Consider #[must_use]
annotation on async fn
as also affecting the Future::Output
No longer lint against `#[must_use] async fn foo()`. When encountering a statement that awaits on a `Future`, check if the `Future`'s parent item is annotated with `#[must_use]` and emit a lint if so. This effectively makes `must_use` an annotation on the `Future::Output` instead of only the `Future` itself. Fix #78149.
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@ -9,7 +9,7 @@
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use rustc_hir::def_id::DefId;
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use rustc_infer::traits::util::elaborate_predicates_with_span;
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use rustc_middle::ty::adjustment;
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use rustc_middle::ty::{self, Ty};
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use rustc_middle::ty::{self, DefIdTree, Ty};
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use rustc_span::symbol::Symbol;
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use rustc_span::symbol::{kw, sym};
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use rustc_span::{BytePos, Span};
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@ -93,6 +93,18 @@ fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
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return;
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}
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if let hir::ExprKind::Match(await_expr, _arms, hir::MatchSource::AwaitDesugar) = expr.kind
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&& let ty = cx.typeck_results().expr_ty(&await_expr)
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&& let ty::Opaque(def_id, _) = ty.kind()
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&& cx.tcx.ty_is_opaque_future(ty)
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&& let parent = cx.tcx.parent(*def_id)
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&& check_must_use_def(cx, parent, expr.span, "awaited future returned by ", "")
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{
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// We have a bare `foo().await;` on an opaque type from an async function that was
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// annotated with `#[must_use]`.
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return;
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}
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let ty = cx.typeck_results().expr_ty(&expr);
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let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, expr.span, "", "", 1);
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@ -139,7 +139,7 @@ fn check_attributes(
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sym::collapse_debuginfo => self.check_collapse_debuginfo(attr, span, target),
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sym::const_trait => self.check_const_trait(attr, span, target),
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sym::must_not_suspend => self.check_must_not_suspend(&attr, span, target),
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sym::must_use => self.check_must_use(hir_id, &attr, span, target),
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sym::must_use => self.check_must_use(hir_id, &attr, target),
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sym::rustc_pass_by_value => self.check_pass_by_value(&attr, span, target),
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sym::rustc_allow_incoherent_impl => {
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self.check_allow_incoherent_impl(&attr, span, target)
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@ -1163,17 +1163,7 @@ fn check_has_incoherent_inherent_impls(
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}
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/// Warns against some misuses of `#[must_use]`
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fn check_must_use(&self, hir_id: HirId, attr: &Attribute, span: Span, target: Target) -> bool {
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let node = self.tcx.hir().get(hir_id);
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if let Some(kind) = node.fn_kind() && let rustc_hir::IsAsync::Async = kind.asyncness() {
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self.tcx.emit_spanned_lint(
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UNUSED_ATTRIBUTES,
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hir_id,
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attr.span,
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errors::MustUseAsync { span }
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);
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}
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fn check_must_use(&self, hir_id: HirId, attr: &Attribute, target: Target) -> bool {
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if !matches!(
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target,
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Target::Fn
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@ -1,24 +1,43 @@
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// edition:2018
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// run-pass
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#![allow(dead_code)]
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#![deny(unused_must_use)]
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#[must_use]
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//~^ WARNING `must_use`
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async fn test() -> i32 {
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async fn foo() -> i32 {
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1
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}
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#[must_use]
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fn bar() -> impl std::future::Future<Output=i32> {
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async {
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42
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}
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}
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async fn baz() -> i32 {
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0
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}
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struct Wowee {}
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impl Wowee {
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#[must_use]
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//~^ WARNING `must_use`
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async fn test_method() -> i32 {
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1
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}
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}
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async fn test() {
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foo(); //~ ERROR unused return value of `foo` that must be used
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//~^ ERROR unused implementer of `Future` that must be used
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foo().await; //~ ERROR unused awaited future returned by `foo` that must be used
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bar(); //~ ERROR unused return value of `bar` that must be used
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//~^ ERROR unused implementer of `Future` that must be used
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bar().await; //~ ERROR unused awaited future returned by `bar` that must be used
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baz(); //~ ERROR unused implementer of `Future` that must be used
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baz().await; // ok
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}
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/* FIXME(guswynn) update this test when async-fn-in-traits works
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trait Doer {
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#[must_use]
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@ -1,26 +1,55 @@
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warning: `must_use` attribute on `async` functions applies to the anonymous `Future` returned by the function, not the value within
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--> $DIR/unused-async.rs:5:1
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error: unused implementer of `Future` that must be used
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--> $DIR/unused-async.rs:31:5
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LL | #[must_use]
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| ^^^^^^^^^^^
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LL |
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LL | / async fn test() -> i32 {
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LL | | 1
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LL | | }
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| |_- this attribute does nothing, the `Future`s returned by async functions are already `must_use`
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LL | foo();
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| ^^^^^
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= note: `#[warn(unused_attributes)]` on by default
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warning: `must_use` attribute on `async` functions applies to the anonymous `Future` returned by the function, not the value within
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--> $DIR/unused-async.rs:15:5
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note: the lint level is defined here
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--> $DIR/unused-async.rs:2:9
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LL | #[must_use]
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| ^^^^^^^^^^^
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LL |
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LL | / async fn test_method() -> i32 {
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LL | | 1
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LL | | }
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| |_____- this attribute does nothing, the `Future`s returned by async functions are already `must_use`
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LL | #![deny(unused_must_use)]
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| ^^^^^^^^^^^^^^^
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= note: futures do nothing unless you `.await` or poll them
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warning: 2 warnings emitted
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error: unused return value of `foo` that must be used
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--> $DIR/unused-async.rs:31:5
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LL | foo();
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| ^^^^^
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error: unused awaited future returned by `foo` that must be used
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--> $DIR/unused-async.rs:33:5
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LL | foo().await;
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| ^^^^^^^^^^^
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error: unused implementer of `Future` that must be used
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--> $DIR/unused-async.rs:34:5
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LL | bar();
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| ^^^^^
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= note: futures do nothing unless you `.await` or poll them
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error: unused return value of `bar` that must be used
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--> $DIR/unused-async.rs:34:5
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LL | bar();
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| ^^^^^
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error: unused awaited future returned by `bar` that must be used
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--> $DIR/unused-async.rs:36:5
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LL | bar().await;
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| ^^^^^^^^^^^
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error: unused implementer of `Future` that must be used
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--> $DIR/unused-async.rs:37:5
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LL | baz();
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| ^^^^^
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= note: futures do nothing unless you `.await` or poll them
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error: aborting due to 7 previous errors
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