explicit tail call tests with indirect operands in LLVM, small test for indexing into a function table as described by RFC 3407

Author: tnuha

tests/ui/explicit-tail-calls/drop-order.rs

@@ -1,5 +1,3 @@
-// FIXME(explicit_tail_calls): enable this test once rustc_codegen_ssa supports tail calls
-//@ ignore-test: tail calls are not implemented in rustc_codegen_ssa yet, so this causes 🧊
 //@ run-pass
 #![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]

tests/ui/explicit-tail-calls/indexer.rs

@@ -0,0 +1,22 @@
+//@ run-pass
+// Indexing taken from
+// https://github.com/phi-go/rfcs/blob/guaranteed-tco/text%2F0000-explicit-tail-calls.md#tail-call-elimination
+// no other test has utilized the "function table"
+// described in the RFC aside from this one at this point.
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn f0(_: usize) {}
+fn f1(_: usize) {}
+fn f2(_: usize) {}
+
+fn indexer(idx: usize) {
+    let v: [fn(usize); 3] = [f0, f1, f2];
+    become v[idx](idx)
+}
+
+fn main() {
+    for idx in 0..3 {
+        indexer(idx);
+    }
+}

tests/ui/explicit-tail-calls/recursion-etc-u64wrapper.rs

@@ -0,0 +1,48 @@
+//@ build-fail
+//@ normalize-stderr: "note: .*\n\n" -> ""
+//@ normalize-stderr: "thread 'rustc' panicked.*\n" -> ""
+//@ normalize-stderr: "(error: internal compiler error: [^:]+):\d+:\d+: " -> "$1:LL:CC: "
+//@ rustc-env:RUST_BACKTRACE=0
+//@ known-bug: #144293
+//@ failure-status: 101
+// Same as recursion-etc but eggs LLVM emission into giving indirect arguments.
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+use std::hint::black_box;
+
+struct U64Wrapper {
+    pub x: u64,
+    pub arbitrary: String,
+}
+
+fn count(curr: U64Wrapper, top: U64Wrapper) -> U64Wrapper {
+    if black_box(curr.x) >= top.x {
+        curr
+    } else {
+        become count(
+            U64Wrapper {
+                x: curr.x + 1,
+                arbitrary: curr.arbitrary,
+            },
+            top,
+        )
+    }
+}
+
+fn main() {
+    println!(
+        "{}",
+        count(
+            U64Wrapper {
+                x: 0,
+                arbitrary: "hello!".into()
+            },
+            black_box(U64Wrapper {
+                x: 1000000,
+                arbitrary: "goodbye!".into()
+            })
+        )
+        .x
+    );
+}

tests/ui/explicit-tail-calls/recursion-etc-u64wrapper.stderr

@@ -0,0 +1,19 @@
+error: internal compiler error: $COMPILER_DIR_REAL/rustc_codegen_ssa/src/mir/block.rs:LL:CC: arguments using PassMode::Indirect are currently not supported for tail calls
+  --> $DIR/recursion-etc-u64wrapper.rs:23:16
+   |
+LL |           become count(
+   |  ________________^
+LL | |             U64Wrapper {
+LL | |                 x: curr.x + 1,
+LL | |                 arbitrary: curr.arbitrary,
+LL | |             },
+LL | |             top,
+LL | |         )
+   | |_________^
+
+
+Box<dyn Any>
+query stack during panic:
+end of query stack
+error: aborting due to 1 previous error
+