Reject tail calls of #[track_caller] functions

compiler/rustc_codegen_llvm/src/builder.rs

@@ -1442,6 +1442,18 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         funclet: Option<&Self::Funclet>,
         instance: Option<Instance<'tcx>>,
     ) {
+        if let Some(instance) = instance
+            && instance.def.requires_caller_location(self.tcx)
+        {
+            self.tcx
+                .dcx()
+                .struct_span_fatal(
+                    self.tcx.def_span(instance.def.def_id()),
+                    "a function marked with `#[track_caller]` cannot be tail-called (llvm)",
+                )
+                .emit();
+        }
+
         let call = self.call(llty, fn_attrs, Some(fn_abi), llfn, args, funclet, instance);
         llvm::LLVMRustSetTailCallKind(call, llvm::TailCallKind::MustTail);
 

compiler/rustc_mir_build/src/check_tail_calls.rs

@@ -8,7 +8,7 @@ use rustc_middle::thir::visit::{self, Visitor};
 use rustc_middle::thir::{BodyTy, Expr, ExprId, ExprKind, Thir};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::{DefId, LocalDefId};
-use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
+use rustc_span::{ErrorGuaranteed, Span};
 
 pub(crate) fn check_tail_calls(tcx: TyCtxt<'_>, def: LocalDefId) -> Result<(), ErrorGuaranteed> {
     let (thir, expr) = tcx.thir_body(def)?;
@@ -135,22 +135,19 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
             // `#[track_caller]` affects the ABI of a function (by adding a location argument),
             // so a `track_caller` can only tail call other `track_caller` functions.
             //
-            // The issue is however that we can't know if a function is `track_caller` or not at
-            // this point (THIR can be polymorphic, we may have an unresolved trait function).
-            // We could only allow functions that we *can* resolve and *are* `track_caller`,
-            // but that would turn changing `track_caller`-ness into a breaking change,
-            // which is probably undesirable.
+            // The issue is however that we can't always know if a function is `track_caller` or not
+            // at this point (THIR can be polymorphic, we may have an unresolved trait function).
+            // To prevent problems we deny code if we detect `#[track_caller]`.
             //
-            // Also note that we don't check callee's `track_caller`-ness at all, mostly for the
-            // reasons above, but also because we can always tailcall the shim we'd generate for
-            // coercing the function to an `fn()` pointer. (although in that case the tailcall is
-            // basically useless -- the shim calls the actual function, so tailcalling the shim is
-            // equivalent to calling the function)
-            let caller_needs_location = self.needs_location(self.caller_ty);
-
-            if caller_needs_location {
+            // N.B. because we can't always know if the callee needs location,
+            // backends have to re-check that.
+            if self.needs_location(self.caller_ty) {
                 self.report_track_caller_caller(expr.span);
             }
+
+            if self.needs_location(ty) {
+                self.report_track_caller_callee(expr.span);
+            }
         }
 
         if caller_sig.c_variadic {
@@ -165,12 +162,13 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
     /// Returns true if function of type `ty` needs location argument
     /// (i.e. if a function is marked as `#[track_caller]`).
     ///
-    /// Panics if the function's instance can't be immediately resolved.
+    /// N.B. might spuriously return `false` in cases when it's not known if the function is
+    /// `#[track_caller]` or not (e.g. polymorphic thir)
     fn needs_location(&self, ty: Ty<'tcx>) -> bool {
-        if let &ty::FnDef(did, substs) = ty.kind() {
-            let instance =
-                ty::Instance::expect_resolve(self.tcx, self.typing_env, did, substs, DUMMY_SP);
-
+        if let &ty::FnDef(did, args) = ty.kind()
+            && let Ok(Some(instance)) =
+                ty::Instance::try_resolve(self.tcx, self.typing_env, did, args)
+        {
             instance.def.requires_caller_location(self.tcx)
         } else {
             false
@@ -321,6 +319,16 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
         self.found_errors = Err(err);
     }
 
+    fn report_track_caller_callee(&mut self, sp: Span) {
+        let err = self
+            .tcx
+            .dcx()
+            .struct_span_err(sp, "a function marked with `#[track_caller]` cannot be tail-called")
+            .emit();
+
+        self.found_errors = Err(err);
+    }
+
     fn report_c_variadic_caller(&mut self, sp: Span) {
         let err = self
             .tcx

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

@@ -1,15 +1,11 @@
-//@ check-pass
-// FIXME(explicit_tail_calls): make this run-pass, once tail calls are properly implemented
 #![expect(incomplete_features)]
 #![feature(explicit_tail_calls)]
 
-fn a(x: u32) -> u32 {
-    become b(x);
+fn a() {
+    become b(); //~ error: a function marked with `#[track_caller]` cannot be tail-called
 }
 
 #[track_caller]
-fn b(x: u32) -> u32 { x + 42 }
+fn b() {}
 
-fn main() {
-    assert_eq!(a(12), 54);
-}
+fn main() {}

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

@@ -0,0 +1,8 @@
+error: a function marked with `#[track_caller]` cannot be tail-called
+  --> $DIR/callee_is_track_caller.rs:5:5
+   |
+LL |     become b();
+   |     ^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

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

@@ -0,0 +1,21 @@
+//@ build-fail
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn c<T: Trait>() {
+    // `T::f` is not known when checking tail calls,
+    // so this has to be checked by the backend.
+    become T::f();
+}
+
+trait Trait {
+    #[track_caller]
+    // FIXME(explicit_tail_calls): make error point to the tail call, not callee definition
+    fn f() {} //~ error: a function marked with `#[track_caller]` cannot be tail-called
+}
+
+impl Trait for () {}
+
+fn main() {
+    c::<()>();
+}

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

@@ -0,0 +1,8 @@
+error: a function marked with `#[track_caller]` cannot be tail-called (llvm)
+  --> $DIR/callee_is_track_caller_polymorphic.rs:14:5
+   |
+LL |     fn f() {}
+   |     ^^^^^^
+
+error: aborting due to 1 previous error
+

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

@@ -3,14 +3,17 @@
 
 #[track_caller]
 fn a() {
-    become b(); //~ error: a function marked with `#[track_caller]` cannot perform a tail-call
+    become b();
+    //~^ error: a function marked with `#[track_caller]` cannot perform a tail-call
 }
 
 fn b() {}
 
 #[track_caller]
 fn c() {
-    become a(); //~ error: a function marked with `#[track_caller]` cannot perform a tail-call
+    become a();
+    //~^ error: a function marked with `#[track_caller]` cannot perform a tail-call
+    //~| error: a function marked with `#[track_caller]` cannot be tail-called
 }
 
 fn main() {}

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

@@ -5,10 +5,16 @@ LL |     become b();
    |     ^^^^^^^^^^
 
 error: a function marked with `#[track_caller]` cannot perform a tail-call
-  --> $DIR/caller_is_track_caller.rs:13:5
+  --> $DIR/caller_is_track_caller.rs:14:5
    |
 LL |     become a();
    |     ^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: a function marked with `#[track_caller]` cannot be tail-called
+  --> $DIR/caller_is_track_caller.rs:14:5
+   |
+LL |     become a();
+   |     ^^^^^^^^^^
+
+error: aborting due to 3 previous errors