2 files changed, 54 insertions, 28 deletions

diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index b29c7d9f253..6665329d114 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -103,7 +103,7 @@ mod zst_offset;
 use bind_instead_of_map::BindInsteadOfMap;
 use clippy_utils::consts::{constant, Constant};
 use clippy_utils::diagnostics::{span_lint, span_lint_and_help};
-use clippy_utils::ty::{contains_adt_constructor, implements_trait, is_copy, is_type_diagnostic_item};
+use clippy_utils::ty::{contains_ty_adt_constructor_opaque, implements_trait, is_copy, is_type_diagnostic_item};
 use clippy_utils::{contains_return, is_trait_method, iter_input_pats, meets_msrv, msrvs, return_ty};
 use if_chain::if_chain;
 use rustc_hir as hir;
@@ -3394,36 +3394,10 @@ impl<'tcx> LateLintPass<'tcx> for Methods {
         if let hir::ImplItemKind::Fn(_, _) = impl_item.kind {
             let ret_ty = return_ty(cx, impl_item.hir_id());
 
-            // walk the return type and check for Self (this does not check associated types)
-            if let Some(self_adt) = self_ty.ty_adt_def() {
-                if contains_adt_constructor(ret_ty, self_adt) {
-                    return;
-                }
-            } else if ret_ty.contains(self_ty) {
+            if contains_ty_adt_constructor_opaque(cx, ret_ty, self_ty) {
                 return;
             }
 
-            // if return type is impl trait, check the associated types
-            if let ty::Opaque(def_id, _) = *ret_ty.kind() {
-                // one of the associated types must be Self
-                for &(predicate, _span) in cx.tcx.explicit_item_bounds(def_id) {
-                    if let ty::PredicateKind::Projection(projection_predicate) = predicate.kind().skip_binder() {
-                        let assoc_ty = match projection_predicate.term.unpack() {
-                            ty::TermKind::Ty(ty) => ty,
-                            ty::TermKind::Const(_c) => continue,
-                        };
-                        // walk the associated type and check for Self
-                        if let Some(self_adt) = self_ty.ty_adt_def() {
-                            if contains_adt_constructor(assoc_ty, self_adt) {
-                                return;
-                            }
-                        } else if assoc_ty.contains(self_ty) {
-                            return;
-                        }
-                    }
-                }
-            }
-
             if name == "new" && ret_ty != self_ty {
                 span_lint(
                     cx,
diff --git a/clippy_utils/src/ty.rs b/clippy_utils/src/ty.rs
index f4a135f2b5b..2dd8c826ab3 100644
--- a/clippy_utils/src/ty.rs
+++ b/clippy_utils/src/ty.rs
@@ -59,6 +59,58 @@ pub fn contains_adt_constructor<'tcx>(ty: Ty<'tcx>, adt: AdtDef<'tcx>) -> bool {
     })
 }
 
+/// Walks into `ty` and returns `true` if any inner type is an instance of the given type, or adt
+/// constructor of the same type.
+///
+/// This method also recurses into opaque type predicates, so call it with `impl Trait<U>` and `U`
+/// will also return `true`.
+pub fn contains_ty_adt_constructor_opaque<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, needle: Ty<'tcx>) -> bool {
+    ty.walk().any(|inner| match inner.unpack() {
+        GenericArgKind::Type(inner_ty) => {
+            if inner_ty == needle {
+                return true;
+            }
+
+            if inner_ty.ty_adt_def() == needle.ty_adt_def() {
+                return true;
+            }
+
+            if let ty::Opaque(def_id, _) = *inner_ty.kind() {
+                for &(predicate, _span) in cx.tcx.explicit_item_bounds(def_id) {
+                    match predicate.kind().skip_binder() {
+                        // For `impl Trait<U>`, it will register a predicate of `T: Trait<U>`, so we go through
+                        // and check substituions to find `U`.
+                        ty::PredicateKind::Trait(trait_predicate) => {
+                            if trait_predicate
+                                .trait_ref
+                                .substs
+                                .types()
+                                .skip(1) // Skip the implicit `Self` generic parameter
+                                .any(|ty| contains_ty_adt_constructor_opaque(cx, ty, needle))
+                            {
+                                return true;
+                            }
+                        },
+                        // For `impl Trait<Assoc=U>`, it will register a predicate of `<T as Trait>::Assoc = U`,
+                        // so we check the term for `U`.
+                        ty::PredicateKind::Projection(projection_predicate) => {
+                            if let ty::TermKind::Ty(ty) = projection_predicate.term.unpack() {
+                                if contains_ty_adt_constructor_opaque(cx, ty, needle) {
+                                    return true;
+                                }
+                            };
+                        },
+                        _ => (),
+                    }
+                }
+            }
+
+            false
+        },
+        GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
+    })
+}
+
 /// Resolves `<T as Iterator>::Item` for `T`
 /// Do not invoke without first verifying that the type implements `Iterator`
 pub fn get_iterator_item_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {