add error-in-impl-trait const generics test

src/test/rustdoc-ui/error-in-impl-trait/const-generics.rs

@@ -0,0 +1,24 @@
+// check-pass
+// edition:2018
+#![feature(min_const_generics)]
+trait ValidTrait {}
+
+/// This has docs
+pub fn extern_fn<const N: usize>() -> impl Iterator<Item = [u8; N]> {
+    loop {}
+}
+
+pub trait Trait<const N: usize> {}
+impl Trait<1> for u8 {}
+impl Trait<2> for u8 {}
+impl<const N: usize> Trait<N> for [u8; N] {}
+
+/// This also has docs
+pub fn test<const N: usize>() -> impl Trait<N> where u8: Trait<N> {
+    loop {}
+}
+
+/// Document all the functions
+pub async fn a_sink<const N: usize>(v: [u8; N]) -> impl Trait<N> {
+    loop {}
+}