Add [`unused_peekable`] lint

2 files changed, 170 insertions, 0 deletions

diff --git a/tests/ui/unused_peekable.rs b/tests/ui/unused_peekable.rs
new file mode 100644
index 00000000000..12cab995621
--- /dev/null
+++ b/tests/ui/unused_peekable.rs
@@ -0,0 +1,119 @@
+#![warn(clippy::unused_peekable)]
+#![allow(clippy::no_effect)]
+
+use std::iter::Empty;
+use std::iter::Peekable;
+
+fn main() {
+    invalid();
+    valid();
+}
+
+#[allow(clippy::unused_unit)]
+fn invalid() {
+    let peekable = std::iter::empty::<u32>().peekable();
+
+    // Only lint `new_local`
+    let old_local = std::iter::empty::<u32>().peekable();
+    let new_local = old_local;
+
+    // Behind mut ref
+    let mut by_mut_ref_test = std::iter::empty::<u32>().peekable();
+    let by_mut_ref = &mut by_mut_ref_test;
+
+    // Explicitly returns `Peekable`
+    fn returns_peekable() -> Peekable<Empty<u32>> {
+        std::iter::empty().peekable()
+    }
+
+    let peekable_from_fn = returns_peekable();
+
+    // Using a method not exclusive to `Peekable`
+    let mut peekable_using_iterator_method = std::iter::empty::<u32>().peekable();
+    peekable_using_iterator_method.next();
+
+    let mut peekable_in_for_loop = std::iter::empty::<u32>().peekable();
+    for x in peekable_in_for_loop {}
+}
+
+fn valid() {
+    fn takes_peekable(_peek: Peekable<Empty<u32>>) {}
+
+    // Passed to another function
+    let passed_along = std::iter::empty::<u32>().peekable();
+    takes_peekable(passed_along);
+
+    // `peek` called in another block
+    let mut peekable_in_block = std::iter::empty::<u32>().peekable();
+    {
+        peekable_in_block.peek();
+    }
+
+    // Check the other `Peekable` methods :)
+    {
+        let mut peekable_with_peek_mut = std::iter::empty::<u32>().peekable();
+        peekable_with_peek_mut.peek_mut();
+
+        let mut peekable_with_next_if = std::iter::empty::<u32>().peekable();
+        peekable_with_next_if.next_if(|_| true);
+
+        let mut peekable_with_next_if_eq = std::iter::empty::<u32>().peekable();
+        peekable_with_next_if_eq.next_if_eq(&3);
+    }
+
+    let mut peekable_in_closure = std::iter::empty::<u32>().peekable();
+    let call_peek = |p: &mut Peekable<Empty<u32>>| {
+        p.peek();
+    };
+    call_peek(&mut peekable_in_closure);
+
+    // From a macro
+    macro_rules! make_me_a_peekable_please {
+        () => {
+            std::iter::empty::<u32>().peekable()
+        };
+    }
+
+    let _unsuspecting_macro_user = make_me_a_peekable_please!();
+
+    // Generic Iterator returned
+    fn return_an_iter() -> impl Iterator<Item = u32> {
+        std::iter::empty::<u32>().peekable()
+    }
+
+    let _unsuspecting_user = return_an_iter();
+
+    // Call `peek` in a macro
+    macro_rules! peek_iter {
+        ($iter:ident) => {
+            $iter.peek();
+        };
+    }
+
+    let mut peek_in_macro = std::iter::empty::<u32>().peekable();
+    peek_iter!(peek_in_macro);
+
+    // Behind mut ref
+    let mut by_mut_ref_test = std::iter::empty::<u32>().peekable();
+    let by_mut_ref = &mut by_mut_ref_test;
+    by_mut_ref.peek();
+
+    // Behind ref
+    let mut by_ref_test = std::iter::empty::<u32>().peekable();
+    let by_ref = &by_ref_test;
+    by_ref_test.peek();
+
+    // In struct
+    struct PeekableWrapper {
+        f: Peekable<Empty<u32>>,
+    }
+
+    let struct_test = std::iter::empty::<u32>().peekable();
+    PeekableWrapper { f: struct_test };
+
+    // `peek` called in another block as the last expression
+    let mut peekable_last_expr = std::iter::empty::<u32>().peekable();
+    {
+        peekable_last_expr.peek();
+    }
+}
diff --git a/tests/ui/unused_peekable.stderr b/tests/ui/unused_peekable.stderr
new file mode 100644
index 00000000000..bd087f56e4c
--- /dev/null
+++ b/tests/ui/unused_peekable.stderr
@@ -0,0 +1,51 @@
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:14:9
+   |
+LL |     let peekable = std::iter::empty::<u32>().peekable();
+   |         ^^^^^^^^
+   |
+   = note: `-D clippy::unused-peekable` implied by `-D warnings`
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:18:9
+   |
+LL |     let new_local = old_local;
+   |         ^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:22:9
+   |
+LL |     let by_mut_ref = &mut by_mut_ref_test;
+   |         ^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:29:9
+   |
+LL |     let peekable_from_fn = returns_peekable();
+   |         ^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:32:13
+   |
+LL |     let mut peekable_using_iterator_method = std::iter::empty::<u32>().peekable();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: `peek` never called on `Peekable` iterator
+  --> $DIR/unused_peekable.rs:35:13
+   |
+LL |     let mut peekable_in_for_loop = std::iter::empty::<u32>().peekable();
+   |             ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing the call to `peekable`
+
+error: aborting due to 6 previous errors
+