Auto merge of #55986 - cjgillot:issue-45510, r=nikomatsakis

Allow to dispatch fn traits depending on number of parameters

Hello,

By following @eddyb's advise on issue #45510, I managed to have the snippets of code in #45510 and #18952 passing without breaking older diagnostics.

EDIT: the codegen tests breakage I experienced is due to the poor quality of my laptop.

If any kind reviewer has any advice, you are very welcome.

src/test/run-pass/issue-18952.rs

@@ -0,0 +1,56 @@
+// This issue tests fn_traits overloading on arity.
+// run-pass
+
+#![feature(fn_traits)]
+#![feature(unboxed_closures)]
+
+struct Foo;
+
+impl Fn<(isize, isize)> for Foo {
+    extern "rust-call" fn call(&self, args: (isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+        (args.0 + 1, args.1 + 1)
+    }
+}
+
+impl FnMut<(isize, isize)> for Foo {
+    extern "rust-call" fn call_mut(&mut self, args: (isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+        (args.0 + 1, args.1 + 1)
+    }
+}
+
+impl FnOnce<(isize, isize)> for Foo {
+    type Output = (isize, isize);
+    extern "rust-call" fn call_once(self, args: (isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+        (args.0 + 1, args.1 + 1)
+    }
+}
+
+impl Fn<(isize, isize, isize)> for Foo {
+    extern "rust-call" fn call(&self, args: (isize, isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+        (args.0 + 3, args.1 + 3, args.2 + 3)
+    }
+}
+
+impl FnMut<(isize, isize, isize)> for Foo {
+    extern "rust-call" fn call_mut(&mut self, args: (isize, isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+        (args.0 + 3, args.1 + 3, args.2 + 3)
+    }
+}
+impl FnOnce<(isize, isize, isize)> for Foo {
+    type Output = (isize, isize, isize);
+    extern "rust-call" fn call_once(self, args: (isize, isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+        (args.0 + 3, args.1 + 3, args.2 + 3)
+    }
+}
+
+fn main() {
+    let foo = Foo;
+    assert_eq!(foo(1, 1), (2, 2));
+    assert_eq!(foo(1, 1, 1), (4, 4, 4));
+}

src/test/run-pass/issue-45510.rs

@@ -0,0 +1,32 @@
+// Test overloaded resolution of fn_traits.
+// run-pass
+
+#![feature(fn_traits)]
+#![feature(unboxed_closures)]
+
+#[derive(Debug, PartialEq, Eq)]
+struct Ishmael;
+#[derive(Debug, PartialEq, Eq)]
+struct Maybe;
+struct CallMe;
+
+impl FnOnce<(Ishmael,)> for CallMe {
+    type Output = Ishmael;
+    extern "rust-call" fn call_once(self, _args: (Ishmael,)) -> Ishmael {
+        println!("Split your lungs with blood and thunder!");
+        Ishmael
+    }
+}
+
+impl FnOnce<(Maybe,)> for CallMe {
+    type Output = Maybe;
+    extern "rust-call" fn call_once(self, _args: (Maybe,)) -> Maybe {
+        println!("So we just met, and this is crazy");
+        Maybe
+    }
+}
+
+fn main() {
+    assert_eq!(CallMe(Ishmael), Ishmael);
+    assert_eq!(CallMe(Maybe), Maybe);
+}