Extend #[must_use] lint to arrays

src/test/ui/lint/must_use-array.rs

@@ -0,0 +1,47 @@
+#![deny(unused_must_use)]
+
+#[must_use]
+struct S;
+
+struct A;
+
+#[must_use]
+trait T {}
+
+impl T for A {}
+
+fn empty() -> [S; 0] {
+    []
+}
+
+fn singleton() -> [S; 1] {
+    [S]
+}
+
+fn many() -> [S; 4] {
+    [S, S, S, S]
+}
+
+fn array_of_impl_trait() -> [impl T; 2] {
+    [A, A]
+}
+
+fn impl_array() -> [(u8, Box<dyn T>); 2] {
+    [(0, Box::new(A)), (0, Box::new(A))]
+}
+
+fn array_of_arrays_of_arrays() -> [[[S; 1]; 2]; 1] {
+    [[[S], [S]]]
+}
+
+fn main() {
+    empty(); // ok
+    singleton(); //~ ERROR unused array of `S` that must be used
+    many(); //~ ERROR unused array of `S` that must be used
+    ([S], 0, ()); //~ ERROR unused array of `S` in tuple element 0 that must be used
+    array_of_impl_trait(); //~ ERROR unused array of implementers of `T` that must be used
+    impl_array();
+    //~^ ERROR unused array of boxed `T` trait objects in tuple element 1 that must be used
+    array_of_arrays_of_arrays();
+    //~^ ERROR unused array of arrays of arrays of `S` that must be used
+}

src/test/ui/lint/must_use-array.stderr

@@ -0,0 +1,44 @@
+error: unused array of `S` that must be used
+  --> $DIR/must_use-array.rs:39:5
+   |
+LL |     singleton();
+   |     ^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/must_use-array.rs:1:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+
+error: unused array of `S` that must be used
+  --> $DIR/must_use-array.rs:40:5
+   |
+LL |     many();
+   |     ^^^^^^^
+
+error: unused array of `S` in tuple element 0 that must be used
+  --> $DIR/must_use-array.rs:41:6
+   |
+LL |     ([S], 0, ());
+   |      ^^^
+
+error: unused array of implementers of `T` that must be used
+  --> $DIR/must_use-array.rs:42:5
+   |
+LL |     array_of_impl_trait();
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: unused array of boxed `T` trait objects in tuple element 1 that must be used
+  --> $DIR/must_use-array.rs:43:5
+   |
+LL |     impl_array();
+   |     ^^^^^^^^^^^^^
+
+error: unused array of arrays of arrays of `S` that must be used
+  --> $DIR/must_use-array.rs:45:5
+   |
+LL |     array_of_arrays_of_arrays();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+