Support manual impl of fn traits in mir interpreter

crates/hir-ty/src/consteval/tests.rs

@@ -1550,6 +1550,30 @@ fn closures() {
     );
 }
 
+#[test]
+fn manual_fn_trait_impl() {
+    check_number(
+        r#"
+//- minicore: fn, copy
+struct S(i32);
+
+impl FnOnce<(i32, i32)> for S {
+    type Output = i32;
+
+    extern "rust-call" fn call_once(self, arg: (i32, i32)) -> i32 {
+        arg.0 + arg.1 + self.0
+    }
+}
+
+const GOAL: i32 = {
+    let s = S(1);
+    s(2, 3)
+};
+"#,
+        6,
+    );
+}
+
 #[test]
 fn closure_and_impl_fn() {
     check_number(

crates/hir-ty/src/consteval/tests/intrinsics.rs

@@ -32,6 +32,7 @@ fn transmute() {
 fn const_eval_select() {
     check_number(
         r#"
+        //- minicore: fn
         extern "rust-intrinsic" {
             pub fn const_eval_select<ARG, F, G, RET>(arg: ARG, called_in_const: F, called_at_rt: G) -> RET
             where