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.
This commit is contained in:
Esteban Küber 2022-08-16 07:56:42 -07:00
parent 50bb7a40e1
commit 243496e129
4 changed files with 89 additions and 39 deletions

View File

@ -9,7 +9,7 @@
use rustc_hir::def_id::DefId;
use rustc_infer::traits::util::elaborate_predicates_with_span;
use rustc_middle::ty::adjustment;
use rustc_middle::ty::{self, Ty};
use rustc_middle::ty::{self, DefIdTree, Ty};
use rustc_span::symbol::Symbol;
use rustc_span::symbol::{kw, sym};
use rustc_span::{BytePos, Span};
@ -93,6 +93,18 @@ fn check_stmt(&mut self, cx: &LateContext<'_>, s: &hir::Stmt<'_>) {
return;
}
if let hir::ExprKind::Match(await_expr, _arms, hir::MatchSource::AwaitDesugar) = expr.kind
&& let ty = cx.typeck_results().expr_ty(&await_expr)
&& let ty::Opaque(def_id, _) = ty.kind()
&& cx.tcx.ty_is_opaque_future(ty)
&& let parent = cx.tcx.parent(*def_id)
&& check_must_use_def(cx, parent, expr.span, "awaited future returned by ", "")
{
// We have a bare `foo().await;` on an opaque type from an async function that was
// annotated with `#[must_use]`.
return;
}
let ty = cx.typeck_results().expr_ty(&expr);
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(
sym::collapse_debuginfo => self.check_collapse_debuginfo(attr, span, target),
sym::const_trait => self.check_const_trait(attr, span, target),
sym::must_not_suspend => self.check_must_not_suspend(&attr, span, target),
sym::must_use => self.check_must_use(hir_id, &attr, span, target),
sym::must_use => self.check_must_use(hir_id, &attr, target),
sym::rustc_pass_by_value => self.check_pass_by_value(&attr, span, target),
sym::rustc_allow_incoherent_impl => {
self.check_allow_incoherent_impl(&attr, span, target)
@ -1163,17 +1163,7 @@ fn check_has_incoherent_inherent_impls(
}
/// Warns against some misuses of `#[must_use]`
fn check_must_use(&self, hir_id: HirId, attr: &Attribute, span: Span, target: Target) -> bool {
let node = self.tcx.hir().get(hir_id);
if let Some(kind) = node.fn_kind() && let rustc_hir::IsAsync::Async = kind.asyncness() {
self.tcx.emit_spanned_lint(
UNUSED_ATTRIBUTES,
hir_id,
attr.span,
errors::MustUseAsync { span }
);
}
fn check_must_use(&self, hir_id: HirId, attr: &Attribute, target: Target) -> bool {
if !matches!(
target,
Target::Fn

View File

@ -1,24 +1,43 @@
// edition:2018
// run-pass
#![allow(dead_code)]
#![deny(unused_must_use)]
#[must_use]
//~^ WARNING `must_use`
async fn test() -> i32 {
async fn foo() -> i32 {
1
}
#[must_use]
fn bar() -> impl std::future::Future<Output=i32> {
async {
42
}
}
async fn baz() -> i32 {
0
}
struct Wowee {}
impl Wowee {
#[must_use]
//~^ WARNING `must_use`
async fn test_method() -> i32 {
1
}
}
async fn test() {
foo(); //~ ERROR unused return value of `foo` that must be used
//~^ ERROR unused implementer of `Future` that must be used
foo().await; //~ ERROR unused awaited future returned by `foo` that must be used
bar(); //~ ERROR unused return value of `bar` that must be used
//~^ ERROR unused implementer of `Future` that must be used
bar().await; //~ ERROR unused awaited future returned by `bar` that must be used
baz(); //~ ERROR unused implementer of `Future` that must be used
baz().await; // ok
}
/* FIXME(guswynn) update this test when async-fn-in-traits works
trait Doer {
#[must_use]

View File

@ -1,26 +1,55 @@
warning: `must_use` attribute on `async` functions applies to the anonymous `Future` returned by the function, not the value within
--> $DIR/unused-async.rs:5:1
error: unused implementer of `Future` that must be used
--> $DIR/unused-async.rs:31:5
|
LL | #[must_use]
| ^^^^^^^^^^^
LL |
LL | / async fn test() -> i32 {
LL | | 1
LL | | }
| |_- this attribute does nothing, the `Future`s returned by async functions are already `must_use`
LL | foo();
| ^^^^^
|
= note: `#[warn(unused_attributes)]` on by default
warning: `must_use` attribute on `async` functions applies to the anonymous `Future` returned by the function, not the value within
--> $DIR/unused-async.rs:15:5
note: the lint level is defined here
--> $DIR/unused-async.rs:2:9
|
LL | #[must_use]
| ^^^^^^^^^^^
LL |
LL | / async fn test_method() -> i32 {
LL | | 1
LL | | }
| |_____- this attribute does nothing, the `Future`s returned by async functions are already `must_use`
LL | #![deny(unused_must_use)]
| ^^^^^^^^^^^^^^^
= note: futures do nothing unless you `.await` or poll them
warning: 2 warnings emitted
error: unused return value of `foo` that must be used
--> $DIR/unused-async.rs:31:5
|
LL | foo();
| ^^^^^
error: unused awaited future returned by `foo` that must be used
--> $DIR/unused-async.rs:33:5
|
LL | foo().await;
| ^^^^^^^^^^^
error: unused implementer of `Future` that must be used
--> $DIR/unused-async.rs:34:5
|
LL | bar();
| ^^^^^
|
= note: futures do nothing unless you `.await` or poll them
error: unused return value of `bar` that must be used
--> $DIR/unused-async.rs:34:5
|
LL | bar();
| ^^^^^
error: unused awaited future returned by `bar` that must be used
--> $DIR/unused-async.rs:36:5
|
LL | bar().await;
| ^^^^^^^^^^^
error: unused implementer of `Future` that must be used
--> $DIR/unused-async.rs:37:5
|
LL | baz();
| ^^^^^
|
= note: futures do nothing unless you `.await` or poll them
error: aborting due to 7 previous errors