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

Author: bryangarza

compiler/rustc_ast_lowering/src/expr.rs

@@ -645,17 +645,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
             hir::ExprKind::Closure(c)
         };
 
-        let track_caller = outer_hir_id
-            .and_then(|id| self.attrs.get(&id.local_id))
-            .map_or(false, |attrs| attrs.into_iter().any(|attr| attr.has_name(sym::track_caller)));
-
         let hir_id = self.lower_node_id(closure_node_id);
-        if track_caller {
-            let unstable_span = self.mark_span_with_reason(
-                DesugaringKind::Async,
-                span,
-                self.allow_gen_future.clone(),
-            );
+        let unstable_span =
+            self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
+
+        if self.tcx.features().closure_track_caller
+            && let Some(outer_hir_id) = outer_hir_id
+            && let Some(attrs) = self.attrs.get(&outer_hir_id.local_id)
+            && attrs.into_iter().any(|attr| attr.has_name(sym::track_caller))
+        {
             self.lower_attrs(
                 hir_id,
                 &[Attribute {

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

@@ -350,6 +350,9 @@ 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
+     .label = this function will not propagate the caller location
+
 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};
@@ -1337,6 +1341,72 @@ impl<'tcx> LateLintPass<'tcx> for UnstableFeatures {
     }
 }
 
+declare_lint! {
+    /// The `ungated_async_fn_track_caller` lint warns when the
+    /// `#[track_caller]` attribute is used on an async function, method, or
+    /// closure, without enabling the corresponding unstable feature flag.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #[track_caller]
+    /// async fn foo() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The attribute must be used in conjunction with the
+    /// [`closure_track_caller` feature flag]. Otherwise, the `#[track_caller]`
+    /// annotation will function as as no-op.
+    ///
+    /// [`closure_track_caller` feature flag]: https://doc.rust-lang.org/beta/unstable-book/language-features/closure-track-caller.html
+    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 fn_kind.asyncness() == IsAsync::Async
+            && !cx.tcx.features().closure_track_caller
+            && let attrs = cx.tcx.hir().attrs(hir_id)
+            // Now, check if the function has the `#[track_caller]` attribute
+            && let Some(attr) = attrs.iter().find(|attr| attr.has_name(sym::track_caller))
+            {
+                cx.struct_span_lint(
+                    UNGATED_ASYNC_FN_TRACK_CALLER,
+                    attr.span,
+                    fluent::lint_ungated_async_fn_track_caller,
+                    |lint| {
+                        lint.span_label(span, fluent::label);
+                        rustc_session::parse::add_feature_diagnostics(
+                            lint,
+                            &cx.tcx.sess.parse_sess,
+                            sym::closure_track_caller,
+                        );
+                        lint
+                    },
+                );
+            }
+    }
+}
+
 declare_lint! {
     /// The `unreachable_pub` lint triggers for `pub` items not reachable from
     /// the crate root.

compiler/rustc_lint/src/lib.rs

@@ -219,6 +219,7 @@ late_lint_methods!(
             // May Depend on constants elsewhere
             UnusedBrokenConst: UnusedBrokenConst,
             UnstableFeatures: UnstableFeatures,
+            UngatedAsyncFnTrackCaller: UngatedAsyncFnTrackCaller,
             ArrayIntoIter: ArrayIntoIter::default(),
             DropTraitConstraints: DropTraitConstraints,
             TemporaryCStringAsPtr: TemporaryCStringAsPtr,

src/test/ui/async-await/track-caller/async-closure-gate.rs

@@ -0,0 +1,9 @@
+// edition:2021
+
+#![feature(async_closure, stmt_expr_attributes)]
+
+fn main() {
+    let _ = #[track_caller] async || {
+        //~^ ERROR `#[track_caller]` on closures is currently unstable [E0658]
+    };
+}

src/test/ui/async-await/track-caller/async-closure-gate.stderr

@@ -0,0 +1,12 @@
+error[E0658]: `#[track_caller]` on closures is currently unstable
+  --> $DIR/async-closure-gate.rs:6:13
+   |
+LL |     let _ = #[track_caller] async || {
+   |             ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #87417 <https://github.com/rust-lang/rust/issues/87417> for more information
+   = help: add `#![feature(closure_track_caller)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/async-await/track-caller/panic-track-caller.nofeat.stderr

@@ -0,0 +1,29 @@
+warning: `#[track_caller]` on async functions is a no-op
+  --> $DIR/panic-track-caller.rs:50:1
+   |
+LL |   #[track_caller]
+   |   ^^^^^^^^^^^^^^^
+LL | / async fn bar_track_caller() {
+LL | |     panic!()
+LL | | }
+   | |_- this function will not propagate the caller location
+   |
+   = note: see issue #87417 <https://github.com/rust-lang/rust/issues/87417> for more information
+   = help: add `#![feature(closure_track_caller)]` to the crate attributes to enable
+   = note: `#[warn(ungated_async_fn_track_caller)]` on by default
+
+warning: `#[track_caller]` on async functions is a no-op
+  --> $DIR/panic-track-caller.rs:62:5
+   |
+LL |       #[track_caller]
+   |       ^^^^^^^^^^^^^^^
+LL | /     async fn bar_assoc() {
+LL | |         panic!();
+LL | |     }
+   | |_____- this function will not propagate the caller location
+   |
+   = note: see issue #87417 <https://github.com/rust-lang/rust/issues/87417> for more information
+   = help: add `#![feature(closure_track_caller)]` to the crate attributes to enable
+
+warning: 2 warnings emitted
+

src/test/ui/async-await/track-caller/panic-track-caller.rs

@@ -1,7 +1,9 @@
 // run-pass
 // edition:2021
+// revisions: feat nofeat
 // needs-unwind
-#![feature(closure_track_caller)]
+#![feature(async_closure, stmt_expr_attributes)]
+#![cfg_attr(feat, feature(closure_track_caller))]
 
 use std::future::Future;
 use std::panic;
@@ -45,7 +47,7 @@ async fn foo() {
     bar().await
 }
 
-#[track_caller]
+#[track_caller] //[nofeat]~ WARN `#[track_caller]` on async functions is a no-op
 async fn bar_track_caller() {
     panic!()
 }
@@ -57,7 +59,7 @@ async fn foo_track_caller() {
 struct Foo;
 
 impl Foo {
-    #[track_caller]
+    #[track_caller] //[nofeat]~ WARN `#[track_caller]` on async functions is a no-op
     async fn bar_assoc() {
         panic!();
     }
@@ -84,7 +86,15 @@ fn panicked_at(f: impl FnOnce() + panic::UnwindSafe) -> u32 {
 }
 
 fn main() {
-    assert_eq!(panicked_at(|| block_on(foo())), 41);
-    assert_eq!(panicked_at(|| block_on(foo_track_caller())), 54);
-    assert_eq!(panicked_at(|| block_on(foo_assoc())), 67);
+    assert_eq!(panicked_at(|| block_on(foo())), 43);
+
+    #[cfg(feat)]
+    assert_eq!(panicked_at(|| block_on(foo_track_caller())), 56);
+    #[cfg(nofeat)]
+    assert_eq!(panicked_at(|| block_on(foo_track_caller())), 52);
+
+    #[cfg(feat)]
+    assert_eq!(panicked_at(|| block_on(foo_assoc())), 69);
+    #[cfg(nofeat)]
+    assert_eq!(panicked_at(|| block_on(foo_assoc())), 64);
 }