Fix an ICE when using become in a default trait method

The logic determining whether the relevant function is marked as `#[track_caller]`
only worked with functions that could be resolved to a concrete instance, which default
trait methods cannot. We need to check the codegen attribute on the default method itself.

Co-authored-by: Folkert de Vries <folkert@folkertdev.nl>

compiler/rustc_lint_defs/src/builtin.rs

@@ -109,6 +109,7 @@
         SHADOWING_SUPERTRAIT_ITEMS,
         SINGLE_USE_LIFETIMES,
         STABLE_FEATURES,
+        TAIL_CALL_TRACK_CALLER,
         TAIL_EXPR_DROP_ORDER,
         TEST_UNSTABLE_LINT,
         TEXT_DIRECTION_CODEPOINT_IN_COMMENT,

compiler/rustc_mir_build/src/check_tail_calls.rs

@@ -4,12 +4,13 @@
 use rustc_hir::LangItem;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::CRATE_DEF_ID;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::span_bug;
 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)?;
@@ -21,7 +22,6 @@ pub(crate) fn check_tail_calls(tcx: TyCtxt<'_>, def: LocalDefId) -> Result<(), E
     }
 
     let is_closure = matches!(tcx.def_kind(def), DefKind::Closure);
-    let caller_ty = tcx.type_of(def).skip_binder();
 
     let mut visitor = TailCallCkVisitor {
         tcx,
@@ -30,7 +30,7 @@ pub(crate) fn check_tail_calls(tcx: TyCtxt<'_>, def: LocalDefId) -> Result<(), E
         // FIXME(#132279): we're clearly in a body here.
         typing_env: ty::TypingEnv::non_body_analysis(tcx, def),
         is_closure,
-        caller_ty,
+        caller_def_id: def,
     };
 
     visitor.visit_expr(&thir[expr]);
@@ -47,8 +47,8 @@ struct TailCallCkVisitor<'a, 'tcx> {
     /// The result of the checks, `Err(_)` if there was a problem with some
     /// tail call, `Ok(())` if all of them were fine.
     found_errors: Result<(), ErrorGuaranteed>,
-    /// Type of the caller function.
-    caller_ty: Ty<'tcx>,
+    /// `LocalDefId` of the caller function.
+    caller_def_id: LocalDefId,
 }
 
 impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
@@ -148,11 +148,13 @@ fn check_tail_call(&mut self, call: &Expr<'_>, expr: &Expr<'_>) {
         // we should think what is the expected behavior here.
         // (we should probably just accept this by revealing opaques?)
         if caller_sig.inputs_and_output != callee_sig.inputs_and_output {
+            let caller_ty = self.tcx.type_of(self.caller_def_id).skip_binder();
+
             self.report_signature_mismatch(
                 expr.span,
                 self.tcx.liberate_late_bound_regions(
                     CRATE_DEF_ID.to_def_id(),
-                    self.caller_ty.fn_sig(self.tcx),
+                    caller_ty.fn_sig(self.tcx),
                 ),
                 self.tcx.liberate_late_bound_regions(CRATE_DEF_ID.to_def_id(), ty.fn_sig(self.tcx)),
             );
@@ -173,7 +175,7 @@ fn check_tail_call(&mut self, call: &Expr<'_>, expr: &Expr<'_>) {
             // 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);
+            let caller_needs_location = self.caller_needs_location();
 
             if caller_needs_location {
                 self.report_track_caller_caller(expr.span);
@@ -189,19 +191,11 @@ fn check_tail_call(&mut self, call: &Expr<'_>, expr: &Expr<'_>) {
         }
     }
 
-    /// 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.
-    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);
-
-            instance.def.requires_caller_location(self.tcx)
-        } else {
-            false
-        }
+    /// Returns true if the caller function needs a location argument
+    /// (i.e. if a function is marked as `#[track_caller]`)
+    fn caller_needs_location(&self) -> bool {
+        let flags = self.tcx.codegen_fn_attrs(self.caller_def_id).flags;
+        flags.contains(CodegenFnAttrFlags::TRACK_CALLER)
     }
 
     fn report_in_closure(&mut self, expr: &Expr<'_>) {

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

@@ -13,4 +13,13 @@ fn c() {
     become a(); //~ error: a function marked with `#[track_caller]` cannot perform a tail-call
 }
 
+trait Trait {
+    fn d(&self);
+
+    #[track_caller]
+    fn e(&self) {
+        become self.d(); //~ error: a function marked with `#[track_caller]` cannot perform a tail-call
+    }
+}
+
 fn main() {}

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

@@ -10,5 +10,11 @@ error: a function marked with `#[track_caller]` cannot perform a tail-call
 LL |     become a();
    |     ^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: a function marked with `#[track_caller]` cannot perform a tail-call
+  --> $DIR/caller_is_track_caller.rs:21:9
+   |
+LL |         become self.d();
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
 

tests/ui/explicit-tail-calls/default-trait-method.rs

@@ -0,0 +1,38 @@
+// A regression test for <https://github.com/rust-lang/rust/issues/144985>.
+// Previously, using `become` in a default trait method would lead to an ICE
+// in a path determining whether the method in question is marked as `#[track_caller]`.
+//
+//@ run-pass
+//@ ignore-backends: gcc
+
+#![feature(explicit_tail_calls)]
+#![expect(incomplete_features)]
+
+trait Trait {
+    fn bar(&self) -> usize {
+        123
+    }
+
+    fn foo(&self) -> usize {
+        #[allow(tail_call_track_caller)]
+        become self.bar();
+    }
+}
+
+struct Struct;
+
+impl Trait for Struct {}
+
+struct OtherStruct;
+
+impl Trait for OtherStruct {
+    #[track_caller]
+    fn bar(&self) -> usize {
+        456
+    }
+}
+
+fn main() {
+    assert_eq!(Struct.foo(), 123);
+    assert_eq!(OtherStruct.foo(), 456);
+}