Update references

23 files changed, 533 insertions, 21 deletions

diff --git a/tests/ui/borrow_box.stderr b/tests/ui/borrow_box.stderr
index 3eac32815be..a40789cd426 100644
--- a/tests/ui/borrow_box.stderr
+++ b/tests/ui/borrow_box.stderr
@@ -22,6 +22,15 @@ error: you seem to be trying to use `&Box<T>`. Consider using just `&T`
 LL |     fn test4(a: &Box<bool>);
    |                 ^^^^^^^^^^ help: try: `&bool`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/borrow_box.rs:63:5
+   |
+LL |     let _ = foo;
+   |     ^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: you seem to be trying to use `&Box<T>`. Consider using just `&T`
   --> $DIR/borrow_box.rs:95:25
    |
@@ -64,5 +73,5 @@ error: you seem to be trying to use `&Box<T>`. Consider using just `&T`
 LL | pub fn test20(_display: &Box<(dyn Display + Send)>) {}
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&(dyn Display + Send)`
 
-error: aborting due to 10 previous errors
+error: aborting due to 11 previous errors
 
diff --git a/tests/ui/borrow_interior_mutable_const/others.stderr b/tests/ui/borrow_interior_mutable_const/others.stderr
index 9a908cf30e9..976c412c7a8 100644
--- a/tests/ui/borrow_interior_mutable_const/others.stderr
+++ b/tests/ui/borrow_interior_mutable_const/others.stderr
@@ -47,6 +47,15 @@ LL |     let _once_mut = &mut ONCE_INIT; //~ ERROR interior mutability
    |
    = help: assign this const to a local or static variable, and use the variable here
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/others.rs:72:5
+   |
+LL |     let _ = &ATOMIC_TUPLE; //~ ERROR interior mutability
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: a `const` item with interior mutability should not be borrowed
   --> $DIR/others.rs:72:14
    |
@@ -95,6 +104,22 @@ LL |     let _ = ATOMIC_TUPLE.0[0]; //~ ERROR interior mutability
    |
    = help: assign this const to a local or static variable, and use the variable here
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/others.rs:83:5
+   |
+LL |     let _ = ATOMIC_TUPLE.1.into_iter();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/others.rs:85:5
+   |
+LL |     let _ = &{ ATOMIC_TUPLE };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: a `const` item with interior mutability should not be borrowed
   --> $DIR/others.rs:87:5
    |
@@ -111,5 +136,5 @@ LL |     assert_eq!(CELL.get(), 6); //~ ERROR interior mutability
    |
    = help: assign this const to a local or static variable, and use the variable here
 
-error: aborting due to 14 previous errors
+error: aborting due to 17 previous errors
 
diff --git a/tests/ui/box_vec.stderr b/tests/ui/box_vec.stderr
index fca12eddd57..a4983df1d30 100644
--- a/tests/ui/box_vec.stderr
+++ b/tests/ui/box_vec.stderr
@@ -1,3 +1,16 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/box_vec.rs:7:9
+   |
+LL |         let _: Box<$x> = Box::new($init);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     boxit!(Vec::new(), Vec<u8>);
+   |     ---------------------------- in this macro invocation
+   |
+   = note: `-D clippy::let-underscore-drop` implied by `-D warnings`
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error: you seem to be trying to use `Box<Vec<T>>`. Consider using just `Vec<T>`
   --> $DIR/box_vec.rs:14:18
    |
@@ -7,5 +20,5 @@ LL | pub fn test(foo: Box<Vec<bool>>) {
    = note: `-D clippy::box-vec` implied by `-D warnings`
    = help: `Vec<T>` is already on the heap, `Box<Vec<T>>` makes an extra allocation.
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
diff --git a/tests/ui/crashes/ice-4968.stderr b/tests/ui/crashes/ice-4968.stderr
new file mode 100644
index 00000000000..9ce39027451
--- /dev/null
+++ b/tests/ui/crashes/ice-4968.stderr
@@ -0,0 +1,11 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/ice-4968.rs:16:9
+   |
+LL |         let _: Vec<ManuallyDrop<T::Assoc>> = mem::transmute(slice);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to previous error
+
diff --git a/tests/ui/crashes/ice-5223.stderr b/tests/ui/crashes/ice-5223.stderr
new file mode 100644
index 00000000000..3ae2dd5f770
--- /dev/null
+++ b/tests/ui/crashes/ice-5223.stderr
@@ -0,0 +1,11 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/ice-5223.rs:14:9
+   |
+LL |         let _ = self.arr.iter().cloned().collect::<Vec<_>>();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to previous error
+
diff --git a/tests/ui/escape_analysis.stderr b/tests/ui/escape_analysis.stderr
index c86a769a3da..6e1c1c07b6e 100644
--- a/tests/ui/escape_analysis.stderr
+++ b/tests/ui/escape_analysis.stderr
@@ -12,5 +12,26 @@ error: local variable doesn't need to be boxed here
 LL | pub fn new(_needs_name: Box<PeekableSeekable<&()>>) -> () {}
    |            ^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/escape_analysis.rs:166:9
+   |
+LL | /         let _ = move || {
+LL | |             consume(x);
+LL | |         };
+   | |__________^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/escape_analysis.rs:172:9
+   |
+LL | /         let _ = || {
+LL | |             borrow(&x);
+LL | |         };
+   | |__________^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to 4 previous errors
 
diff --git a/tests/ui/eta.stderr b/tests/ui/eta.stderr
index c4713ca8083..bc79caee887 100644
--- a/tests/ui/eta.stderr
+++ b/tests/ui/eta.stderr
@@ -64,6 +64,15 @@ error: redundant closure found
 LL |     let e: std::vec::Vec<char> = vec!['a', 'b', 'c'].iter().map(|c| c.to_ascii_uppercase()).collect();
    |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove closure as shown: `char::to_ascii_uppercase`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/eta.rs:107:5
+   |
+LL |     let _: Vec<_> = arr.iter().map(|x| x.map_err(|e| some.take().unwrap()(e))).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: redundant closure found
   --> $DIR/eta.rs:172:27
    |
@@ -76,5 +85,13 @@ error: redundant closure found
 LL |     let a = Some(1u8).map(|a| closure(a));
    |                           ^^^^^^^^^^^^^^ help: remove closure as shown: `closure`
 
-error: aborting due to 12 previous errors
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/eta.rs:203:5
+   |
+LL |     let _ = [Bar].iter().map(|s| s.to_string()).collect::<Vec<_>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to 14 previous errors
 
diff --git a/tests/ui/filter_methods.stderr b/tests/ui/filter_methods.stderr
index 91718dd1175..08b781d7363 100644
--- a/tests/ui/filter_methods.stderr
+++ b/tests/ui/filter_methods.stderr
@@ -1,3 +1,12 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/filter_methods.rs:5:5
+   |
+LL |     let _: Vec<_> = vec![5; 6].into_iter().filter(|&x| x == 0).map(|x| x * 2).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::let-underscore-drop` implied by `-D warnings`
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `filter(..).map(..)` on an `Iterator`
   --> $DIR/filter_methods.rs:5:21
    |
@@ -7,6 +16,18 @@ LL |     let _: Vec<_> = vec![5; 6].into_iter().filter(|&x| x == 0).map(|x| x *
    = note: `-D clippy::filter-map` implied by `-D warnings`
    = help: this is more succinctly expressed by calling `.filter_map(..)` instead
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/filter_methods.rs:7:5
+   |
+LL | /     let _: Vec<_> = vec![5_i8; 6]
+LL | |         .into_iter()
+LL | |         .filter(|&x| x == 0)
+LL | |         .flat_map(|x| x.checked_mul(2))
+LL | |         .collect();
+   | |___________________^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `filter(..).flat_map(..)` on an `Iterator`
   --> $DIR/filter_methods.rs:7:21
    |
@@ -19,6 +40,18 @@ LL | |         .flat_map(|x| x.checked_mul(2))
    |
    = help: this is more succinctly expressed by calling `.flat_map(..)` and filtering by returning `iter::empty()`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/filter_methods.rs:13:5
+   |
+LL | /     let _: Vec<_> = vec![5_i8; 6]
+LL | |         .into_iter()
+LL | |         .filter_map(|x| x.checked_mul(2))
+LL | |         .flat_map(|x| x.checked_mul(2))
+LL | |         .collect();
+   | |___________________^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `filter_map(..).flat_map(..)` on an `Iterator`
   --> $DIR/filter_methods.rs:13:21
    |
@@ -31,6 +64,18 @@ LL | |         .flat_map(|x| x.checked_mul(2))
    |
    = help: this is more succinctly expressed by calling `.flat_map(..)` and filtering by returning `iter::empty()`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/filter_methods.rs:19:5
+   |
+LL | /     let _: Vec<_> = vec![5_i8; 6]
+LL | |         .into_iter()
+LL | |         .filter_map(|x| x.checked_mul(2))
+LL | |         .map(|x| x.checked_mul(2))
+LL | |         .collect();
+   | |___________________^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `filter_map(..).map(..)` on an `Iterator`
   --> $DIR/filter_methods.rs:19:21
    |
@@ -43,5 +88,5 @@ LL | |         .map(|x| x.checked_mul(2))
    |
    = help: this is more succinctly expressed by only calling `.filter_map(..)` instead
 
-error: aborting due to 4 previous errors
+error: aborting due to 8 previous errors
 
diff --git a/tests/ui/get_unwrap.stderr b/tests/ui/get_unwrap.stderr
index 76a098df82a..6aa5452bac1 100644
--- a/tests/ui/get_unwrap.stderr
+++ b/tests/ui/get_unwrap.stderr
@@ -70,17 +70,34 @@ error: called `.get_mut().unwrap()` on a VecDeque. Using `[]` is more clear and
 LL |         *some_vecdeque.get_mut(0).unwrap() = 1;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `some_vecdeque[0]`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/get_unwrap.rs:59:9
+   |
+LL |         let _ = some_vec.get(0..1).unwrap().to_vec();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `.get().unwrap()` on a Vec. Using `[]` is more clear and more concise
   --> $DIR/get_unwrap.rs:59:17
    |
 LL |         let _ = some_vec.get(0..1).unwrap().to_vec();
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `some_vec[0..1]`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/get_unwrap.rs:60:9
+   |
+LL |         let _ = some_vec.get_mut(0..1).unwrap().to_vec();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `.get_mut().unwrap()` on a Vec. Using `[]` is more clear and more concise
   --> $DIR/get_unwrap.rs:60:17
    |
 LL |         let _ = some_vec.get_mut(0..1).unwrap().to_vec();
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `some_vec[0..1]`
 
-error: aborting due to 13 previous errors
+error: aborting due to 15 previous errors
 
diff --git a/tests/ui/into_iter_on_ref.stderr b/tests/ui/into_iter_on_ref.stderr
index 28003b365bb..efe9d20920b 100644
--- a/tests/ui/into_iter_on_ref.stderr
+++ b/tests/ui/into_iter_on_ref.stderr
@@ -1,3 +1,12 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/into_iter_on_ref.rs:13:5
+   |
+LL |     let _ = vec![1, 2, 3].into_iter();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: this `.into_iter()` call is equivalent to `.iter()` and will not consume the `Vec`
   --> $DIR/into_iter_on_ref.rs:14:30
    |
@@ -162,5 +171,5 @@ error: this `.into_iter()` call is equivalent to `.iter()` and will not consume
 LL |     let _ = (&[1, 2, 3]).into_iter().next(); //~ WARN equivalent to .iter()
    |                          ^^^^^^^^^ help: call directly: `iter`
 
-error: aborting due to 27 previous errors
+error: aborting due to 28 previous errors
 
diff --git a/tests/ui/iter_cloned_collect.stderr b/tests/ui/iter_cloned_collect.stderr
index b90a1e6c919..f5cd43b3da5 100644
--- a/tests/ui/iter_cloned_collect.stderr
+++ b/tests/ui/iter_cloned_collect.stderr
@@ -6,12 +6,33 @@ LL |     let v2: Vec<isize> = v.iter().cloned().collect();
    |
    = note: `-D clippy::iter-cloned-collect` implied by `-D warnings`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/iter_cloned_collect.rs:15:5
+   |
+LL |     let _: Vec<isize> = vec![1, 2, 3].iter().cloned().collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `iter().cloned().collect()` on a slice to create a `Vec`. Calling `to_vec()` is both faster and more readable
   --> $DIR/iter_cloned_collect.rs:15:38
    |
 LL |     let _: Vec<isize> = vec![1, 2, 3].iter().cloned().collect();
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `.to_vec()`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/iter_cloned_collect.rs:19:9
+   |
+LL | /         let _: Vec<u8> = std::ffi::CStr::from_ptr(std::ptr::null())
+LL | |             .to_bytes()
+LL | |             .iter()
+LL | |             .cloned()
+LL | |             .collect();
+   | |_______________________^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `iter().cloned().collect()` on a slice to create a `Vec`. Calling `to_vec()` is both faster and more readable
   --> $DIR/iter_cloned_collect.rs:20:24
    |
@@ -22,5 +43,5 @@ LL | |             .cloned()
 LL | |             .collect();
    | |______________________^ help: try: `.to_vec()`
 
-error: aborting due to 3 previous errors
+error: aborting due to 5 previous errors
 
diff --git a/tests/ui/map_clone.stderr b/tests/ui/map_clone.stderr
index 4f43cff5024..122a678f118 100644
--- a/tests/ui/map_clone.stderr
+++ b/tests/ui/map_clone.stderr
@@ -1,3 +1,12 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_clone.rs:10:5
+   |
+LL |     let _: Vec<i8> = vec![5_i8; 6].iter().map(|x| *x).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::let-underscore-drop` implied by `-D warnings`
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: you are using an explicit closure for copying elements
   --> $DIR/map_clone.rs:10:22
    |
@@ -6,12 +15,28 @@ LL |     let _: Vec<i8> = vec![5_i8; 6].iter().map(|x| *x).collect();
    |
    = note: `-D clippy::map-clone` implied by `-D warnings`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_clone.rs:11:5
+   |
+LL |     let _: Vec<String> = vec![String::new()].iter().map(|x| x.clone()).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: you are using an explicit closure for cloning elements
   --> $DIR/map_clone.rs:11:26
    |
 LL |     let _: Vec<String> = vec![String::new()].iter().map(|x| x.clone()).collect();
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling the dedicated `cloned` method: `vec![String::new()].iter().cloned()`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_clone.rs:12:5
+   |
+LL |     let _: Vec<u32> = vec![42, 43].iter().map(|&x| x).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: you are using an explicit closure for copying elements
   --> $DIR/map_clone.rs:12:23
    |
@@ -36,5 +61,37 @@ error: you are needlessly cloning iterator elements
 LL |     let _ = std::env::args().map(|v| v.clone());
    |                             ^^^^^^^^^^^^^^^^^^^ help: remove the `map` call
 
-error: aborting due to 6 previous errors
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_clone.rs:35:9
+   |
+LL |         let _: Vec<u32> = v.into_iter().map(|x| *x).collect();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_clone.rs:42:9
+   |
+LL |         let _: Vec<u32> = v.into_iter().map(|x| *x).collect();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_clone.rs:45:9
+   |
+LL |         let _: Vec<u32> = v.into_iter().map(|&mut x| x).collect();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_clone.rs:53:9
+   |
+LL |         let _: Vec<_> = items.into_iter().map(|x| x.clone()).collect();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to 13 previous errors
 
diff --git a/tests/ui/map_collect_result_unit.stderr b/tests/ui/map_collect_result_unit.stderr
index 8b06e13baa6..26e876b1808 100644
--- a/tests/ui/map_collect_result_unit.stderr
+++ b/tests/ui/map_collect_result_unit.stderr
@@ -12,5 +12,22 @@ error: `.map().collect()` can be replaced with `.try_for_each()`
 LL |         let _: Result<(), _> = (0..3).map(|t| Err(t + 1)).collect();
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `(0..3).try_for_each(|t| Err(t + 1))`
 
-error: aborting due to 2 previous errors
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_collect_result_unit.rs:14:5
+   |
+LL |     let _ = (0..3).map(|t| Err(t + 1)).collect::<Result<Vec<i32>, _>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_collect_result_unit.rs:15:5
+   |
+LL |     let _ = (0..3).map(|t| Err(t + 1)).collect::<Vec<Result<(), _>>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to 4 previous errors
 
diff --git a/tests/ui/map_flatten.stderr b/tests/ui/map_flatten.stderr
index b6479cd69ea..6159b5256ee 100644
--- a/tests/ui/map_flatten.stderr
+++ b/tests/ui/map_flatten.stderr
@@ -1,3 +1,12 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_flatten.rs:14:5
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id).flatten().collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::let-underscore-drop` implied by `-D warnings`
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `map(..).flatten()` on an `Iterator`
   --> $DIR/map_flatten.rs:14:46
    |
@@ -6,24 +15,56 @@ LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id).flatten().coll
    |
    = note: `-D clippy::map-flatten` implied by `-D warnings`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_flatten.rs:15:5
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_ref).flatten().collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `map(..).flatten()` on an `Iterator`
   --> $DIR/map_flatten.rs:15:46
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_ref).flatten().collect();
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `filter_map` instead: `.filter_map(option_id_ref)`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_flatten.rs:16:5
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_closure).flatten().collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `map(..).flatten()` on an `Iterator`
   --> $DIR/map_flatten.rs:16:46
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_closure).flatten().collect();
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `filter_map` instead: `.filter_map(option_id_closure)`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_flatten.rs:17:5
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| x.checked_add(1)).flatten().collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `map(..).flatten()` on an `Iterator`
   --> $DIR/map_flatten.rs:17:46
    |
 LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| x.checked_add(1)).flatten().collect();
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `filter_map` instead: `.filter_map(|x| x.checked_add(1))`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_flatten.rs:20:5
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: called `map(..).flatten()` on an `Iterator`
   --> $DIR/map_flatten.rs:20:46
    |
@@ -36,5 +77,5 @@ error: called `map(..).flatten()` on an `Option`
 LL |     let _: Option<_> = (Some(Some(1))).map(|x| x).flatten();
    |                                       ^^^^^^^^^^^^^^^^^^^^^ help: try using `and_then` instead: `.and_then(|x| x)`
 
-error: aborting due to 6 previous errors
+error: aborting due to 11 previous errors
 
diff --git a/tests/ui/map_identity.stderr b/tests/ui/map_identity.stderr
index e4a0320cbda..6bfeb186bad 100644
--- a/tests/ui/map_identity.stderr
+++ b/tests/ui/map_identity.stderr
@@ -1,3 +1,12 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_identity.rs:8:5
+   |
+LL |     let _: Vec<_> = x.iter().map(not_identity).map(|x| return x).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: unnecessary map of the identity function
   --> $DIR/map_identity.rs:8:47
    |
@@ -6,6 +15,14 @@ LL |     let _: Vec<_> = x.iter().map(not_identity).map(|x| return x).collect();
    |
    = note: `-D clippy::map-identity` implied by `-D warnings`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_identity.rs:9:5
+   |
+LL |     let _: Vec<_> = x.iter().map(std::convert::identity).map(|y| y).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: unnecessary map of the identity function
   --> $DIR/map_identity.rs:9:57
    |
@@ -33,5 +50,21 @@ LL | |         return x;
 LL | |     });
    | |______^ help: remove the call to `map`
 
-error: aborting due to 5 previous errors
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_identity.rs:15:5
+   |
+LL |     let _: Vec<_> = x.iter().map(|x| 2 * x).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/map_identity.rs:16:5
+   |
+LL |     let _: Vec<_> = x.iter().map(not_identity).map(|x| return x - 4).collect();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to 9 previous errors
 
diff --git a/tests/ui/match_single_binding.stderr b/tests/ui/match_single_binding.stderr
index 795c8c3e24d..8b07599817b 100644
--- a/tests/ui/match_single_binding.stderr
+++ b/tests/ui/match_single_binding.stderr
@@ -150,6 +150,20 @@ LL |     let Point { x, y } = coords();
 LL |     let product = x * y;
    |
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/match_single_binding.rs:96:5
+   |
+LL | /     let _ = v
+LL | |         .iter()
+LL | |         .map(|i| match i.unwrap() {
+LL | |             unwrapped => unwrapped,
+LL | |         })
+LL | |         .collect::<Vec<u8>>();
+   | |______________________________^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: this match could be written as a `let` statement
   --> $DIR/match_single_binding.rs:98:18
    |
@@ -167,5 +181,5 @@ LL |             unwrapped
 LL |         })
    |
 
-error: aborting due to 11 previous errors
+error: aborting due to 12 previous errors
 
diff --git a/tests/ui/needless_pass_by_value.stderr b/tests/ui/needless_pass_by_value.stderr
index 9aa783bf904..cf04ee4b257 100644
--- a/tests/ui/needless_pass_by_value.stderr
+++ b/tests/ui/needless_pass_by_value.stderr
@@ -90,6 +90,15 @@ help: change `v.clone()` to
 LL |     let _ = v.to_owned();
    |             ^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/needless_pass_by_value.rs:85:5
+   |
+LL |     let _ = v.clone();
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: this argument is passed by value, but not consumed in the function body
   --> $DIR/needless_pass_by_value.rs:94:12
    |
@@ -174,5 +183,5 @@ error: this argument is passed by value, but not consumed in the function body
 LL | fn more_fun(_item: impl Club<'static, i32>) {}
    |                    ^^^^^^^^^^^^^^^^^^^^^^^ help: consider taking a reference instead: `&impl Club<'static, i32>`
 
-error: aborting due to 22 previous errors
+error: aborting due to 23 previous errors
 
diff --git a/tests/ui/redundant_clone.stderr b/tests/ui/redundant_clone.stderr
index 89b39254299..270c3fac990 100644
--- a/tests/ui/redundant_clone.stderr
+++ b/tests/ui/redundant_clone.stderr
@@ -167,5 +167,14 @@ note: cloned value is neither consumed nor mutated
 LL |     let y = x.clone().join("matthias");
    |             ^^^^^^^^^
 
-error: aborting due to 14 previous errors
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/redundant_clone.rs:180:5
+   |
+LL |     let _ = a.clone(); // OK
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
+error: aborting due to 15 previous errors
 
diff --git a/tests/ui/reversed_empty_ranges_fixable.stderr b/tests/ui/reversed_empty_ranges_fixable.stderr
index de83c4f3d63..707a5d5032e 100644
--- a/tests/ui/reversed_empty_ranges_fixable.stderr
+++ b/tests/ui/reversed_empty_ranges_fixable.stderr
@@ -10,6 +10,15 @@ help: consider using the following if you are attempting to iterate over this ra
 LL |     (21..=42).rev().for_each(|x| println!("{}", x));
    |     ^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/reversed_empty_ranges_fixable.rs:10:5
+   |
+LL |     let _ = (ANSWER..21).filter(|x| x % 2 == 0).take(10).collect::<Vec<_>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: this range is empty so it will yield no values
   --> $DIR/reversed_empty_ranges_fixable.rs:10:13
    |
@@ -43,5 +52,5 @@ help: consider using the following if you are attempting to iterate over this ra
 LL |     for _ in (21u32..42u32).rev() {}
    |              ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
diff --git a/tests/ui/transmute.stderr b/tests/ui/transmute.stderr
index ad9953d12bc..d6767dc9f15 100644
--- a/tests/ui/transmute.stderr
+++ b/tests/ui/transmute.stderr
@@ -24,30 +24,71 @@ error: transmute from a reference to a pointer
 LL |     let _: *const U = core::intrinsics::transmute(t);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `t as *const T as *const U`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute.rs:34:9
+   |
+LL |         let _: Vec<i32> = core::intrinsics::transmute(my_vec());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from a type (`std::vec::Vec<i32>`) to itself
   --> $DIR/transmute.rs:34:27
    |
 LL |         let _: Vec<i32> = core::intrinsics::transmute(my_vec());
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute.rs:36:9
+   |
+LL |         let _: Vec<i32> = core::mem::transmute(my_vec());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from a type (`std::vec::Vec<i32>`) to itself
   --> $DIR/transmute.rs:36:27
    |
 LL |         let _: Vec<i32> = core::mem::transmute(my_vec());
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute.rs:38:9
+   |
+LL |         let _: Vec<i32> = std::intrinsics::transmute(my_vec());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from a type (`std::vec::Vec<i32>`) to itself
   --> $DIR/transmute.rs:38:27
    |
 LL |         let _: Vec<i32> = std::intrinsics::transmute(my_vec());
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute.rs:40:9
+   |
+LL |         let _: Vec<i32> = std::mem::transmute(my_vec());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from a type (`std::vec::Vec<i32>`) to itself
   --> $DIR/transmute.rs:40:27
    |
 LL |         let _: Vec<i32> = std::mem::transmute(my_vec());
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute.rs:42:9
+   |
+LL |         let _: Vec<i32> = my_transmute(my_vec());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from a type (`std::vec::Vec<i32>`) to itself
   --> $DIR/transmute.rs:42:27
    |
@@ -154,5 +195,5 @@ error: transmute from a `&mut [u8]` to a `&mut str`
 LL |     let _: &mut str = unsafe { std::mem::transmute(mb) };
    |                                ^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::str::from_utf8_mut(mb).unwrap()`
 
-error: aborting due to 24 previous errors
+error: aborting due to 29 previous errors
 
diff --git a/tests/ui/transmute_collection.stderr b/tests/ui/transmute_collection.stderr
index ebc05c402ab..e89f6d8539f 100644
--- a/tests/ui/transmute_collection.stderr
+++ b/tests/ui/transmute_collection.stderr
@@ -1,3 +1,12 @@
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:9:9
+   |
+LL |         let _ = transmute::<_, Vec<u32>>(vec![0u8]);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::vec::Vec<u8>` to `std::vec::Vec<u32>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:9:17
    |
@@ -6,18 +15,42 @@ LL |         let _ = transmute::<_, Vec<u32>>(vec![0u8]);
    |
    = note: `-D clippy::unsound-collection-transmute` implied by `-D warnings`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:11:9
+   |
+LL |         let _ = transmute::<_, Vec<[u8; 4]>>(vec![1234u32]);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::vec::Vec<u32>` to `std::vec::Vec<[u8; 4]>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:11:17
    |
 LL |         let _ = transmute::<_, Vec<[u8; 4]>>(vec![1234u32]);
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:14:9
+   |
+LL |         let _ = transmute::<_, VecDeque<u32>>(VecDeque::<u8>::new());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::collections::VecDeque<u8>` to `std::collections::VecDeque<u32>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:14:17
    |
 LL |         let _ = transmute::<_, VecDeque<u32>>(VecDeque::<u8>::new());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:16:9
+   |
+LL |         let _ = transmute::<_, VecDeque<u32>>(VecDeque::<[u8; 4]>::new());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::collections::VecDeque<[u8; 4]>` to `std::collections::VecDeque<u32>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:16:17
    |
@@ -60,24 +93,56 @@ error: transmute from `std::collections::HashSet<[u8; 4]>` to `std::collections:
 LL |         let _ = transmute::<_, HashSet<u32>>(HashSet::<[u8; 4]>::new());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:34:9
+   |
+LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, u8>::new());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::collections::BTreeMap<u8, u8>` to `std::collections::BTreeMap<u8, u32>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:34:17
    |
 LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, u8>::new());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:35:9
+   |
+LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u32, u32>::new());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::collections::BTreeMap<u32, u32>` to `std::collections::BTreeMap<u8, u32>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:35:17
    |
 LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u32, u32>::new());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:37:9
+   |
+LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, [u8; 4]>::new());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::collections::BTreeMap<u8, [u8; 4]>` to `std::collections::BTreeMap<u8, u32>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:37:17
    |
 LL |         let _ = transmute::<_, BTreeMap<u8, u32>>(BTreeMap::<u8, [u8; 4]>::new());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/transmute_collection.rs:38:9
+   |
+LL |         let _ = transmute::<_, BTreeMap<u32, u32>>(BTreeMap::<[u8; 4], u32>::new());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: transmute from `std::collections::BTreeMap<[u8; 4], u32>` to `std::collections::BTreeMap<u32, u32>` with mismatched layout is unsound
   --> $DIR/transmute_collection.rs:38:17
    |
@@ -108,5 +173,5 @@ error: transmute from `std::collections::HashMap<[u8; 4], u32>` to `std::collect
 LL |         let _ = transmute::<_, HashMap<u32, u32>>(HashMap::<[u8; 4], u32>::new());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 18 previous errors
+error: aborting due to 26 previous errors
 
diff --git a/tests/ui/unnecessary_clone.stderr b/tests/ui/unnecessary_clone.stderr
index 5ffa6c4fd06..1f89cb0cdef 100644
--- a/tests/ui/unnecessary_clone.stderr
+++ b/tests/ui/unnecessary_clone.stderr
@@ -24,6 +24,15 @@ error: using `.clone()` on a ref-counted pointer
 LL |     arc_weak.clone();
    |     ^^^^^^^^^^^^^^^^ help: try this: `Weak::<bool>::clone(&arc_weak)`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/unnecessary_clone.rs:36:5
+   |
+LL |     let _: Arc<dyn SomeTrait> = x.clone();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: using `.clone()` on a ref-counted pointer
   --> $DIR/unnecessary_clone.rs:36:33
    |
@@ -102,5 +111,5 @@ error: using `.clone()` on a ref-counted pointer
 LL |         Some(try_opt!(Some(rc)).clone())
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `Rc::<u8>::clone(&try_opt!(Some(rc)))`
 
-error: aborting due to 12 previous errors
+error: aborting due to 13 previous errors
 
diff --git a/tests/ui/useless_conversion.stderr b/tests/ui/useless_conversion.stderr
index 11c6efb25cc..ea3e96111cb 100644
--- a/tests/ui/useless_conversion.stderr
+++ b/tests/ui/useless_conversion.stderr
@@ -52,6 +52,15 @@ error: useless conversion to the same type: `std::str::Lines`
 LL |     let _ = "".lines().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `"".lines()`
 
+error: non-binding let on a type that implements `Drop`
+  --> $DIR/useless_conversion.rs:65:5
+   |
+LL |     let _ = vec![1, 2, 3].into_iter().into_iter();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(clippy::let_underscore_drop)]` on by default
+   = help: consider using an underscore-prefixed named binding or dropping explicitly with `std::mem::drop`
+
 error: useless conversion to the same type: `std::vec::IntoIter<i32>`
   --> $DIR/useless_conversion.rs:65:13
    |
@@ -70,5 +79,5 @@ error: useless conversion to the same type: `i32`
 LL |     let _ = i32::from(a + b) * 3;
    |             ^^^^^^^^^^^^^^^^ help: consider removing `i32::from()`: `(a + b)`
 
-error: aborting due to 11 previous errors
+error: aborting due to 12 previous errors