoxedGreeter = (Box<dyn Greeter0>, Box<dyn Greeter1>);
+//~^ HELP to declare that the trait object captures data from argument `self`, you can add a lifetime parameter `'a` in the type alias
+
+struct FixedGreeter<'a>(pub &'a str);
+
+impl Greeter0 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("0 {}", self.0)
+    }
+}
+
+impl Greeter1 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("1 {}", self.0)
+    }
+}
+
+struct Greetings(pub Vec<String>);
+
+impl Greetings {
+    pub fn get(&self, i: usize) -> BoxedGreeter {
+        (Box::new(FixedGreeter(&self.0[i])), Box::new(FixedGreeter(&self.0[i])))
+        //~^ ERROR lifetime may not live long enough
+    }
+}
+
+fn main() {
+    let mut g = Greetings {0 : vec!()};
+    g.0.push("a".to_string());
+    g.0.push("b".to_string());
+    g.get(0).0.greet();
+    g.get(0).1.greet();
+    g.get(1).0.greet();
+    g.get(1).1.greet();
+}

tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.stderr

@@ -0,0 +1,15 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.rs:32:9
+   |
+LL |     pub fn get(&self, i: usize) -> BoxedGreeter {
+   |                - let's call the lifetime of this reference `'1`
+LL |         (Box::new(FixedGreeter(&self.0[i])), Box::new(FixedGreeter(&self.0[i])))
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`
+   |
+help: to declare that the trait object captures data from argument `self`, you can add a lifetime parameter `'a` in the type alias
+   |
+LL | type BoxedGreeter<'a> = (Box<dyn Greeter0 + 'a>, Box<dyn Greeter1 + 'a>);
+   |                  ++++                     ++++                    ++++
+
+error: aborting due to previous error
+