From 008ba7326b315abd1d6ef693d773b88ccb044ba4 Mon Sep 17 00:00:00 2001
From: Michael Schubart <michael@schubart.net>
Date: Sun, 12 Mar 2023 14:56:14 +0000
Subject: [PATCH] Ignore fake read access

---
 clippy_lints/src/collection_is_never_read.rs |  9 +++++++++
 tests/ui/collection_is_never_read.rs         |  3 +--
 tests/ui/collection_is_never_read.stderr     | 20 +++++++++++++-------
 3 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/clippy_lints/src/collection_is_never_read.rs b/clippy_lints/src/collection_is_never_read.rs
index a5feffa3106..5e2eb5789f6 100644
--- a/clippy_lints/src/collection_is_never_read.rs
+++ b/clippy_lints/src/collection_is_never_read.rs
@@ -120,6 +120,15 @@ fn has_no_read_access<'tcx>(cx: &LateContext<'tcx>, id: HirId, block: &'tcx Bloc
             if let Some(Node::Stmt(..)) = get_parent_node(cx.tcx, parent.hir_id) {
                 return ControlFlow::Continue(());
             }
+
+            // The method call is not a statement, so its return value is used somehow but its type is the
+            // unit type, so this is not a real read access. Examples:
+            //
+            // let y = x.clear();
+            // println!("{:?}", x.clear());
+            if cx.typeck_results().expr_ty(parent).is_unit() {
+                return ControlFlow::Continue(());
+            }
         }
 
         // Any other access to `id` is a read access. Stop searching.
diff --git a/tests/ui/collection_is_never_read.rs b/tests/ui/collection_is_never_read.rs
index ca20031bfbe..4f9431be72a 100644
--- a/tests/ui/collection_is_never_read.rs
+++ b/tests/ui/collection_is_never_read.rs
@@ -85,9 +85,8 @@ fn shadowing_2() {
 
 #[allow(clippy::let_unit_value)]
 fn fake_read() {
-    let mut x = vec![1, 2, 3]; // Ok
+    let mut x = vec![1, 2, 3]; // WARNING
     x.reverse();
-    // `collection_is_never_read` gets fooled, but other lints should catch this.
     let _: () = x.clear();
 }
 
diff --git a/tests/ui/collection_is_never_read.stderr b/tests/ui/collection_is_never_read.stderr
index f5dea96116f..cfb39298339 100644
--- a/tests/ui/collection_is_never_read.stderr
+++ b/tests/ui/collection_is_never_read.stderr
@@ -25,40 +25,46 @@ LL |     let mut x = HashMap::new(); // WARNING
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: collection is never read
-  --> $DIR/collection_is_never_read.rs:95:5
+  --> $DIR/collection_is_never_read.rs:88:5
    |
 LL |     let mut x = vec![1, 2, 3]; // WARNING
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: collection is never read
-  --> $DIR/collection_is_never_read.rs:102:5
+  --> $DIR/collection_is_never_read.rs:94:5
    |
 LL |     let mut x = vec![1, 2, 3]; // WARNING
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: collection is never read
-  --> $DIR/collection_is_never_read.rs:119:5
+  --> $DIR/collection_is_never_read.rs:101:5
+   |
+LL |     let mut x = vec![1, 2, 3]; // WARNING
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: collection is never read
+  --> $DIR/collection_is_never_read.rs:118:5
    |
 LL |     let mut x = HashSet::new(); // WARNING
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: collection is never read
-  --> $DIR/collection_is_never_read.rs:133:5
+  --> $DIR/collection_is_never_read.rs:132:5
    |
 LL |     let x = vec![1, 2, 3]; // WARNING
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 error: collection is never read
-  --> $DIR/collection_is_never_read.rs:169:5
+  --> $DIR/collection_is_never_read.rs:168:5
    |
 LL |     let mut s = String::new();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: collection is never read
-  --> $DIR/collection_is_never_read.rs:182:5
+  --> $DIR/collection_is_never_read.rs:181:5
    |
 LL |     let mut s = String::from("Hello, World!");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 10 previous errors
+error: aborting due to 11 previous errors