Add discarded_futures lint

Author: Manishearth

CHANGELOG.md

@@ -5918,6 +5918,7 @@ Released 2018-09-13
 [`disallowed_script_idents`]: https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_script_idents
 [`disallowed_type`]: https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_type
 [`disallowed_types`]: https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_types
+[`discarded_future`]: https://rust-lang.github.io/rust-clippy/master/index.html#discarded_future
 [`diverging_sub_expression`]: https://rust-lang.github.io/rust-clippy/master/index.html#diverging_sub_expression
 [`doc_broken_link`]: https://rust-lang.github.io/rust-clippy/master/index.html#doc_broken_link
 [`doc_comment_double_space_linebreaks`]: https://rust-lang.github.io/rust-clippy/master/index.html#doc_comment_double_space_linebreaks

clippy_lints/src/declared_lints.rs

@@ -113,6 +113,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::disallowed_names::DISALLOWED_NAMES_INFO,
     crate::disallowed_script_idents::DISALLOWED_SCRIPT_IDENTS_INFO,
     crate::disallowed_types::DISALLOWED_TYPES_INFO,
+    crate::discarded_future::DISCARDED_FUTURE_INFO,
     crate::doc::DOC_BROKEN_LINK_INFO,
     crate::doc::DOC_COMMENT_DOUBLE_SPACE_LINEBREAKS_INFO,
     crate::doc::DOC_INCLUDE_WITHOUT_CFG_INFO,

clippy_lints/src/discarded_future.rs

@@ -0,0 +1,56 @@
+use rustc_hir::*;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet;
+use clippy_utils::ty::implements_trait;
+use rustc_errors::Applicability;
+use rustc_middle::ty;
+use rustc_span::sym;
+use clippy_utils::ty::is_type_diagnostic_item;
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// ### Why restrict this?
+    ///
+    /// ### Example
+    /// ```no_run
+    /// // example code where clippy issues a warning
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// // example code which does not raise clippy warning
+    /// ```
+    #[clippy::version = "1.91.0"]
+    pub DISCARDED_FUTURE,
+    restriction,
+    "default lint description"
+}
+declare_lint_pass!(DiscardedFuture => [DISCARDED_FUTURE]);
+
+impl<'tcx> LateLintPass<'tcx> for DiscardedFuture {
+    fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'tcx>) {
+        if let StmtKind::Let(let_stmt) = stmt.kind
+            && let PatKind::Wild = let_stmt.pat.kind
+            && let Some(expr) = let_stmt.init
+            && let ty = cx.typeck_results().expr_ty(expr)
+            && is_type_diagnostic_item(cx, ty, sym::Result)
+            && let ty::Adt(_, substs) = ty.kind()
+            && let Some(inner_ty) = substs[0].as_type()
+            && let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait()
+            && implements_trait(cx, inner_ty, future_trait_def_id, &[])
+        {
+            span_lint_and_sugg(
+                cx,
+                DISCARDED_FUTURE,
+                expr.span,
+                format!("Discarding a Result<Future>: did you mean to call .await on this first?"),
+                "consider `.await` on it",
+                format!("{}.await", snippet(cx, expr.span, "..")),
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+}

clippy_lints/src/discarded_futures.rs

@@ -118,6 +118,7 @@ mod disallowed_methods;
 mod disallowed_names;
 mod disallowed_script_idents;
 mod disallowed_types;
+mod discarded_future;
 mod doc;
 mod double_parens;
 mod drop_forget_ref;
@@ -831,5 +832,6 @@ pub fn register_lint_passes(store: &mut rustc_lint::LintStore, conf: &'static Co
     store.register_late_pass(|_| Box::new(cloned_ref_to_slice_refs::ClonedRefToSliceRefs::new(conf)));
     store.register_late_pass(|_| Box::new(infallible_try_from::InfallibleTryFrom));
     store.register_late_pass(|_| Box::new(coerce_container_to_any::CoerceContainerToAny));
+    store.register_late_pass(|_| Box::new(discarded_future::DiscardedFuture));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

clippy_lints/src/lib.rs

@@ -0,0 +1,19 @@
+#![warn(clippy::discarded_future)]
+#![allow(clippy::result_unit_err)]
+
+pub fn result_future() -> Result<impl std::future::Future<Output = ()>, ()> {
+    Ok(async {
+        dbg!("hello im in a future");
+    })
+}
+
+pub async fn calls_result_future() {
+    // some async stuff
+    let _ = result_future().await;
+    //~^ discarded_future
+    // more async stuff
+}
+
+fn main() {
+
+}

tests/ui/discarded_future.fixed

@@ -0,0 +1,19 @@
+#![warn(clippy::discarded_future)]
+#![allow(clippy::result_unit_err)]
+
+pub fn result_future() -> Result<impl std::future::Future<Output = ()>, ()> {
+    Ok(async {
+        dbg!("hello im in a future");
+    })
+}
+
+pub async fn calls_result_future() {
+    // some async stuff
+    let _ = result_future();
+    //~^ discarded_future
+    // more async stuff
+}
+
+fn main() {
+
+}

tests/ui/discarded_future.rs

@@ -0,0 +1,11 @@
+error: Discarding a Result<Future>: did you mean to call .await on this first?
+  --> tests/ui/discarded_future.rs:12:13
+   |
+LL |     let _ = result_future();
+   |             ^^^^^^^^^^^^^^^ help: consider `.await` on it: `result_future().await`
+   |
+   = note: `-D clippy::discarded-future` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::discarded_future)]`
+
+error: aborting due to 1 previous error
+