Auto merge of rust-lang#11879 - samueltardieu:issue-11876, r=Alexendoo

`manual_try_fold`: check that `fold` is really `Iterator::fold`

Fix rust-lang#11876

changelog: [`manual_try_fold`]: suggest using `try_fold` only for `Iterator::fold` uses

clippy_lints/src/methods/manual_try_fold.rs

@@ -1,14 +1,14 @@
 use clippy_config::msrvs::{self, Msrv};
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::is_from_proc_macro;
 use clippy_utils::source::snippet_opt;
 use clippy_utils::ty::implements_trait;
+use clippy_utils::{is_from_proc_macro, is_trait_method};
 use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LintContext};
 use rustc_middle::lint::in_external_macro;
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 
 use super::MANUAL_TRY_FOLD;
 
@@ -22,6 +22,7 @@ pub(super) fn check<'tcx>(
 ) {
     if !in_external_macro(cx.sess(), fold_span)
         && msrv.meets(msrvs::ITERATOR_TRY_FOLD)
+        && is_trait_method(cx, expr, sym::Iterator)
         && let init_ty = cx.typeck_results().expr_ty(init)
         && let Some(try_trait) = cx.tcx.lang_items().try_trait()
         && implements_trait(cx, init_ty, try_trait, &[])

tests/ui/manual_try_fold.rs

@@ -96,3 +96,33 @@ fn msrv_juust_right() {
         .fold(Some(0i32), |sum, i| sum?.checked_add(*i))
         .unwrap();
 }
+
+mod issue11876 {
+    struct Foo;
+
+    impl Bar for Foo {
+        type Output = u32;
+    }
+
+    trait Bar: Sized {
+        type Output;
+        fn fold<A, F>(self, init: A, func: F) -> Fold<Self, A, F>
+        where
+            A: Clone,
+            F: Fn(A, Self::Output) -> A,
+        {
+            Fold { this: self, init, func }
+        }
+    }
+
+    #[allow(dead_code)]
+    struct Fold<S, A, F> {
+        this: S,
+        init: A,
+        func: F,
+    }
+
+    fn main() {
+        Foo.fold(Some(0), |acc, entry| Some(acc? + entry));
+    }
+}