Suggest creating unary tuples when types don't match a trait

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -466,6 +466,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         }
 
                         self.try_to_add_help_message(
+                            &root_obligation,
                             &obligation,
                             leaf_trait_predicate,
                             &mut err,
@@ -2428,6 +2429,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
     fn try_to_add_help_message(
         &self,
+        root_obligation: &PredicateObligation<'tcx>,
         obligation: &PredicateObligation<'tcx>,
         trait_predicate: ty::PolyTraitPredicate<'tcx>,
         err: &mut Diag<'_>,
@@ -2511,6 +2513,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 impl_candidates.as_slice(),
                 span,
             );
+
+            self.suggest_tuple_wrapping(err, root_obligation, obligation);
         }
     }
 

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -4436,6 +4436,41 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         }
     }
 
+    /// If the type failed selection but the trait is implemented for `(T,)`, suggest that the user
+    /// creates a unary tuple
+    ///
+    /// This is a common gotcha when using libraries that emulate variadic functions with traits for tuples.
+    pub(super) fn suggest_tuple_wrapping(
+        &self,
+        err: &mut Diag<'_>,
+        root_obligation: &PredicateObligation<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) {
+        let ObligationCauseCode::FunctionArg { arg_hir_id, .. } = obligation.cause.code() else {
+            return;
+        };
+
+        let Some(root_pred) = root_obligation.predicate.as_trait_clause() else { return };
+
+        let trait_ref = root_pred.map_bound(|root_pred| {
+            root_pred
+                .trait_ref
+                .with_self_ty(self.tcx, Ty::new_tup(self.tcx, &[root_pred.trait_ref.self_ty()]))
+        });
+
+        let obligation =
+            Obligation::new(self.tcx, obligation.cause.clone(), obligation.param_env, trait_ref);
+
+        if self.predicate_must_hold_modulo_regions(&obligation) {
+            let arg_span = self.tcx.hir().span(*arg_hir_id);
+            err.multipart_suggestion_verbose(
+                format!("use a unary tuple instead"),
+                vec![(arg_span.shrink_to_lo(), "(".into()), (arg_span.shrink_to_hi(), ",)".into())],
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+
     pub(super) fn explain_hrtb_projection(
         &self,
         diag: &mut Diag<'_>,

tests/ui/diagnostic_namespace/do_not_recommend/do_not_apply_attribute_without_feature_flag.stderr

@@ -15,6 +15,10 @@ note: required by a bound in `check`
    |
 LL | fn check(a: impl Foo) {}
    |                  ^^^ required by this bound in `check`
+help: use a unary tuple instead
+   |
+LL |     check(((),));
+   |           +  ++
 
 error: aborting due to 1 previous error
 

tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.current.stderr

@@ -12,6 +12,10 @@ note: required by a bound in `check`
    |
 LL | fn check(a: impl Foo) {}
    |                  ^^^ required by this bound in `check`
+help: use a unary tuple instead
+   |
+LL |     check(((),));
+   |           +  ++
 
 error: aborting due to 1 previous error
 

tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.next.stderr

@@ -12,6 +12,10 @@ note: required by a bound in `check`
    |
 LL | fn check(a: impl Foo) {}
    |                  ^^^ required by this bound in `check`
+help: use a unary tuple instead
+   |
+LL |     check(((),));
+   |           +  ++
 
 error: aborting due to 1 previous error
 

tests/ui/layout/rust-call-abi-not-a-tuple-ice-81974.stderr

@@ -71,6 +71,10 @@ LL |     cachedcoso.call_once(1);
    |
 note: required by a bound in `call_once`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+help: use a unary tuple instead
+   |
+LL |     cachedcoso.call_once((1,));
+   |                          + ++
 
 error: aborting due to 6 previous errors
 

tests/ui/on-unimplemented/suggest_tuple_wrap.rs

@@ -0,0 +1,19 @@
+pub trait Argument {}
+impl Argument for u8 {}
+impl Argument for i8 {}
+impl Argument for String {}
+impl Argument for &str {}
+
+pub trait TupleArgs {}
+impl<A: Argument> TupleArgs for (A,) {}
+impl<A: Argument, B: Argument> TupleArgs for (A, B) {}
+impl<A: Argument, B: Argument, C: Argument> TupleArgs for (A, B, C) {}
+
+fn convert_into_tuple(_x: impl TupleArgs) {}
+
+fn main() {
+    convert_into_tuple(42_u8);
+    //~^ ERROR E0277
+    //~| HELP the following other types implement trait `TupleArgs`
+    //~| HELP use a unary tuple instead
+}

tests/ui/on-unimplemented/suggest_tuple_wrap.stderr

@@ -0,0 +1,25 @@
+error[E0277]: the trait bound `u8: TupleArgs` is not satisfied
+  --> $DIR/suggest_tuple_wrap.rs:15:24
+   |
+LL |     convert_into_tuple(42_u8);
+   |     ------------------ ^^^^^ the trait `TupleArgs` is not implemented for `u8`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the following other types implement trait `TupleArgs`:
+             (A, B)
+             (A, B, C)
+             (A,)
+note: required by a bound in `convert_into_tuple`
+  --> $DIR/suggest_tuple_wrap.rs:12:32
+   |
+LL | fn convert_into_tuple(_x: impl TupleArgs) {}
+   |                                ^^^^^^^^^ required by this bound in `convert_into_tuple`
+help: use a unary tuple instead
+   |
+LL |     convert_into_tuple((42_u8,));
+   |                        +     ++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/on-unimplemented/suggest_tuple_wrap_root_obligation.rs

@@ -0,0 +1,26 @@
+struct Tuple;
+
+impl From<(u8,)> for Tuple {
+    fn from(_: (u8,)) -> Self {
+        todo!()
+    }
+}
+impl From<(u8, u8)> for Tuple {
+    fn from(_: (u8, u8)) -> Self {
+        todo!()
+    }
+}
+impl From<(u8, u8, u8)> for Tuple {
+    fn from(_: (u8, u8, u8)) -> Self {
+        todo!()
+    }
+}
+
+fn convert_into_tuple(_x: impl Into<Tuple>) {}
+
+fn main() {
+    convert_into_tuple(42_u8);
+    //~^ ERROR E0277
+    //~| HELP use a unary tuple instead
+    //~| HELP the following other types implement trait `From<T>`
+}

tests/ui/on-unimplemented/suggest_tuple_wrap_root_obligation.stderr

@@ -0,0 +1,26 @@
+error[E0277]: the trait bound `Tuple: From<u8>` is not satisfied
+  --> $DIR/suggest_tuple_wrap_root_obligation.rs:22:24
+   |
+LL |     convert_into_tuple(42_u8);
+   |     ------------------ ^^^^^ the trait `From<u8>` is not implemented for `Tuple`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the following other types implement trait `From<T>`:
+             `Tuple` implements `From<(u8, u8)>`
+             `Tuple` implements `From<(u8, u8, u8)>`
+             `Tuple` implements `From<(u8,)>`
+   = note: required for `u8` to implement `Into<Tuple>`
+note: required by a bound in `convert_into_tuple`
+  --> $DIR/suggest_tuple_wrap_root_obligation.rs:19:32
+   |
+LL | fn convert_into_tuple(_x: impl Into<Tuple>) {}
+   |                                ^^^^^^^^^^^ required by this bound in `convert_into_tuple`
+help: use a unary tuple instead
+   |
+LL |     convert_into_tuple((42_u8,));
+   |                        +     ++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/overloaded/overloaded-calls-nontuple.stderr

@@ -38,6 +38,10 @@ LL |         self.call_mut(z)
    |
 note: required by a bound in `call_mut`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+help: use a unary tuple instead
+   |
+LL |         self.call_mut((z,))
+   |                       + ++
 
 error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
   --> $DIR/overloaded-calls-nontuple.rs:29:10