Add some more tail call tests

Author: WaffleLapkin

Diff for: tests/ui/explicit-tail-calls/dyn-fn-once.rs

@@ -0,0 +1,15 @@
+#![feature(explicit_tail_calls)]
+
+fn f() {
+    become (Box::new(|| ()) as Box<dyn FnOnce() -> ()>)();
+    //~^ error: mismatched function ABIs
+    //~| error: mismatched signatures
+}
+
+fn g() {
+    become (&g as &dyn FnOnce() -> ())();
+    //~^ error: mismatched function ABIs
+    //~| error: mismatched signatures
+}
+
+fn main() {}

Diff for: tests/ui/explicit-tail-calls/dyn-fn-once.stderr

@@ -0,0 +1,40 @@
+error: mismatched function ABIs
+  --> $DIR/dyn-fn-once.rs:4:5
+   |
+LL |     become (Box::new(|| ()) as Box<dyn FnOnce() -> ()>)();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have the same ABI
+   = note: caller ABI is `"Rust"`, while callee ABI is `"rust-call"`
+
+error: mismatched signatures
+  --> $DIR/dyn-fn-once.rs:4:5
+   |
+LL |     become (Box::new(|| ()) as Box<dyn FnOnce() -> ()>)();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have matching signatures
+   = note: caller signature: `fn()`
+   = note: callee signature: `extern "rust-call" fn(Box<dyn FnOnce()>, ())`
+
+error: mismatched function ABIs
+  --> $DIR/dyn-fn-once.rs:10:5
+   |
+LL |     become (&g as &dyn FnOnce() -> ())();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have the same ABI
+   = note: caller ABI is `"Rust"`, while callee ABI is `"rust-call"`
+
+error: mismatched signatures
+  --> $DIR/dyn-fn-once.rs:10:5
+   |
+LL |     become (&g as &dyn FnOnce() -> ())();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have matching signatures
+   = note: caller signature: `fn()`
+   = note: callee signature: `extern "rust-call" fn(dyn FnOnce(), ())`
+
+error: aborting due to 4 previous errors
+

Diff for: tests/ui/explicit-tail-calls/fn-pointers.rs

@@ -0,0 +1,43 @@
+// run-pass
+#![feature(explicit_tail_calls)]
+
+fn main() {
+    use Inst::*;
+
+    let program = [
+        Inc,       // st + 1 = 1
+        ShiftLeft, // st * 2 = 2
+        ShiftLeft, // st * 2 = 4
+        ShiftLeft, // st * 2 = 8
+        Inc,       // st + 1 = 9
+        Inc,       // st + 1 = 10
+        Inc,       // st + 1 = 11
+        ShiftLeft, // st * 2 = 22
+        Halt,      // st = 22
+    ];
+
+    assert_eq!(run(0, &program), 22);
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum Inst {
+    Inc,
+    ShiftLeft,
+    Halt,
+}
+
+fn run(state: u32, program: &[Inst]) -> u32 {
+    const DISPATCH_TABLE: [fn(u32, &[Inst]) -> u32; 3] = [inc, shift_left, |st, _| st];
+
+    become DISPATCH_TABLE[program[0] as usize](state, program);
+}
+
+fn inc(state: u32, program: &[Inst]) -> u32 {
+    assert_eq!(program[0], Inst::Inc);
+    become run(state + 1, &program[1..])
+}
+
+fn shift_left(state: u32, program: &[Inst]) -> u32 {
+    assert_eq!(program[0], Inst::ShiftLeft);
+    become run(state << 1, &program[1..])
+}

Diff for: tests/ui/explicit-tail-calls/from-main-1.rs

@@ -0,0 +1,8 @@
+// run-pass
+#![feature(explicit_tail_calls)]
+
+fn main() {
+    become f();
+}
+
+fn f() {}

Diff for: tests/ui/explicit-tail-calls/from-main-2.rs

@@ -0,0 +1,10 @@
+// run-pass
+#![feature(explicit_tail_calls)]
+
+fn main() -> Result<(), i32> {
+    become f();
+}
+
+fn f() -> Result<(), i32> {
+    Ok(())
+}