Auto merge of #10589 - blyxyas:fix-double_must_use, r=giraffate

Mini-fix `double_must_use` for async functions

From Rust 1.67 onwards, the `#[must_use]` attribute also applies to the `Future::Output` (rust-lang/rust#100633). So the lint `double_must_use` was linting all async functions. This PR changes the `double_must_use` lint so it ignores `async` functions.

---

Closes #10486
changelog: [`double_must_use`]: Fix false positive in async function

3 files changed, 35 insertions, 4 deletions

diff --git a/clippy_lints/src/functions/must_use.rs b/clippy_lints/src/functions/must_use.rs
index 1e9e826631c..d0ad2628264 100644
--- a/clippy_lints/src/functions/must_use.rs
+++ b/clippy_lints/src/functions/must_use.rs
@@ -1,7 +1,9 @@
+use hir::FnSig;
 use rustc_ast::ast::Attribute;
 use rustc_errors::Applicability;
 use rustc_hir::def_id::DefIdSet;
 use rustc_hir::{self as hir, def::Res, QPath};
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::{LateContext, LintContext};
 use rustc_middle::{
     lint::in_external_macro,
@@ -27,7 +29,7 @@ pub(super) fn check_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>
         let is_public = cx.effective_visibilities.is_exported(item.owner_id.def_id);
         let fn_header_span = item.span.with_hi(sig.decl.output.span().hi());
         if let Some(attr) = attr {
-            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr);
+            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr, sig);
         } else if is_public && !is_proc_macro(attrs) && !attrs.iter().any(|a| a.has_name(sym::no_mangle)) {
             check_must_use_candidate(
                 cx,
@@ -49,7 +51,7 @@ pub(super) fn check_impl_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Imp
         let attrs = cx.tcx.hir().attrs(item.hir_id());
         let attr = cx.tcx.get_attr(item.owner_id, sym::must_use);
         if let Some(attr) = attr {
-            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr);
+            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr, sig);
         } else if is_public && !is_proc_macro(attrs) && trait_ref_of_method(cx, item.owner_id.def_id).is_none() {
             check_must_use_candidate(
                 cx,
@@ -72,7 +74,7 @@ pub(super) fn check_trait_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Tr
         let attrs = cx.tcx.hir().attrs(item.hir_id());
         let attr = cx.tcx.get_attr(item.owner_id, sym::must_use);
         if let Some(attr) = attr {
-            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr);
+            check_needless_must_use(cx, sig.decl, item.owner_id, item.span, fn_header_span, attr, sig);
         } else if let hir::TraitFn::Provided(eid) = *eid {
             let body = cx.tcx.hir().body(eid);
             if attr.is_none() && is_public && !is_proc_macro(attrs) {
@@ -97,6 +99,7 @@ fn check_needless_must_use(
     item_span: Span,
     fn_header_span: Span,
     attr: &Attribute,
+    sig: &FnSig<'_>,
 ) {
     if in_external_macro(cx.sess(), item_span) {
         return;
@@ -112,6 +115,15 @@ fn check_needless_must_use(
             },
         );
     } else if attr.value_str().is_none() && is_must_use_ty(cx, return_ty(cx, item_id)) {
+        // Ignore async functions unless Future::Output type is a must_use type
+        if sig.header.is_async() {
+            let infcx = cx.tcx.infer_ctxt().build();
+            if let Some(future_ty) = infcx.get_impl_future_output_ty(return_ty(cx, item_id))
+			&& !is_must_use_ty(cx, future_ty) {
+				return;
+			}
+        }
+
         span_lint_and_help(
             cx,
             DOUBLE_MUST_USE,
diff --git a/tests/ui/double_must_use.rs b/tests/ui/double_must_use.rs
index 05e087b08bc..26a387b3cf0 100644
--- a/tests/ui/double_must_use.rs
+++ b/tests/ui/double_must_use.rs
@@ -21,6 +21,17 @@ pub fn must_use_with_note() -> Result<(), ()> {
     unimplemented!();
 }
 
+// vvvv Should not lint (#10486)
+#[must_use]
+async fn async_must_use() -> usize {
+    unimplemented!();
+}
+
+#[must_use]
+async fn async_must_use_result() -> Result<(), ()> {
+    Ok(())
+}
+
 fn main() {
     must_use_result();
     must_use_tuple();
diff --git a/tests/ui/double_must_use.stderr b/tests/ui/double_must_use.stderr
index 3d34557a881..49ab2ea3e12 100644
--- a/tests/ui/double_must_use.stderr
+++ b/tests/ui/double_must_use.stderr
@@ -23,5 +23,13 @@ LL | pub fn must_use_array() -> [Result<(), ()>; 1] {
    |
    = help: either add some descriptive text or remove the attribute
 
-error: aborting due to 3 previous errors
+error: this function has an empty `#[must_use]` attribute, but returns a type already marked as `#[must_use]`
+  --> $DIR/double_must_use.rs:31:1
+   |
+LL | async fn async_must_use_result() -> Result<(), ()> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: either add some descriptive text or remove the attribute
+
+error: aborting due to 4 previous errors