properly reject tail calls to &FnPtr or &FnDef

Author: WaffleLapkin

compiler/rustc_mir_build/src/check_tail_calls.rs

@@ -105,6 +105,13 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
             };
         }
 
+        let (ty::FnDef(..) | ty::FnPtr(..)) = ty.kind() else {
+            self.report_nonfn_callee(fn_span, self.thir[fun].span, ty);
+
+            // `fn_sig` below panics otherwise
+            return;
+        };
+
         // Erase regions since tail calls don't care about lifetimes
         let callee_sig =
             self.tcx.normalize_erasing_late_bound_regions(self.typing_env, ty.fn_sig(self.tcx));
@@ -280,6 +287,31 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
         self.found_errors = Err(err);
     }
 
+    fn report_nonfn_callee(&mut self, call_sp: Span, fun_sp: Span, ty: Ty<'_>) {
+        let mut err = self
+            .tcx
+            .dcx()
+            .struct_span_err(
+                call_sp,
+                "tail calls can only be performed with function definitions or pointers",
+            )
+            .with_note(format!("callee has type `{ty}`"));
+
+        if ty.is_ref() {
+            err.multipart_suggestion(
+                "try dereferencing here",
+                vec![
+                    (fun_sp.shrink_to_lo(), "(*".to_owned()),
+                    (fun_sp.shrink_to_hi(), ")".to_owned()),
+                ],
+                Applicability::MachineApplicable,
+            );
+        }
+
+        let err = err.emit();
+        self.found_errors = Err(err);
+    }
+
     fn report_abi_mismatch(&mut self, sp: Span, caller_abi: ExternAbi, callee_abi: ExternAbi) {
         let err = self
             .tcx

tests/ui/explicit-tail-calls/callee_is_ref.fixed

@@ -0,0 +1,22 @@
+//@ run-rustfix
+#![feature(explicit_tail_calls)]
+#![expect(incomplete_features)]
+
+fn f() {}
+
+fn g() {
+    become (*(&f))() //~ error: tail calls can only be performed with function definitions or pointers
+}
+
+fn h() {
+    let table = [f as fn()];
+    if let Some(fun) = table.get(0) {
+        become (*fun)(); //~ error: tail calls can only be performed with function definitions or pointers
+    }
+}
+
+fn main() {
+    g();
+    h();
+}
+

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

@@ -0,0 +1,21 @@
+//@ run-rustfix
+#![feature(explicit_tail_calls)]
+#![expect(incomplete_features)]
+
+fn f() {}
+
+fn g() {
+    become (&f)() //~ error: tail calls can only be performed with function definitions or pointers
+}
+
+fn h() {
+    let table = [f as fn()];
+    if let Some(fun) = table.get(0) {
+        become fun(); //~ error: tail calls can only be performed with function definitions or pointers
+    }
+}
+
+fn main() {
+    g();
+    h();
+}

tests/ui/explicit-tail-calls/callee_is_ref.stderr

@@ -0,0 +1,26 @@
+error: tail calls can only be performed with function definitions or pointers
+  --> $DIR/callee_is_ref.rs:8:12
+   |
+LL |     become (&f)()
+   |            ^^^^^^
+   |
+   = note: callee has type `&fn() {f}`
+help: try dereferencing here
+   |
+LL |     become (*(&f))()
+   |            ++    +
+
+error: tail calls can only be performed with function definitions or pointers
+  --> $DIR/callee_is_ref.rs:14:16
+   |
+LL |         become fun();
+   |                ^^^^^
+   |
+   = note: callee has type `&fn()`
+help: try dereferencing here
+   |
+LL |         become (*fun)();
+   |                ++   +
+
+error: aborting due to 2 previous errors
+