Add collection_is_never_read

tests/ui/collection_is_never_read.rs

@@ -0,0 +1,116 @@
+#![allow(unused)]
+#![warn(clippy::collection_is_never_read)]
+
+use std::collections::HashMap;
+
+fn main() {}
+
+fn not_a_collection() {
+    // TODO: Expand `collection_is_never_read` beyond collections?
+    let mut x = 10; // Ok
+    x += 1;
+}
+
+fn no_access_at_all() {
+    // Other lints should catch this.
+    let x = vec![1, 2, 3]; // Ok
+}
+
+fn write_without_read() {
+    // The main use case for `collection_is_never_read`.
+    let mut x = HashMap::new(); // WARNING
+    x.insert(1, 2);
+}
+
+fn read_without_write() {
+    let mut x = vec![1, 2, 3]; // Ok
+    let _ = x.len();
+}
+
+fn write_and_read() {
+    let mut x = vec![1, 2, 3]; // Ok
+    x.push(4);
+    let _ = x.len();
+}
+
+fn write_after_read() {
+    // TODO: Warn here, but this requires more extensive data flow analysis.
+    let mut x = vec![1, 2, 3]; // Ok
+    let _ = x.len();
+    x.push(4); // Pointless
+}
+
+fn write_before_reassign() {
+    // TODO: Warn here, but this requires more extensive data flow analysis.
+    let mut x = HashMap::new(); // Ok
+    x.insert(1, 2); // Pointless
+    x = HashMap::new();
+    let _ = x.len();
+}
+
+fn read_in_closure() {
+    let mut x = HashMap::new(); // Ok
+    x.insert(1, 2);
+    let _ = || {
+        let _ = x.len();
+    };
+}
+
+fn write_in_closure() {
+    let mut x = vec![1, 2, 3]; // WARNING
+    let _ = || {
+        x.push(4);
+    };
+}
+
+fn read_in_format() {
+    let mut x = HashMap::new(); // Ok
+    x.insert(1, 2);
+    format!("{x:?}");
+}
+
+fn shadowing_1() {
+    let x = HashMap::<usize, usize>::new(); // Ok
+    let _ = x.len();
+    let mut x = HashMap::new(); // WARNING
+    x.insert(1, 2);
+}
+
+fn shadowing_2() {
+    let mut x = HashMap::new(); // WARNING
+    x.insert(1, 2);
+    let x = HashMap::<usize, usize>::new(); // Ok
+    let _ = x.len();
+}
+
+#[allow(clippy::let_unit_value)]
+fn fake_read() {
+    let mut x = vec![1, 2, 3]; // Ok
+    x.reverse();
+    // `collection_is_never_read` gets fooled, but other lints should catch this.
+    let _: () = x.clear();
+}
+
+fn assignment() {
+    let mut x = vec![1, 2, 3]; // WARNING
+    let y = vec![4, 5, 6]; // Ok
+    x = y;
+}
+
+#[allow(clippy::self_assignment)]
+fn self_assignment() {
+    let mut x = vec![1, 2, 3]; // WARNING
+    x = x;
+}
+
+fn method_argument_but_not_target() {
+    struct MyStruct;
+    impl MyStruct {
+        fn my_method(&self, _argument: &[usize]) {}
+    }
+    let my_struct = MyStruct;
+
+    let mut x = vec![1, 2, 3]; // Ok
+    x.reverse();
+    my_struct.my_method(&x);
+}

tests/ui/collection_is_never_read.stderr

@@ -0,0 +1,40 @@
+error: collection is never read
+  --> $DIR/collection_is_never_read.rs:21:5
+   |
+LL |     let mut x = HashMap::new(); // WARNING
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::collection-is-never-read` implied by `-D warnings`
+
+error: collection is never read
+  --> $DIR/collection_is_never_read.rs:60:5
+   |
+LL |     let mut x = vec![1, 2, 3]; // WARNING
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: collection is never read
+  --> $DIR/collection_is_never_read.rs:75:5
+   |
+LL |     let mut x = HashMap::new(); // WARNING
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: collection is never read
+  --> $DIR/collection_is_never_read.rs:80:5
+   |
+LL |     let mut x = HashMap::new(); // WARNING
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: collection is never read
+  --> $DIR/collection_is_never_read.rs:95:5
+   |
+LL |     let mut x = vec![1, 2, 3]; // WARNING
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: collection is never read
+  --> $DIR/collection_is_never_read.rs:102:5
+   |
+LL |     let mut x = vec![1, 2, 3]; // WARNING
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+