add miri tests and a fixme

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -992,6 +992,10 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             bug!("can't tailcall as root");
         };
 
+        // FIXME(explicit_tail_calls):
+        //   we should check if both caller&callee can/n't unwind,
+        //   see <https://github.com/rust-lang/rust/pull/113128#issuecomment-1614979803>
+
         self.eval_fn_call(
             fn_val,
             (caller_abi, caller_fn_abi),

src/tools/miri/tests/fail/tail_calls/cc-mismatch.rs

@@ -0,0 +1,10 @@
+//@error-in-other-file: Undefined Behavior: calling a function with calling convention C using calling convention Rust
+#![feature(explicit_tail_calls)]
+#![allow(incomplete_features)]
+
+fn main() {
+    let f = unsafe { std::mem::transmute::<extern "C" fn(), fn()>(f) };
+    become f();
+}
+
+extern "C" fn f() {}

src/tools/miri/tests/fail/tail_calls/cc-mismatch.stderr

@@ -0,0 +1,25 @@
+error: Undefined Behavior: calling a function with calling convention C using calling convention Rust
+  --> RUSTLIB/core/src/ops/function.rs:LL:CC
+   |
+LL |     extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ calling a function with calling convention C using calling convention Rust
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at RUSTLIB/core/src/ops/function.rs:LL:CC
+   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at RUSTLIB/std/src/sys_common/backtrace.rs:LL:CC
+   = note: inside closure at RUSTLIB/std/src/rt.rs:LL:CC
+   = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at RUSTLIB/core/src/ops/function.rs:LL:CC
+   = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at RUSTLIB/std/src/panicking.rs:LL:CC
+   = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at RUSTLIB/std/src/panicking.rs:LL:CC
+   = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at RUSTLIB/std/src/panic.rs:LL:CC
+   = note: inside closure at RUSTLIB/std/src/rt.rs:LL:CC
+   = note: inside `std::panicking::r#try::do_call::<{closure@std::rt::lang_start_internal::{closure#2}}, isize>` at RUSTLIB/std/src/panicking.rs:LL:CC
+   = note: inside `std::panicking::r#try::<isize, {closure@std::rt::lang_start_internal::{closure#2}}>` at RUSTLIB/std/src/panicking.rs:LL:CC
+   = note: inside `std::panic::catch_unwind::<{closure@std::rt::lang_start_internal::{closure#2}}, isize>` at RUSTLIB/std/src/panic.rs:LL:CC
+   = note: inside `std::rt::lang_start_internal` at RUSTLIB/std/src/rt.rs:LL:CC
+   = note: inside `std::rt::lang_start::<()>` at RUSTLIB/std/src/rt.rs:LL:CC
+
+error: aborting due to 1 previous error
+

src/tools/miri/tests/fail/tail_calls/signature-mismatch-arg.rs

@@ -0,0 +1,17 @@
+#![feature(explicit_tail_calls)]
+#![allow(incomplete_features)]
+
+fn main() {
+    // FIXME(explicit_tail_calls):
+    //   the error should point to `become f(x)`,
+    //   but tail calls mess up the backtrace it seems like...
+    f(0);
+    //~^ error: Undefined Behavior: calling a function with argument of type i32 passing data of type u32
+}
+
+fn f(x: u32) {
+    let g = unsafe { std::mem::transmute::<fn(i32), fn(u32)>(g) };
+    become g(x);
+}
+
+fn g(_: i32) {}

src/tools/miri/tests/fail/tail_calls/signature-mismatch-arg.stderr

@@ -0,0 +1,17 @@
+error: Undefined Behavior: calling a function with argument of type i32 passing data of type u32
+  --> $DIR/signature-mismatch-arg.rs:LL:CC
+   |
+LL |     f(0);
+   |     ^^^^ calling a function with argument of type i32 passing data of type u32
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = help: this means these two types are not *guaranteed* to be ABI-compatible across all targets
+   = help: if you think this code should be accepted anyway, please report an issue
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/signature-mismatch-arg.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

src/tools/miri/tests/pass/tail_call.rs

@@ -0,0 +1,39 @@
+#![allow(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn main() {
+    assert_eq!(factorial(10), 3_628_800);
+    assert_eq!(mutually_recursive_identity(1000), 1000);
+}
+
+fn factorial(n: u32) -> u32 {
+    fn factorial_acc(n: u32, acc: u32) -> u32 {
+        match n {
+            0 => acc,
+            _ => become factorial_acc(n - 1, acc * n),
+        }
+    }
+
+    factorial_acc(n, 1)
+}
+
+// this is of course very silly, but we need to demonstrate mutual recursion somehow so...
+fn mutually_recursive_identity(x: u32) -> u32 {
+    fn switch(src: u32, tgt: u32) -> u32 {
+        match src {
+            0 => tgt,
+            _ if src % 7 == 0 => become advance_with_extra_steps(src, tgt),
+            _ => become advance(src, tgt),
+        }
+    }
+
+    fn advance(src: u32, tgt: u32) -> u32 {
+        become switch(src - 1, tgt + 1)
+    }
+
+    fn advance_with_extra_steps(src: u32, tgt: u32) -> u32 {
+        become advance(src, tgt)
+    }
+
+    switch(x, 0)
+}