Add regression test for impl associated types source link

tests/rustdoc/assoc-type-source-link.rs

@@ -0,0 +1,26 @@
+// This test ensures that the source links are generated for impl associated types.
+
+#![crate_name = "foo"]
+#![feature(inherent_associated_types)]
+#![allow(incomplete_features)]
+
+//@ has 'foo/struct.Bar.html'
+pub struct Bar;
+
+impl Bar {
+    //@ has - '//*[@id="implementations-list"]//*[@id="associatedtype.Y"]/a' 'source'
+    //@ has - '//*[@id="implementations-list"]//*[@id="associatedtype.Y"]/a/@href' \
+    // '../src/foo/assoc-type-source-link.rs.html#14'
+    pub type Y = u8;
+}
+
+pub trait Foo {
+    type Z;
+}
+
+impl Foo for Bar {
+    //@ has - '//*[@id="trait-implementations-list"]//*[@id="associatedtype.Z"]/a' 'source'
+    //@ has - '//*[@id="trait-implementations-list"]//*[@id="associatedtype.Z"]/a/@href' \
+    // '../src/foo/assoc-type-source-link.rs.html#25'
+    type Z = u8;
+}