unused_must_use: Suppress warnings for ControlFlow<Uninhabited, ()> too

Author: joshtriplett

compiler/rustc_lint/src/unused.rs

@@ -303,6 +303,18 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                 {
                     Some(MustUsePath::Suppressed)
                 }
+                // Suppress warnings on `ControlFlow<Uninhabited, ()>` (e.g. `ControlFlow<!, ()>`).
+                ty::Adt(def, args)
+                    if cx.tcx.is_diagnostic_item(sym::ControlFlow, def.did())
+                        && args.type_at(1).is_unit()
+                        && !args.type_at(0).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/must_use-result-unit-uninhabited.rs

@@ -4,6 +4,7 @@
 #![deny(unused_must_use)]
 #![feature(never_type)]
 
+use core::ops::{ControlFlow, ControlFlow::Continue};
 use dep::{MyUninhabited, MyUninhabitedNonexhaustive};
 
 fn f1() -> Result<(), ()> {
@@ -63,6 +64,18 @@ impl UsesAssocType for S2 {
     }
 }
 
+fn c1() -> ControlFlow<()> {
+    Continue(())
+}
+
+fn c2() -> ControlFlow<core::convert::Infallible, ()> {
+    Continue(())
+}
+
+fn c3() -> ControlFlow<!> {
+    Continue(())
+}
+
 fn main() {
     f1(); //~ ERROR: unused `Result` that must be used
     f2();
@@ -73,4 +86,8 @@ fn main() {
     f6(S2); //~ ERROR: unused `Result` that must be used
     S1.method();
     S2.method(); //~ ERROR: unused `Result` that must be used
+
+    c1(); //~ ERROR: unused `ControlFlow` that must be used
+    c2();
+    c3();
 }

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

@@ -1,5 +1,5 @@
 error: unused `Result` that must be used
-  --> $DIR/must_use-result-unit-uninhabited.rs:67:5
+  --> $DIR/must_use-result-unit-uninhabited.rs:80:5
    |
 LL |     f1();
    |     ^^^^
@@ -16,7 +16,7 @@ LL |     let _ = f1();
    |     +++++++
 
 error: unused `Result` that must be used
-  --> $DIR/must_use-result-unit-uninhabited.rs:71:5
+  --> $DIR/must_use-result-unit-uninhabited.rs:84:5
    |
 LL |     f5();
    |     ^^^^
@@ -28,7 +28,7 @@ LL |     let _ = f5();
    |     +++++++
 
 error: unused `Result` that must be used
-  --> $DIR/must_use-result-unit-uninhabited.rs:73:5
+  --> $DIR/must_use-result-unit-uninhabited.rs:86:5
    |
 LL |     f6(S2);
    |     ^^^^^^
@@ -40,7 +40,7 @@ LL |     let _ = f6(S2);
    |     +++++++
 
 error: unused `Result` that must be used
-  --> $DIR/must_use-result-unit-uninhabited.rs:75:5
+  --> $DIR/must_use-result-unit-uninhabited.rs:88:5
    |
 LL |     S2.method();
    |     ^^^^^^^^^^^
@@ -51,5 +51,16 @@ help: use `let _ = ...` to ignore the resulting value
 LL |     let _ = S2.method();
    |     +++++++
 
-error: aborting due to 4 previous errors
+error: unused `ControlFlow` that must be used
+  --> $DIR/must_use-result-unit-uninhabited.rs:90:5
+   |
+LL |     c1();
+   |     ^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = c1();
+   |     +++++++
+
+error: aborting due to 5 previous errors