gccrs: add testcase to prove issue has already been fixed

gcc/testsuite/rust/compile/issue-1483.rs

@@ -0,0 +1,28 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+pub fn takes_fn_generic<F: FnOnce(i32) -> i32>(a: i32, f: F) -> i32 {
+    f(a)
+}
+
+pub fn takes_fn_generic_where<F>(a: i32, f: F) -> i32
+where
+    F: FnOnce(i32) -> i32,
+{
+    f(a)
+}
+
+pub fn test() {
+    let foo = |x: i32| -> i32 { x + 1 };
+
+    takes_fn_generic(1, foo);
+    takes_fn_generic_where(2, foo);
+}