unused_must_use: Don't warn on Result<(), Uninhabited>

This suppresses warnings on things like `Result<(), !>`, which helps
simplify code using the common pattern of having an `Error` associated
type: code will only have to check the error if there is a possibility
of error.

compiler/rustc_lint/src/unused.rs

@@ -291,6 +291,18 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                     is_ty_must_use(cx, pinned_ty, expr, span)
                         .map(|inner| MustUsePath::Pinned(Box::new(inner)))
                 }
+                // Suppress warnings on `Result<(), Uninhabited>` (e.g. `Result<(), !>`).
+                ty::Adt(def, args)
+                    if cx.tcx.is_diagnostic_item(sym::Result, def.did())
+                        && args.type_at(0).is_unit()
+                        && !args.type_at(1).is_inhabited_from(
+                            cx.tcx,
+                            parent_mod_did,
+                            cx.typing_env(),
+                        ) =>
+                {
+                    Some(MustUsePath::Suppressed)
+                }
                 ty::Adt(def, _) => is_def_must_use(cx, def.did(), span),
                 ty::Alias(ty::Opaque | ty::Projection, ty::AliasTy { def_id: def, .. }) => {
                     elaborate(cx.tcx, cx.tcx.explicit_item_self_bounds(def).iter_identity_copied())

tests/ui/lint/unused/auxiliary/must_use_result_unit_uninhabited_extern_crate.rs

@@ -0,0 +1,4 @@
+pub enum MyUninhabited {}
+
+#[non_exhaustive]
+pub enum MyUninhabitedNonexhaustive {}

tests/ui/lint/unused/must_use-result-unit-uninhabited.rs

@@ -0,0 +1,55 @@
+//@ edition: 2024
+//@ aux-crate:dep=must_use_result_unit_uninhabited_extern_crate.rs
+
+#![deny(unused_must_use)]
+#![feature(never_type)]
+
+use dep::{MyUninhabited, MyUninhabitedNonexhaustive};
+
+fn f1() -> Result<(), ()> {
+    Ok(())
+}
+
+fn f2() -> Result<(), core::convert::Infallible> {
+    Ok(())
+}
+
+fn f3() -> Result<(), !> {
+    Ok(())
+}
+
+fn f4() -> Result<(), MyUninhabited> {
+    Ok(())
+}
+
+fn f5() -> Result<(), MyUninhabitedNonexhaustive> {
+    Ok(())
+}
+
+trait AssocType {
+    type Error;
+}
+
+struct S1;
+impl AssocType for S1 {
+    type Error = !;
+}
+
+struct S2;
+impl AssocType for S2 {
+    type Error = ();
+}
+
+fn f6<AT: AssocType>(_: AT) -> Result<(), AT::Error> {
+    Ok(())
+}
+
+fn main() {
+    f1(); //~ ERROR: unused `Result` that must be used
+    f2();
+    f3();
+    f4();
+    f5(); //~ ERROR: unused `Result` that must be used
+    f6(S1);
+    f6(S2); //~ ERROR: unused `Result` that must be used
+}

tests/ui/lint/unused/must_use-result-unit-uninhabited.stderr

@@ -0,0 +1,43 @@
+error: unused `Result` that must be used
+  --> $DIR/must_use-result-unit-uninhabited.rs:48:5
+   |
+LL |     f1();
+   |     ^^^^
+   |
+   = note: this `Result` may be an `Err` variant, which should be handled
+note: the lint level is defined here
+  --> $DIR/must_use-result-unit-uninhabited.rs:4:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = f1();
+   |     +++++++
+
+error: unused `Result` that must be used
+  --> $DIR/must_use-result-unit-uninhabited.rs:52:5
+   |
+LL |     f5();
+   |     ^^^^
+   |
+   = note: this `Result` may be an `Err` variant, which should be handled
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = f5();
+   |     +++++++
+
+error: unused `Result` that must be used
+  --> $DIR/must_use-result-unit-uninhabited.rs:54:5
+   |
+LL |     f6(S2);
+   |     ^^^^^^
+   |
+   = note: this `Result` may be an `Err` variant, which should be handled
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = f6(S2);
+   |     +++++++
+
+error: aborting due to 3 previous errors
+