Fix false negative on Result<(), ()>

Author: blyxyas

clippy_lints/src/functions/must_use.rs

@@ -3,6 +3,7 @@ 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,
@@ -113,7 +114,16 @@ fn check_needless_must_use(
                 diag.span_suggestion(attr.span, "remove the attribute", "", Applicability::MachineApplicable);
             },
         );
-    } else if attr.value_str().is_none() && is_must_use_ty(cx, return_ty(cx, item_id)) && !sig.header.is_async() {
+    } 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,

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