Rollup merge of #132583 - mejrs:tuples, r=compiler-errors

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

When you want to have a variadic function, a common workaround to implement this is to create a trait and then implement that trait for various tuples. For example in `pyo3` there exists
```rust
/// Calls the object with only positional arguments.
pub fn call1(&self, args: impl IntoPy<Py<PyTuple>>) -> PyResult<&PyAny> {
   ...
}
```

with various impls like
```rust
impl<A: IntoPy<PyObject> IntoPy<Py<PyAny>> for (A,)
impl<A: IntoPy<PyObject, B: IntoPy<PyObject> IntoPy<Py<PyAny>> for (A, B)
... etc
```

This means that if you want to call the method with a single item you have to create a unary tuple, like `(x,)`, rather than just `x`.

This PR implements a suggestion to do that, if applicable.

9 files changed, 116 insertions, 0 deletions

diff --git a/tests/ui/diagnostic_namespace/do_not_recommend/do_not_apply_attribute_without_feature_flag.stderr b/tests/ui/diagnostic_namespace/do_not_recommend/do_not_apply_attribute_without_feature_flag.stderr
index e56af28f3fb..be17476524a 100644
--- a/tests/ui/diagnostic_namespace/do_not_recommend/do_not_apply_attribute_without_feature_flag.stderr
+++ b/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
 
diff --git a/tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.current.stderr b/tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.current.stderr
index 629fc59361d..ca9a6ebc1c4 100644
--- a/tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.current.stderr
+++ b/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
 
diff --git a/tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.next.stderr b/tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.next.stderr
index 629fc59361d..ca9a6ebc1c4 100644
--- a/tests/ui/diagnostic_namespace/do_not_recommend/supress_suggestions_in_help.next.stderr
+++ b/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
 
diff --git a/tests/ui/layout/rust-call-abi-not-a-tuple-ice-81974.stderr b/tests/ui/layout/rust-call-abi-not-a-tuple-ice-81974.stderr
index cceaddf7803..206a6801065 100644
--- a/tests/ui/layout/rust-call-abi-not-a-tuple-ice-81974.stderr
+++ b/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
 
diff --git a/tests/ui/on-unimplemented/suggest_tuple_wrap.rs b/tests/ui/on-unimplemented/suggest_tuple_wrap.rs
new file mode 100644
index 00000000000..010a47aef62
--- /dev/null
+++ b/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
+}
diff --git a/tests/ui/on-unimplemented/suggest_tuple_wrap.stderr b/tests/ui/on-unimplemented/suggest_tuple_wrap.stderr
new file mode 100644
index 00000000000..93dd43aafd9
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/on-unimplemented/suggest_tuple_wrap_root_obligation.rs b/tests/ui/on-unimplemented/suggest_tuple_wrap_root_obligation.rs
new file mode 100644
index 00000000000..e0036d30187
--- /dev/null
+++ b/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>`
+}
diff --git a/tests/ui/on-unimplemented/suggest_tuple_wrap_root_obligation.stderr b/tests/ui/on-unimplemented/suggest_tuple_wrap_root_obligation.stderr
new file mode 100644
index 00000000000..6ee08d2cd1b
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/overloaded/overloaded-calls-nontuple.stderr b/tests/ui/overloaded/overloaded-calls-nontuple.stderr
index 45a84fc4d7b..22598f3a390 100644
--- a/tests/ui/overloaded/overloaded-calls-nontuple.stderr
+++ b/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