Use #[must_use] determination from the compiler

Author: samueltardieu

clippy_lints/src/drop_forget_ref.rs

@@ -98,7 +98,7 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetRef {
                 sym::mem_drop
                     if !(arg_ty.needs_drop(cx.tcx, cx.typing_env())
                         || is_must_use_func_call(cx, arg)
-                        || is_must_use_ty(cx, arg_ty)
+                        || is_must_use_ty(cx, arg_ty, expr.hir_id)
                         || drop_is_single_call_in_arm) =>
                 {
                     (DROP_NON_DROP, DROP_NON_DROP_SUMMARY.into(), Some(arg.span))

clippy_lints/src/functions/must_use.rs

@@ -39,6 +39,7 @@ pub(super) fn check_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>
                 cx,
                 sig.decl,
                 item.owner_id,
+                item.hir_id(),
                 item.span,
                 fn_header_span,
                 *attr_span,
@@ -71,6 +72,7 @@ pub(super) fn check_impl_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Imp
                 cx,
                 sig.decl,
                 item.owner_id,
+                item.hir_id(),
                 item.span,
                 fn_header_span,
                 *attr_span,
@@ -104,6 +106,7 @@ pub(super) fn check_trait_item<'tcx>(cx: &LateContext<'tcx>, item: &'tcx hir::Tr
                 cx,
                 sig.decl,
                 item.owner_id,
+                item.hir_id(),
                 item.span,
                 fn_header_span,
                 *attr_span,
@@ -134,6 +137,7 @@ fn check_needless_must_use(
     cx: &LateContext<'_>,
     decl: &hir::FnDecl<'_>,
     item_id: hir::OwnerId,
+    item_hir_id: hir::HirId,
     item_span: Span,
     fn_header_span: Span,
     attr_span: Span,
@@ -170,12 +174,12 @@ fn check_needless_must_use(
                 "remove `must_use`",
             );
         }
-    } else if reason.is_none() && is_must_use_ty(cx, return_ty(cx, item_id)) {
+    } else if reason.is_none() && is_must_use_ty(cx, return_ty(cx, item_id), item_hir_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(cx.typing_mode());
             if let Some(future_ty) = infcx.err_ctxt().get_impl_future_output_ty(return_ty(cx, item_id))
-                && !is_must_use_ty(cx, future_ty)
+                && !is_must_use_ty(cx, future_ty, item_hir_id)
             {
                 return;
             }
@@ -206,7 +210,7 @@ fn check_must_use_candidate<'tcx>(
         || item_span.in_external_macro(cx.sess().source_map())
         || returns_unit(decl)
         || !cx.effective_visibilities.is_exported(item_id.def_id)
-        || is_must_use_ty(cx, return_ty(cx, item_id))
+        || is_must_use_ty(cx, return_ty(cx, item_id), body.value.hir_id)
         || item_span.from_expansion()
         || is_entrypoint_fn(cx, item_id.def_id.to_def_id())
     {

clippy_lints/src/let_underscore.rs

@@ -175,7 +175,7 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
                         diag.help("consider awaiting the future or dropping explicitly with `std::mem::drop`");
                     },
                 );
-            } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init)) {
+            } else if is_must_use_ty(cx, cx.typeck_results().expr_ty(init), init.hir_id) {
                 #[expect(clippy::collapsible_span_lint_calls, reason = "rust-clippy#7797")]
                 span_lint_and_then(
                     cx,

clippy_lints/src/return_self_not_must_use.rs

@@ -87,7 +87,7 @@ fn check_method(cx: &LateContext<'_>, decl: &FnDecl<'_>, fn_def: LocalDefId, spa
         // there is one, we shouldn't emit a warning!
         && self_arg.peel_refs() == ret_ty
         // If `Self` is already marked as `#[must_use]`, no need for the attribute here.
-        && !is_must_use_ty(cx, ret_ty)
+        && !is_must_use_ty(cx, ret_ty, cx.tcx.local_def_id_to_hir_id(fn_def))
     {
         span_lint_and_help(
             cx,

clippy_utils/src/ty/mod.rs

@@ -9,10 +9,11 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
 use rustc_hir::def_id::DefId;
-use rustc_hir::{Expr, FnDecl, LangItem, find_attr};
+use rustc_hir::{Expr, ExprKind, FnDecl, HirId, LangItem};
 use rustc_hir_analysis::lower_ty;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::LateContext;
+use rustc_lint::unused::must_use::{IsTyMustUse, is_ty_must_use};
 use rustc_middle::mir::ConstValue;
 use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::traits::EvaluationResult;
@@ -310,46 +311,13 @@ pub fn has_drop<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
 // Returns whether the `ty` has `#[must_use]` attribute. If `ty` is a `Result`/`ControlFlow`
 // whose `Err`/`Break` payload is an uninhabited type, the `Ok`/`Continue` payload type
 // will be used instead. See <https://github.com/rust-lang/rust/pull/148214>.
-pub fn is_must_use_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
-    match ty.kind() {
-        ty::Adt(adt, args) => match cx.tcx.get_diagnostic_name(adt.did()) {
-            Some(sym::Result) if args.type_at(1).is_privately_uninhabited(cx.tcx, cx.typing_env()) => {
-                is_must_use_ty(cx, args.type_at(0))
-            },
-            Some(sym::ControlFlow) if args.type_at(0).is_privately_uninhabited(cx.tcx, cx.typing_env()) => {
-                is_must_use_ty(cx, args.type_at(1))
-            },
-            _ => find_attr!(cx.tcx, adt.did(), MustUse { .. }),
-        },
-        ty::Foreign(did) => find_attr!(cx.tcx, *did, MustUse { .. }),
-        ty::Slice(ty) | ty::Array(ty, _) | ty::RawPtr(ty, _) | ty::Ref(_, ty, _) => {
-            // for the Array case we don't need to care for the len == 0 case
-            // because we don't want to lint functions returning empty arrays
-            is_must_use_ty(cx, *ty)
-        },
-        ty::Tuple(args) => args.iter().any(|ty| is_must_use_ty(cx, ty)),
-        ty::Alias(ty::Opaque, AliasTy { def_id, .. }) => {
-            for (predicate, _) in cx.tcx.explicit_item_self_bounds(*def_id).skip_binder() {
-                if let ty::ClauseKind::Trait(trait_predicate) = predicate.kind().skip_binder()
-                    && find_attr!(cx.tcx, trait_predicate.trait_ref.def_id, MustUse { .. })
-                {
-                    return true;
-                }
-            }
-            false
-        },
-        ty::Dynamic(binder, _) => {
-            for predicate in *binder {
-                if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate.skip_binder()
-                    && find_attr!(cx.tcx, trait_ref.def_id, MustUse { .. })
-                {
-                    return true;
-                }
-            }
-            false
-        },
-        _ => false,
-    }
+pub fn is_must_use_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, hir_id: HirId) -> bool {
+    let dummy_expr = Expr {
+        hir_id,
+        span: DUMMY_SP,
+        kind: ExprKind::Ret(None),
+    };
+    matches!(is_ty_must_use(cx, ty, &dummy_expr, true), IsTyMustUse::Yes(_))
 }
 
 /// Returns `true` if the given type is a non aggregate primitive (a `bool` or `char`, any

tests/ui/must_use_candidates.fixed

@@ -120,3 +120,9 @@ pub fn result_uninhabited() -> Result<i32, std::convert::Infallible> {
 pub fn controlflow_uninhabited() -> std::ops::ControlFlow<std::convert::Infallible, i32> {
     todo!()
 }
+
+// Do not lint: even though `Box` itself is not `#[must_use]`, if the content is, the
+// compiler will treat the box as-is.
+pub fn with_box() -> Box<Result<u32, u32>> {
+    Box::new(Ok(0))
+}

tests/ui/must_use_candidates.rs

@@ -113,3 +113,9 @@ pub fn result_uninhabited() -> Result<i32, std::convert::Infallible> {
 pub fn controlflow_uninhabited() -> std::ops::ControlFlow<std::convert::Infallible, i32> {
     todo!()
 }
+
+// Do not lint: even though `Box` itself is not `#[must_use]`, if the content is, the
+// compiler will treat the box as a `#[must_use]` type already.
+pub fn with_box() -> Box<Result<u32, u32>> {
+    Box::new(Ok(0))
+}