Add test for issue 127911 and 128839

Author: mu001999

tests/ui/lint/dead-code/alias-type-used-as-generic-arg-in-impl.rs

@@ -0,0 +1,19 @@
+//@ check-pass
+
+#![deny(dead_code)]
+
+struct T<X>(X);
+
+type A<X> = T<X>;
+
+trait Tr {
+    fn foo();
+}
+
+impl<X> Tr for T<A<X>> {
+    fn foo() {}
+}
+
+fn main() {
+   T::<T<()>>::foo();
+}

tests/ui/lint/dead-code/trait-only-used-as-type-bound.rs

@@ -0,0 +1,31 @@
+//@ check-pass
+
+#![deny(dead_code)]
+
+trait UInt: Copy + From<u8> {}
+
+impl UInt for u16 {}
+
+trait Int: Copy {
+    type Unsigned: UInt;
+
+    fn as_unsigned(self) -> Self::Unsigned;
+}
+
+impl Int for i16 {
+    type Unsigned = u16;
+
+    fn as_unsigned(self) -> u16 {
+        self as _
+    }
+}
+
+fn priv_func<T: Int>(x: u8, y: T) -> (T::Unsigned, T::Unsigned) {
+    (T::Unsigned::from(x), y.as_unsigned())
+}
+
+pub fn pub_func(x: u8, y: i16) -> (u16, u16) {
+    priv_func(x, y)
+}
+
+fn main() {}