Fix musttail returns for cast/indirect ABIs

Author: InvalidPathException

compiler/rustc_codegen_llvm/src/builder.rs

@@ -16,7 +16,6 @@ use rustc_codegen_ssa::mir::place::PlaceRef;
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
-use rustc_middle::bug;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::ty::layout::{
     FnAbiError, FnAbiOfHelpers, FnAbiRequest, HasTypingEnv, LayoutError, LayoutOfHelpers,
@@ -1458,10 +1457,7 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
 
         match &fn_abi.ret.mode {
             PassMode::Ignore | PassMode::Indirect { .. } => self.ret_void(),
-            PassMode::Direct(_) | PassMode::Pair { .. } => self.ret(call),
-            mode @ PassMode::Cast { .. } => {
-                bug!("Encountered `PassMode::{mode:?}` during codegen")
-            }
+            PassMode::Direct(_) | PassMode::Pair { .. } | PassMode::Cast { .. } => self.ret(call),
         }
     }
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1055,6 +1055,15 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
         let mut llargs = Vec::with_capacity(arg_count);
 
+        if kind == CallKind::Tail && fn_abi.ret.is_indirect() {
+            match self.make_return_dest(bx, destination, &fn_abi.ret, &mut llargs) {
+                ReturnDest::Nothing => {}
+                _ => bug!(
+                    "tail calls to functions with indirect returns cannot store into a destination"
+                ),
+            }
+        }
+
         // We still need to call `make_return_dest` even if there's no `target`, since
         // `fn_abi.ret` could be `PassMode::Indirect`, even if it is uninhabited,
         // and `make_return_dest` adds the return-place indirect pointer to `llargs`.

tests/ui/explicit-tail-calls/become-cast-return.rs

@@ -0,0 +1,19 @@
+//@ check-pass
+#![allow(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+#[inline(never)]
+fn leaf(_: &Box<u8>) -> [u8; 1] {
+    [1]
+}
+
+#[inline(never)]
+fn dispatch(param: &Box<u8>) -> [u8; 1] {
+    become leaf(param)
+}
+
+fn main() {
+    let data = Box::new(0);
+    let out = dispatch(&data);
+    assert_eq!(out, [1]);
+}

tests/ui/explicit-tail-calls/become-indirect-return.rs

@@ -0,0 +1,20 @@
+//@ check-pass
+#![allow(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+#[inline(never)]
+fn op_dummy(_param: &Box<u8>) -> [u8; 24] {
+    [1; 24]
+}
+
+#[inline(never)]
+fn dispatch(param: &Box<u8>) -> [u8; 24] {
+    become op_dummy(param)
+}
+
+fn main() {
+    let param = Box::new(0);
+    let result = dispatch(&param);
+    eprintln!("dispatch returned: {result:?}");
+    assert_eq!(result, [1; 24]);
+}