tests: add ui fail/pass tests and llvm ir tail codegen tests

Author: xacrimon

tests/codegen/become-musttail.rs

@@ -0,0 +1,18 @@
+//@ compile-flags: -C opt-level=0 -Cpanic=abort -C no-prepopulate-passes
+//@ needs-unwind
+
+#![crate_type = "lib"]
+#![feature(explicit_tail_calls)]
+
+// CHECK-LABEL: define {{.*}}@fibonacci(
+#[no_mangle]
+#[inline(never)]
+pub fn fibonacci(n: u64, a: u64, b: u64) -> u64 {
+    // CHECK: musttail call {{.*}}@fibonacci(
+    // CHECK-NEXT: ret u64
+    match n {
+        0 => a,
+        1 => b,
+        _ => become fibonacci(n - 1, b, a + b),
+    }
+}

tests/ui/explicit-tail-calls/recursion-no-tce.rs

@@ -0,0 +1,16 @@
+//@ run-fail
+
+use std::hint::black_box;
+
+pub fn count(curr: u64, top: u64) -> u64 {
+   if black_box(curr) >= top {
+        curr
+   } else {
+        count(curr + 1, top)
+   }
+}
+
+fn main() {
+    println!("{}",
+    count(0, black_box(1000000)));
+}

tests/ui/explicit-tail-calls/recursion-tce.rs

@@ -0,0 +1,17 @@
+//@ run-pass
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+use std::hint::black_box;
+
+pub fn count(curr: u64, top: u64) -> u64 {
+   if black_box(curr) >= top {
+        curr
+   } else {
+        become count(curr + 1, top)
+   }
+}
+
+fn main() {
+    println!("{}", count(0, black_box(1000000)));
+}