[double_must_use]: make the lint machine-applicable in single-attribute case

clippy_lints/src/functions/must_use.rs

@@ -10,7 +10,7 @@ use rustc_middle::ty::{self, Ty};
 use rustc_span::{Span, sym};
 
 use clippy_utils::attrs::is_proc_macro;
-use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_then};
+use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::snippet_indent;
 use clippy_utils::ty::is_must_use_ty;
 use clippy_utils::visitors::for_each_expr_without_closures;
@@ -145,31 +145,24 @@ fn check_needless_must_use(
         return;
     }
     if returns_unit(decl) {
-        if attrs.len() == 1 {
-            span_lint_and_then(
-                cx,
-                MUST_USE_UNIT,
-                fn_header_span,
-                "this unit-returning function has a `#[must_use]` attribute",
-                |diag| {
+        span_lint_and_then(
+            cx,
+            MUST_USE_UNIT,
+            fn_header_span,
+            "this unit-returning function has a `#[must_use]` attribute",
+            |diag| {
+                // When there are multiple attributes, it is not sufficient to simply make `must_use` empty, see
+                // issue #12320.
+                // FIXME(jdonszelmann): this used to give a machine-applicable fix. However, it was super fragile,
+                // honestly looked incorrect, and is a little hard to support for a little bit now. Some day this
+                // could be re-added.
+                if attrs.len() == 1 {
                     diag.span_suggestion(attr_span, "remove the attribute", "", Applicability::MachineApplicable);
-                },
-            );
-        } else {
-            // When there are multiple attributes, it is not sufficient to simply make `must_use` empty, see
-            // issue #12320.
-            // FIXME(jdonszelmann): this used to give a machine-applicable fix. However, it was super fragile,
-            // honestly looked incorrect, and is a little hard to support for a little bit now. Some day this
-            // could be re-added.
-            span_lint_and_help(
-                cx,
-                MUST_USE_UNIT,
-                fn_header_span,
-                "this unit-returning function has a `#[must_use]` attribute",
-                Some(attr_span),
-                "remove `must_use`",
-            );
-        }
+                } else {
+                    diag.span_help(attr_span, "remove `must_use`");
+                }
+            },
+        );
     } else if reason.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() {
@@ -181,13 +174,24 @@ fn check_needless_must_use(
             }
         }
 
-        span_lint_and_help(
+        span_lint_and_then(
             cx,
             DOUBLE_MUST_USE,
             fn_header_span,
             "this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`",
-            None,
-            "either add some descriptive message or remove the attribute",
+            |diag| {
+                // When there are multiple attributes, it is not sufficient to simply make `must_use` empty, see
+                // issue #12320.
+                // FIXME(jdonszelmann): this used to give a machine-applicable fix. However, it was super fragile,
+                // honestly looked incorrect, and is a little hard to support for a little bit now. Some day this
+                // could be re-added.
+                if attrs.len() == 1 {
+                    diag.span_suggestion(attr_span, "remove the attribute", "", Applicability::MachineApplicable);
+                } else {
+                    diag.span_help(attr_span, "remove `must_use`");
+                }
+                diag.note("alternatively, you may add an explicit reason to the `must_use` attribute");
+            },
         );
     }
 }

tests/ui/double_must_use.fixed

@@ -0,0 +1,65 @@
+#![warn(clippy::double_must_use)]
+#![expect(clippy::result_unit_err)]
+#![feature(never_type)]
+
+use std::ops::ControlFlow;
+
+pub fn must_use_result() -> Result<(), ()> {
+    //~^ double_must_use
+
+    unimplemented!();
+}
+
+pub fn must_use_tuple() -> (Result<(), ()>, u8) {
+    //~^ double_must_use
+
+    unimplemented!();
+}
+
+pub fn must_use_array() -> [Result<(), ()>; 1] {
+    //~^ double_must_use
+
+    unimplemented!();
+}
+
+#[must_use = "With note"]
+pub fn must_use_with_note() -> Result<(), ()> {
+    unimplemented!();
+}
+
+// vvvv Should not lint (#10486)
+#[must_use]
+async fn async_must_use() -> usize {
+    unimplemented!();
+}
+
+async fn async_must_use_result() -> Result<(), ()> {
+    //~^ double_must_use
+
+    Ok(())
+}
+
+#[must_use]
+pub fn must_use_result_with_uninhabited() -> Result<(), !> {
+    unimplemented!();
+}
+
+#[must_use]
+pub struct T;
+
+pub fn must_use_result_with_uninhabited_2() -> Result<T, !> {
+    //~^ double_must_use
+    unimplemented!();
+}
+
+#[must_use]
+pub fn must_use_controlflow_with_uninhabited() -> ControlFlow<std::convert::Infallible> {
+    unimplemented!();
+}
+
+pub fn must_use_controlflow_with_uninhabited_2() -> ControlFlow<std::convert::Infallible, T> {
+    //~^ double_must_use
+    unimplemented!();
+}
+
+fn main() {}

tests/ui/double_must_use.stderr

@@ -1,52 +1,64 @@
 error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
   --> tests/ui/double_must_use.rs:8:1
    |
+LL | #[must_use]
+   | ----------- help: remove the attribute
 LL | pub fn must_use_result() -> Result<(), ()> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: either add some descriptive message or remove the attribute
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
    = note: `-D clippy::double-must-use` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::double_must_use)]`
 
 error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
   --> tests/ui/double_must_use.rs:15:1
    |
+LL | #[must_use]
+   | ----------- help: remove the attribute
 LL | pub fn must_use_tuple() -> (Result<(), ()>, u8) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: either add some descriptive message or remove the attribute
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
 
 error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
   --> tests/ui/double_must_use.rs:22:1
    |
+LL | #[must_use]
+   | ----------- help: remove the attribute
 LL | pub fn must_use_array() -> [Result<(), ()>; 1] {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: either add some descriptive message or remove the attribute
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
 
 error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
   --> tests/ui/double_must_use.rs:40:1
    |
+LL | #[must_use]
+   | ----------- help: remove the attribute
 LL | async fn async_must_use_result() -> Result<(), ()> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: either add some descriptive message or remove the attribute
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
 
 error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
   --> tests/ui/double_must_use.rs:55:1
    |
+LL | #[must_use]
+   | ----------- help: remove the attribute
 LL | pub fn must_use_result_with_uninhabited_2() -> Result<T, !> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: either add some descriptive message or remove the attribute
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
 
 error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
   --> tests/ui/double_must_use.rs:66:1
    |
+LL | #[must_use]
+   | ----------- help: remove the attribute
 LL | pub fn must_use_controlflow_with_uninhabited_2() -> ControlFlow<std::convert::Infallible, T> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: either add some descriptive message or remove the attribute
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
 
 error: aborting due to 6 previous errors
 

tests/ui/double_must_use_unfixable.rs

@@ -0,0 +1,17 @@
+#![warn(clippy::double_must_use)]
+#![expect(clippy::result_unit_err)]
+#![feature(never_type)]
+
+#[cfg_attr(all(), must_use, deprecated)]
+pub fn issue_12320() -> Result<(), ()> {
+    //~^ double_must_use
+    unimplemented!();
+}
+
+#[cfg_attr(all(), deprecated, must_use)]
+pub fn issue_12320_2() -> Result<(), ()> {
+    //~^ double_must_use
+    unimplemented!();
+}
+
+fn main() {}

tests/ui/double_must_use_unfixable.stderr

@@ -0,0 +1,30 @@
+error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
+  --> tests/ui/double_must_use_unfixable.rs:6:1
+   |
+LL | pub fn issue_12320() -> Result<(), ()> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: remove `must_use`
+  --> tests/ui/double_must_use_unfixable.rs:5:19
+   |
+LL | #[cfg_attr(all(), must_use, deprecated)]
+   |                   ^^^^^^^^
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
+   = note: `-D clippy::double-must-use` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::double_must_use)]`
+
+error: this function has a `#[must_use]` attribute with no message, but returns a type already marked as `#[must_use]`
+  --> tests/ui/double_must_use_unfixable.rs:12:1
+   |
+LL | pub fn issue_12320_2() -> Result<(), ()> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: remove `must_use`
+  --> tests/ui/double_must_use_unfixable.rs:11:31
+   |
+LL | #[cfg_attr(all(), deprecated, must_use)]
+   |                               ^^^^^^^^
+   = note: alternatively, you may add an explicit reason to the `must_use` attribute
+
+error: aborting due to 2 previous errors
+