diff --git a/clippy_lints/src/manual_retain.rs b/clippy_lints/src/manual_retain.rs
index 6f15fca089eb..3ddb06a1e08a 100644
--- a/clippy_lints/src/manual_retain.rs
+++ b/clippy_lints/src/manual_retain.rs
@@ -2,7 +2,7 @@ use clippy_config::msrvs::{self, Msrv};
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::snippet;
 use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
-use clippy_utils::{get_parent_expr, match_def_path, paths, SpanlessEq};
+use clippy_utils::{match_def_path, paths, SpanlessEq};
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -69,17 +69,16 @@ impl_lint_pass!(ManualRetain => [MANUAL_RETAIN]);
 
 impl<'tcx> LateLintPass<'tcx> for ManualRetain {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
-        if let Some(parent_expr) = get_parent_expr(cx, expr)
-            && let Assign(left_expr, collect_expr, _) = &parent_expr.kind
+        if let Assign(left_expr, collect_expr, _) = &expr.kind
             && let hir::ExprKind::MethodCall(seg, ..) = &collect_expr.kind
             && seg.args.is_none()
             && let hir::ExprKind::MethodCall(_, target_expr, [], _) = &collect_expr.kind
             && let Some(collect_def_id) = cx.typeck_results().type_dependent_def_id(collect_expr.hir_id)
             && cx.tcx.is_diagnostic_item(sym::iterator_collect_fn, collect_def_id)
         {
-            check_into_iter(cx, left_expr, target_expr, parent_expr.span, &self.msrv);
-            check_iter(cx, left_expr, target_expr, parent_expr.span, &self.msrv);
-            check_to_owned(cx, left_expr, target_expr, parent_expr.span, &self.msrv);
+            check_into_iter(cx, left_expr, target_expr, expr.span, &self.msrv);
+            check_iter(cx, left_expr, target_expr, expr.span, &self.msrv);
+            check_to_owned(cx, left_expr, target_expr, expr.span, &self.msrv);
         }
     }
 
diff --git a/tests/ui/manual_retain.fixed b/tests/ui/manual_retain.fixed
index e359dfbb98c8..5540029bf6b1 100644
--- a/tests/ui/manual_retain.fixed
+++ b/tests/ui/manual_retain.fixed
@@ -1,5 +1,3 @@
-//@compile-flags: -Zdeduplicate-diagnostics=yes
-
 #![warn(clippy::manual_retain)]
 #![allow(unused, clippy::redundant_clone)]
 use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
diff --git a/tests/ui/manual_retain.rs b/tests/ui/manual_retain.rs
index 931814f08b7c..cee641d9d65f 100644
--- a/tests/ui/manual_retain.rs
+++ b/tests/ui/manual_retain.rs
@@ -1,5 +1,3 @@
-//@compile-flags: -Zdeduplicate-diagnostics=yes
-
 #![warn(clippy::manual_retain)]
 #![allow(unused, clippy::redundant_clone)]
 use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
diff --git a/tests/ui/manual_retain.stderr b/tests/ui/manual_retain.stderr
index fdbbc53e4df6..c25c804df758 100644
--- a/tests/ui/manual_retain.stderr
+++ b/tests/ui/manual_retain.stderr
@@ -1,5 +1,5 @@
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:27:5
+  --> tests/ui/manual_retain.rs:25:5
    |
 LL |     binary_heap = binary_heap.into_iter().filter(|x| x % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `binary_heap.retain(|x| x % 2 == 0)`
@@ -8,43 +8,43 @@ LL |     binary_heap = binary_heap.into_iter().filter(|x| x % 2 == 0).collect();
    = help: to override `-D warnings` add `#[allow(clippy::manual_retain)]`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:28:5
+  --> tests/ui/manual_retain.rs:26:5
    |
 LL |     binary_heap = binary_heap.iter().filter(|&x| x % 2 == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `binary_heap.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:29:5
+  --> tests/ui/manual_retain.rs:27:5
    |
 LL |     binary_heap = binary_heap.iter().filter(|&x| x % 2 == 0).cloned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `binary_heap.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:33:5
+  --> tests/ui/manual_retain.rs:31:5
    |
 LL |     tuples = tuples.iter().filter(|(ref x, ref y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(ref x, ref y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:34:5
+  --> tests/ui/manual_retain.rs:32:5
    |
 LL |     tuples = tuples.iter().filter(|(x, y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(x, y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:64:5
+  --> tests/ui/manual_retain.rs:62:5
    |
 LL |     btree_map = btree_map.into_iter().filter(|(k, _)| k % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `btree_map.retain(|k, _| k % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:65:5
+  --> tests/ui/manual_retain.rs:63:5
    |
 LL |     btree_map = btree_map.into_iter().filter(|(_, v)| v % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `btree_map.retain(|_, &mut v| v % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:66:5
+  --> tests/ui/manual_retain.rs:64:5
    |
 LL | /     btree_map = btree_map
 LL | |         .into_iter()
@@ -53,49 +53,49 @@ LL | |         .collect();
    | |__________________^ help: consider calling `.retain()` instead: `btree_map.retain(|k, &mut v| (k % 2 == 0) && (v % 2 == 0))`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:91:5
+  --> tests/ui/manual_retain.rs:89:5
    |
 LL |     btree_set = btree_set.iter().filter(|&x| x % 2 == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `btree_set.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:92:5
+  --> tests/ui/manual_retain.rs:90:5
    |
 LL |     btree_set = btree_set.iter().filter(|&x| x % 2 == 0).cloned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `btree_set.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:93:5
+  --> tests/ui/manual_retain.rs:91:5
    |
 LL |     btree_set = btree_set.into_iter().filter(|x| x % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `btree_set.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:97:5
+  --> tests/ui/manual_retain.rs:95:5
    |
 LL |     tuples = tuples.iter().filter(|(ref x, ref y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(ref x, ref y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:98:5
+  --> tests/ui/manual_retain.rs:96:5
    |
 LL |     tuples = tuples.iter().filter(|(x, y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(x, y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:128:5
+  --> tests/ui/manual_retain.rs:126:5
    |
 LL |     hash_map = hash_map.into_iter().filter(|(k, _)| k % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `hash_map.retain(|k, _| k % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:129:5
+  --> tests/ui/manual_retain.rs:127:5
    |
 LL |     hash_map = hash_map.into_iter().filter(|(_, v)| v % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `hash_map.retain(|_, &mut v| v % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:130:5
+  --> tests/ui/manual_retain.rs:128:5
    |
 LL | /     hash_map = hash_map
 LL | |         .into_iter()
@@ -104,133 +104,133 @@ LL | |         .collect();
    | |__________________^ help: consider calling `.retain()` instead: `hash_map.retain(|k, &mut v| (k % 2 == 0) && (v % 2 == 0))`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:154:5
+  --> tests/ui/manual_retain.rs:152:5
    |
 LL |     hash_set = hash_set.into_iter().filter(|x| x % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `hash_set.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:155:5
+  --> tests/ui/manual_retain.rs:153:5
    |
 LL |     hash_set = hash_set.iter().filter(|&x| x % 2 == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `hash_set.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:156:5
+  --> tests/ui/manual_retain.rs:154:5
    |
 LL |     hash_set = hash_set.iter().filter(|&x| x % 2 == 0).cloned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `hash_set.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:160:5
+  --> tests/ui/manual_retain.rs:158:5
    |
 LL |     tuples = tuples.iter().filter(|(ref x, ref y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(ref x, ref y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:161:5
+  --> tests/ui/manual_retain.rs:159:5
    |
 LL |     tuples = tuples.iter().filter(|(x, y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(x, y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:190:5
+  --> tests/ui/manual_retain.rs:188:5
    |
 LL |     s = s.chars().filter(|&c| c != 'o').to_owned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `s.retain(|c| c != 'o')`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:202:5
+  --> tests/ui/manual_retain.rs:200:5
    |
 LL |     vec = vec.iter().filter(|&x| x % 2 == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:203:5
+  --> tests/ui/manual_retain.rs:201:5
    |
 LL |     vec = vec.iter().filter(|&x| x % 2 == 0).cloned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:204:5
+  --> tests/ui/manual_retain.rs:202:5
    |
 LL |     vec = vec.into_iter().filter(|x| x % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:208:5
+  --> tests/ui/manual_retain.rs:206:5
    |
 LL |     tuples = tuples.iter().filter(|(ref x, ref y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(ref x, ref y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:209:5
+  --> tests/ui/manual_retain.rs:207:5
    |
 LL |     tuples = tuples.iter().filter(|(x, y)| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(x, y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:231:5
+  --> tests/ui/manual_retain.rs:229:5
    |
 LL |     vec_deque = vec_deque.iter().filter(|&x| x % 2 == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec_deque.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:232:5
+  --> tests/ui/manual_retain.rs:230:5
    |
 LL |     vec_deque = vec_deque.iter().filter(|&x| x % 2 == 0).cloned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec_deque.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:233:5
+  --> tests/ui/manual_retain.rs:231:5
    |
 LL |     vec_deque = vec_deque.into_iter().filter(|x| x % 2 == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec_deque.retain(|x| x % 2 == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:290:5
+  --> tests/ui/manual_retain.rs:288:5
    |
 LL |     vec = vec.into_iter().filter(|(x, y)| *x == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|(x, y)| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:294:5
+  --> tests/ui/manual_retain.rs:292:5
    |
 LL |     tuples = tuples.into_iter().filter(|(_, n)| *n > 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `tuples.retain(|(_, n)| *n > 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:311:5
+  --> tests/ui/manual_retain.rs:309:5
    |
 LL |     vec = vec.iter().filter(|&&x| x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|&x| x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:312:5
+  --> tests/ui/manual_retain.rs:310:5
    |
 LL |     vec = vec.iter().filter(|&&x| x == 0).cloned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|&x| x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:313:5
+  --> tests/ui/manual_retain.rs:311:5
    |
 LL |     vec = vec.into_iter().filter(|&x| x == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|&x| x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:316:5
+  --> tests/ui/manual_retain.rs:314:5
    |
 LL |     vec = vec.iter().filter(|&x| *x == 0).copied().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|x| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:317:5
+  --> tests/ui/manual_retain.rs:315:5
    |
 LL |     vec = vec.iter().filter(|&x| *x == 0).cloned().collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|x| *x == 0)`
 
 error: this expression can be written more simply using `.retain()`
-  --> tests/ui/manual_retain.rs:318:5
+  --> tests/ui/manual_retain.rs:316:5
    |
 LL |     vec = vec.into_iter().filter(|x| *x == 0).collect();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling `.retain()` instead: `vec.retain(|x| *x == 0)`