Added test for issue #56835.

Author: Alexander Regueiro

src/test/ui/issues/issue-56835.rs

@@ -0,0 +1,10 @@
+
+pub struct Foo {}
+
+impl Foo {
+    fn bar(Self(foo): Self) {}
+    //~^ ERROR the `Self` constructor can only be used with tuple structs
+    //~^^ ERROR expected tuple struct/variant, found self constructor `Self` [E0164]
+}
+
+fn main() {}

src/test/ui/issues/issue-56835.stderr

@@ -0,0 +1,15 @@
+error: the `Self` constructor can only be used with tuple structs
+  --> $DIR/issue-56835.rs:5:12
+   |
+LL |     fn bar(Self(foo): Self) {}
+   |            ^^^^^^^^^ did you mean `Self { /* fields */ }`?
+
+error[E0164]: expected tuple struct/variant, found self constructor `Self`
+  --> $DIR/issue-56835.rs:5:12
+   |
+LL |     fn bar(Self(foo): Self) {}
+   |            ^^^^^^^^^ not a tuple variant or struct
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0164`.