tests: adjust tests/ui/auto-instantiate.rs

- Reformat the test.
- Document test intention.
- Move test under `tests/ui/inference/`.

Author: jieyouxu

Diff for: tests/ui/auto-instantiate.rs

@@ -0,0 +1,28 @@
+//! Check that type parameters in generic function arg position and in "nested" return type position
+//! can be inferred on an invocation of the generic function.
+//!
+//! See <https://github.com/rust-lang/rust/issues/45>.
+
+//@ run-pass
+
+#![allow(dead_code)]
+#[derive(Debug)]
+struct Pair<T, U> {
+    a: T,
+    b: U,
+}
+
+struct Triple {
+    x: isize,
+    y: isize,
+    z: isize,
+}
+
+fn f<T, U>(x: T, y: U) -> Pair<T, U> {
+    return Pair { a: x, b: y };
+}
+
+pub fn main() {
+    println!("{}", f(Triple {x: 3, y: 4, z: 5}, 4).a.x);
+    println!("{}", f(5, 6).a);
+}