From 3ce8cfbc58a7cbd0cc01e4c4cc63d269fe8e59d4 Mon Sep 17 00:00:00 2001
From: Folkert de Vries <folkert@folkertdev.nl>
Date: Sun, 29 Mar 2026 18:14:53 +0200
Subject: [PATCH] emit error on `#[track_caller]` with `extern fn`

such a function implicitly uses `extern "C"
---
 .../rustc_ast_passes/src/ast_validation.rs    | 33 ++++++++++++-------
 .../error-with-invalid-abi.rs                 |  6 ++++
 .../error-with-invalid-abi.stderr             | 19 ++++++++++-
 3 files changed, 45 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index dd14e9143569..7d83295cfa65 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -1679,19 +1679,28 @@ fn visit_fn(&mut self, fk: FnKind<'a>, attrs: &AttrVec, span: Span, id: NodeId)
             self.check_item_safety(span, safety);
         }
 
-        if let FnKind::Fn(ctxt, _, fun) = fk
-            && let Extern::Explicit(str_lit, extern_abi_span) = fun.sig.header.ext
-            && let Ok(abi) = ExternAbi::from_str(str_lit.symbol.as_str())
-        {
-            self.check_extern_fn_signature(abi, ctxt, &fun.ident, &fun.sig);
+        if let FnKind::Fn(ctxt, _, fun) = fk {
+            let ext = match fun.sig.header.ext {
+                Extern::None => None,
+                Extern::Implicit(span) => Some((ExternAbi::FALLBACK, span)),
+                Extern::Explicit(str_lit, span) => {
+                    ExternAbi::from_str(str_lit.symbol.as_str()).ok().map(|abi| (abi, span))
+                }
+            };
 
-            if let Some(attr) = attr::find_by_name(attrs, sym::track_caller)
-                && abi != ExternAbi::Rust
-            {
-                self.dcx().emit_err(errors::RequiresRustAbi {
-                    track_caller_span: attr.span,
-                    extern_abi_span,
-                });
+            if let Some((extern_abi, extern_abi_span)) = ext {
+                // Some ABIs impose special restrictions on the signature.
+                self.check_extern_fn_signature(extern_abi, ctxt, &fun.ident, &fun.sig);
+
+                // #[track_caller] can only be used with the rust ABI.
+                if let Some(attr) = attr::find_by_name(attrs, sym::track_caller)
+                    && extern_abi != ExternAbi::Rust
+                {
+                    self.dcx().emit_err(errors::RequiresRustAbi {
+                        track_caller_span: attr.span,
+                        extern_abi_span,
+                    });
+                }
             }
         }
 
diff --git a/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.rs b/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.rs
index 53f99760d88f..a8dfa514d070 100644
--- a/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.rs
+++ b/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.rs
@@ -27,4 +27,10 @@ extern "C" fn c_method() {}
     extern "Rust" fn rust_method() {}
 }
 
+#[rustfmt::skip] // rustfmt will insert the implicit "C"
+#[track_caller]
+//~^ ERROR `#[track_caller]` can only be used with the Rust ABI
+pub extern fn extern_missing_abi() {}
+//~^ WARN `extern` declarations without an explicit ABI are deprecated
+
 fn main() {}
diff --git a/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.stderr b/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.stderr
index f5cba7b8a011..2a1fac08b103 100644
--- a/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.stderr
+++ b/tests/ui/rfcs/rfc-2091-track-caller/error-with-invalid-abi.stderr
@@ -24,6 +24,23 @@ LL |
 LL |     extern "C" fn c_method() {}
    |     ---------- not using the Rust ABI because of this
 
-error: aborting due to 3 previous errors
+error[E0737]: `#[track_caller]` can only be used with the Rust ABI
+  --> $DIR/error-with-invalid-abi.rs:31:1
+   |
+LL | #[track_caller]
+   | ^^^^^^^^^^^^^^^ using `#[track_caller]` here
+LL |
+LL | pub extern fn extern_missing_abi() {}
+   |     ------ not using the Rust ABI because of this
+
+warning: `extern` declarations without an explicit ABI are deprecated
+  --> $DIR/error-with-invalid-abi.rs:33:5
+   |
+LL | pub extern fn extern_missing_abi() {}
+   |     ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`
+   |
+   = note: `#[warn(missing_abi)]` on by default
+
+error: aborting due to 4 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0737`.