Add a regression test for unevaluated const in rustdoc

Author: varkor

src/test/rustdoc/const-generics/add-impl.rs

@@ -0,0 +1,21 @@
+// ignore-tidy-linelength
+
+#![feature(const_generics)]
+
+#![crate_name = "foo"]
+
+use std::ops::Add;
+
+// @has foo/struct.Simd.html '//pre[@class="rust struct"]' 'pub struct Simd<T, const WIDTH: usize>'
+pub struct Simd<T, const WIDTH: usize> {
+    inner: T,
+}
+
+// @has foo/struct.Simd.html '//div[@id="implementations-list"]/h3/code' 'impl Add<Simd<u8, 16>> for Simd<u8, 16>'
+impl Add for Simd<u8, 16> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self::Output {
+        Self { inner: 0 }
+    }
+}