Add drop_result lint

CHANGELOG.md

@@ -5040,6 +5040,7 @@ Released 2018-09-13
 [`drop_copy`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_copy
 [`drop_non_drop`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_non_drop
 [`drop_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_ref
+[`drop_result`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_result
 [`duplicate_mod`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicate_mod
 [`duplicate_underscore_argument`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicate_underscore_argument
 [`duration_subsec`]: https://rust-lang.github.io/rust-clippy/master/index.html#duration_subsec

clippy_config/src/conf.rs

@@ -173,7 +173,9 @@ macro_rules! define_Conf {
                             }
                         })*
                         // ignore contents of the third_party key
-                        Ok(Field::third_party) => drop(map.next_value::<IgnoredAny>())
+                        Ok(Field::third_party) => {
+                            let _ = map.next_value::<IgnoredAny>();
+                        }
                     }
                 }
                 let conf = Conf { $($name: $name.unwrap_or_else(defaults::$name),)* };

clippy_dev/src/update_lints.rs

@@ -999,7 +999,7 @@ fn try_rename_file(old_name: &Path, new_name: &Path) -> bool {
     match fs::rename(old_name, new_name) {
         Ok(()) => true,
         Err(e) => {
-            drop(fs::remove_file(new_name));
+            let _ = fs::remove_file(new_name);
             if e.kind() == io::ErrorKind::NotFound {
                 false
             } else {

clippy_lints/src/declared_lints.rs

@@ -145,6 +145,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::drop_forget_ref::DROP_NON_DROP_INFO,
     crate::drop_forget_ref::FORGET_NON_DROP_INFO,
     crate::drop_forget_ref::MEM_FORGET_INFO,
+    crate::drop_result::DROP_RESULT_INFO,
     crate::duplicate_mod::DUPLICATE_MOD_INFO,
     crate::else_if_without_else::ELSE_IF_WITHOUT_ELSE_INFO,
     crate::empty_drop::EMPTY_DROP_INFO,

clippy_lints/src/drop_result.rs

@@ -0,0 +1,44 @@
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::ty::is_type_diagnostic_item;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `std::mem::drop(t)` where `t` is
+    /// a `Result`.
+    ///
+    /// ### Why is this bad?
+    /// Dropping a `Result` means that its value wasn't checked.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// # use std::mem;
+    /// mem::drop(Err::<(), &str>("something went wrong"));
+    /// ```
+    #[clippy::version = "1.76.0"]
+    pub DROP_RESULT,
+    pedantic,
+    "`mem::drop` usage on `Result` types"
+}
+
+declare_lint_pass!(DropResult => [
+    DROP_RESULT,
+]);
+
+impl<'tcx> LateLintPass<'tcx> for DropResult {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if let ExprKind::Call(path, [arg]) = expr.kind
+            && let ExprKind::Path(ref qpath) = path.kind
+            && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
+            && let Some(fn_name) = cx.tcx.get_diagnostic_name(def_id)
+            && fn_name == sym::mem_drop
+            && let arg_ty = cx.typeck_results().expr_ty(arg)
+            && is_type_diagnostic_item(cx, arg_ty, sym::Result)
+        {
+            span_lint(cx, DROP_RESULT, expr.span, "using `drop()` on a `Result` type");
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -108,6 +108,7 @@ mod disallowed_types;
 mod doc;
 mod double_parens;
 mod drop_forget_ref;
+mod drop_result;
 mod duplicate_mod;
 mod else_if_without_else;
 mod empty_drop;
@@ -1069,6 +1070,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(iter_without_into_iter::IterWithoutIntoIter));
     store.register_late_pass(|_| Box::new(iter_over_hash_type::IterOverHashType));
     store.register_late_pass(|_| Box::new(impl_hash_with_borrow_str_and_bytes::ImplHashWithBorrowStrBytes));
+    store.register_late_pass(|_| Box::new(drop_result::DropResult));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

tests/ui/drop_non_drop.rs

@@ -1,4 +1,5 @@
 #![warn(clippy::drop_non_drop)]
+#![allow(clippy::drop_result)]
 
 use core::mem::drop;
 

tests/ui/drop_non_drop.stderr

@@ -1,25 +1,25 @@
 error: call to `std::mem::drop` with a value that does not implement `Drop`. Dropping such a type only extends its contained lifetimes
-  --> $DIR/drop_non_drop.rs:22:5
+  --> $DIR/drop_non_drop.rs:23:5
    |
 LL |     drop(Foo);
    |     ^^^^^^^^^
    |
 note: argument has type `main::Foo`
-  --> $DIR/drop_non_drop.rs:22:10
+  --> $DIR/drop_non_drop.rs:23:10
    |
 LL |     drop(Foo);
    |          ^^^
    = note: `-D clippy::drop-non-drop` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::drop_non_drop)]`
 
 error: call to `std::mem::drop` with a value that does not implement `Drop`. Dropping such a type only extends its contained lifetimes
-  --> $DIR/drop_non_drop.rs:38:5
+  --> $DIR/drop_non_drop.rs:39:5
    |
 LL |     drop(Baz(Foo));
    |     ^^^^^^^^^^^^^^
    |
 note: argument has type `main::Baz<main::Foo>`
-  --> $DIR/drop_non_drop.rs:38:10
+  --> $DIR/drop_non_drop.rs:39:10
    |
 LL |     drop(Baz(Foo));
    |          ^^^^^^^^

tests/ui/drop_result.rs

@@ -0,0 +1,12 @@
+#![warn(clippy::drop_result)]
+
+fn make_result<T>(t: T) -> Result<T, ()> {
+    Ok(t)
+}
+
+fn main() {
+    drop(Ok::<String, ()>("a".to_string())); //~ ERROR: using `drop()` on a `Result` type
+    let x = Err::<(), String>("b".to_string());
+    drop(x); //~ ERROR: using `drop()` on a `Result` type
+    drop(make_result("a".to_string())); //~ ERROR: using `drop()` on a `Result` type
+}

tests/ui/drop_result.stderr

@@ -0,0 +1,23 @@
+error: using `drop()` on a `Result` type
+  --> $DIR/drop_result.rs:8:5
+   |
+LL |     drop(Ok::<String, ()>("a".to_string()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::drop-result` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::drop_result)]`
+
+error: using `drop()` on a `Result` type
+  --> $DIR/drop_result.rs:10:5
+   |
+LL |     drop(x);
+   |     ^^^^^^^
+
+error: using `drop()` on a `Result` type
+  --> $DIR/drop_result.rs:11:5
+   |
+LL |     drop(make_result("a".to_string()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+