Ignore fake read access

Author: schubart

clippy_lints/src/collection_is_never_read.rs

@@ -114,6 +114,15 @@ 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 stement, 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.

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();
 }
 

tests/ui/collection_is_never_read.stderr

@@ -25,28 +25,34 @@ 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: aborting due to 8 previous errors
+error: aborting due to 9 previous errors