Switch #[track_caller] back to a no-op unless feature gate is enabled

This patch fixes a regression, in which `#[track_caller]`, which was
previously a no-op, was changed to actually turn on the behavior. This
should instead only happen behind the `closure_track_caller` feature
gate.

Also, add a warning for the user to understand how their code will
compile depending on the feature gate being turned on or not.

Fixes #104588

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -350,6 +350,8 @@ lint_builtin_mutable_transmutes =
 
 lint_builtin_unstable_features = unstable feature
 
+lint_ungated_async_fn_track_caller = `#[track_caller]` on async functions is a no-op, unless the `closure_track_caller` feature is enabled
+
 lint_builtin_unreachable_pub = unreachable `pub` {$what}
     .suggestion = consider restricting its visibility
     .help = or consider exporting it for use by other crates

compiler/rustc_lint/src/builtin.rs

@@ -25,6 +25,7 @@ use crate::{
     types::{transparent_newtype_field, CItemKind},
     EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext,
 };
+use hir::IsAsync;
 use rustc_ast::attr;
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::visit::{FnCtxt, FnKind};
@@ -40,7 +41,10 @@ use rustc_feature::{deprecated_attributes, AttributeGate, BuiltinAttribute, Gate
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId, LocalDefIdSet, CRATE_DEF_ID};
-use rustc_hir::{ForeignItemKind, GenericParamKind, HirId, Node, PatKind, PredicateOrigin};
+use rustc_hir::intravisit::FnKind as HirFnKind;
+use rustc_hir::{
+    Body, FnDecl, ForeignItemKind, GenericParamKind, HirId, Node, PatKind, PredicateOrigin,
+};
 use rustc_index::vec::Idx;
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::layout::{LayoutError, LayoutOf};
@@ -1370,6 +1374,45 @@ impl<'tcx> LateLintPass<'tcx> for UnstableFeatures {
     }
 }
 
+declare_lint! {
+    /// `#[track_caller]` is a no-op without corresponding feature flag
+    UNGATED_ASYNC_FN_TRACK_CALLER,
+    Warn,
+    "enabling track_caller on an async fn is a no-op unless the closure_track_caller feature is enabled"
+}
+
+declare_lint_pass!(
+    /// Explains corresponding feature flag must be enabled for the `#[track_caller] attribute to
+    /// do anything
+    UngatedAsyncFnTrackCaller => [UNGATED_ASYNC_FN_TRACK_CALLER]
+);
+
+impl<'tcx> LateLintPass<'tcx> for UngatedAsyncFnTrackCaller {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'_>,
+        fn_kind: HirFnKind<'_>,
+        _: &'tcx FnDecl<'_>,
+        _: &'tcx Body<'_>,
+        span: Span,
+        hir_id: HirId,
+    ) {
+        if let HirFnKind::ItemFn(_, _, _) = fn_kind && fn_kind.asyncness() == IsAsync::Async && !cx.tcx.features().closure_track_caller {
+            // Now, check if the function has the `#[track_caller]` attribute
+            let attrs = cx.tcx.hir().attrs(hir_id);
+            let maybe_track_caller = attrs.iter().find(|attr| attr.has_name(sym::track_caller));
+            if let Some(attr) = maybe_track_caller {
+                cx.struct_span_lint(
+                    UNGATED_ASYNC_FN_TRACK_CALLER,
+                    span.with_hi(attr.span.hi()),
+                    fluent::lint_ungated_async_fn_track_caller,
+                    |lint| lint,
+                    );
+            }
+        }
+    }
+}
+
 declare_lint! {
     /// The `unreachable_pub` lint triggers for `pub` items not reachable from
     /// the crate root.

src/test/ui/async-await/track-caller/issue-104588-no-op-panic-track-caller.rs

@@ -0,0 +1,78 @@
+// run-pass
+// edition:2021
+// needs-unwind
+
+
+use std::future::Future;
+use std::panic;
+use std::sync::{Arc, Mutex};
+use std::task::{Context, Poll, Wake};
+use std::thread::{self, Thread};
+
+/// A waker that wakes up the current thread when called.
+struct ThreadWaker(Thread);
+
+impl Wake for ThreadWaker {
+    fn wake(self: Arc<Self>) {
+        self.0.unpark();
+    }
+}
+
+/// Run a future to completion on the current thread.
+fn block_on<T>(fut: impl Future<Output = T>) -> T {
+    // Pin the future so it can be polled.
+    let mut fut = Box::pin(fut);
+
+    // Create a new context to be passed to the future.
+    let t = thread::current();
+    let waker = Arc::new(ThreadWaker(t)).into();
+    let mut cx = Context::from_waker(&waker);
+
+    // Run the future to completion.
+    loop {
+        match fut.as_mut().poll(&mut cx) {
+            Poll::Ready(res) => return res,
+            Poll::Pending => thread::park(),
+        }
+    }
+}
+
+async fn bar() {
+    panic!()
+}
+
+async fn foo() {
+    bar().await
+}
+
+#[track_caller] //~ WARN `#[track_caller]` on async functions is a no-op, unless the `closure_track_caller` feature is enabled [ungated_async_fn_track_caller]
+async fn bar_track_caller() {
+    panic!()
+}
+
+async fn foo_track_caller() {
+    bar_track_caller().await
+}
+
+fn panicked_at(f: impl FnOnce() + panic::UnwindSafe) -> u32 {
+    let loc = Arc::new(Mutex::new(None));
+
+    let hook = panic::take_hook();
+    {
+        let loc = loc.clone();
+        panic::set_hook(Box::new(move |info| {
+            *loc.lock().unwrap() = info.location().map(|loc| loc.line())
+        }));
+    }
+    panic::catch_unwind(f).unwrap_err();
+    panic::set_hook(hook);
+    let x = loc.lock().unwrap().unwrap();
+    x
+}
+
+fn main() {
+    assert_eq!(panicked_at(|| block_on(foo())), 41);
+    // Since the `closure_track_caller` feature is not enabled, the
+    // `track_caller annotation does nothing.
+    assert_eq!(panicked_at(|| block_on(foo_track_caller())), 50);
+}

src/test/ui/async-await/track-caller/issue-104588-no-op-panic-track-caller.stderr

@@ -0,0 +1,12 @@
+warning: `#[track_caller]` on async functions is a no-op, unless the `closure_track_caller` feature is enabled
+  --> $DIR/issue-104588-no-op-panic-track-caller.rs:48:16
+   |
+LL |   #[track_caller]
+   |  ________________^
+LL | | async fn bar_track_caller() {
+   | |_
+   |
+   = note: `#[warn(ungated_async_fn_track_caller)]` on by default
+
+warning: 1 warning emitted
+

src/test/ui/async-await/track-caller/issue-104588-no-op-track-caller.rs

@@ -0,0 +1,9 @@
+// check-pass
+// edition:2021
+
+#[track_caller] //~ WARN `#[track_caller]` on async functions is a no-op, unless the `closure_track_caller` feature is enabled
+async fn foo() {}
+
+fn main() {
+    foo();
+}

src/test/ui/async-await/track-caller/issue-104588-no-op-track-caller.stderr

@@ -0,0 +1,12 @@
+warning: `#[track_caller]` on async functions is a no-op, unless the `closure_track_caller` feature is enabled
+  --> $DIR/issue-104588-no-op-track-caller.rs:4:16
+   |
+LL |   #[track_caller]
+   |  ________________^
+LL | | async fn foo() {}
+   | |_
+   |
+   = note: `#[warn(ungated_async_fn_track_caller)]` on by default
+
+warning: 1 warning emitted
+