feature-gate c-variadic definitions and calls in const contexts

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -698,6 +698,13 @@ impl<'a> AstValidator<'a> {
             unreachable!("C variable argument list cannot be used in closures")
         };
 
+        if let Const::Yes(_) = sig.header.constness
+            && !self.features.enabled(sym::const_c_variadic)
+        {
+            let msg = format!("c-variadic const function definitions are unstable");
+            feature_err(&self.sess, sym::const_c_variadic, sig.span, msg).emit();
+        }
+
         if let Some(coroutine_kind) = sig.header.coroutine_kind {
             self.dcx().emit_err(errors::CoroutineAndCVariadic {
                 spans: vec![coroutine_kind.span(), variadic_param.span],

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -815,6 +815,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     });
                 }
 
+                if self.tcx.fn_sig(callee).skip_binder().c_variadic() {
+                    self.check_op(ops::FnCallCVariadic)
+                }
+
                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
 
                 // `begin_panic` and `panic_display` functions accept generic

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -75,6 +75,27 @@ impl<'tcx> NonConstOp<'tcx> for FnCallIndirect {
     }
 }
 
+/// A c-variadic function call.
+#[derive(Debug)]
+pub(crate) struct FnCallCVariadic;
+impl<'tcx> NonConstOp<'tcx> for FnCallCVariadic {
+    fn status_in_item(&self, _ccx: &ConstCx<'_, 'tcx>) -> Status {
+        Status::Unstable {
+            gate: sym::const_c_variadic,
+            gate_already_checked: false,
+            safe_to_expose_on_stable: false,
+            is_function_call: true,
+        }
+    }
+
+    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
+        ccx.tcx.sess.create_feature_err(
+            errors::NonConstCVariadicCall { span, kind: ccx.const_kind() },
+            sym::const_c_variadic,
+        )
+    }
+}
+
 /// A call to a function that is in a trait, or has trait bounds that make it conditionally-const.
 #[derive(Debug)]
 pub(crate) struct ConditionallyConstCall<'tcx> {

compiler/rustc_const_eval/src/errors.rs

@@ -531,6 +531,19 @@ pub struct NonConstClosure {
     pub non_or_conditionally: &'static str,
 }
 
+#[derive(Diagnostic)]
+#[diag(r#"calling const c-variadic functions is unstable in {$kind ->
+    [const] constant
+    [static] static
+    [const_fn] constant function
+    *[other] {""}
+}s"#, code = E0015)]
+pub struct NonConstCVariadicCall {
+    #[primary_span]
+    pub span: Span,
+    pub kind: ConstContext,
+}
+
 #[derive(Subdiagnostic)]
 pub enum NonConstClosureNote {
     #[note("function defined here, but it is not `const`")]

compiler/rustc_feature/src/unstable.rs

@@ -420,6 +420,8 @@ declare_features! (
     (unstable, const_async_blocks, "1.53.0", Some(85368)),
     /// Allows `const { ... }` as a shorthand for `const _: () = const { ... };` for module items.
     (unstable, const_block_items, "CURRENT_RUSTC_VERSION", Some(149226)),
+    /// Allows defining and calling c-variadic functions in const contexts.
+    (unstable, const_c_variadic, "CURRENT_RUSTC_VERSION", Some(151787)),
     /// Allows `const || {}` closures in const contexts.
     (incomplete, const_closures, "1.68.0", Some(106003)),
     /// Allows using `[const] Destruct` bounds and calling drop impls in const contexts.

compiler/rustc_span/src/symbol.rs

@@ -740,6 +740,7 @@ symbols! {
         const_allocate,
         const_async_blocks,
         const_block_items,
+        const_c_variadic,
         const_closures,
         const_compare_raw_pointers,
         const_constructor,

library/core/src/ffi/va_list.rs

@@ -205,7 +205,7 @@ impl VaList<'_> {
     }
 }
 
-#[rustc_const_unstable(feature = "c_variadic_const", issue = "none")]
+#[rustc_const_unstable(feature = "const_c_variadic", issue = "151787")]
 impl<'f> const Clone for VaList<'f> {
     #[inline]
     fn clone(&self) -> Self {
@@ -217,7 +217,7 @@ impl<'f> const Clone for VaList<'f> {
     }
 }
 
-#[rustc_const_unstable(feature = "c_variadic_const", issue = "none")]
+#[rustc_const_unstable(feature = "const_c_variadic", issue = "151787")]
 impl<'f> const Drop for VaList<'f> {
     fn drop(&mut self) {
         // SAFETY: this variable argument list is being dropped, so won't be read from again.
@@ -293,7 +293,7 @@ impl<'f> VaList<'f> {
     ///
     /// [valid]: https://doc.rust-lang.org/nightly/nomicon/what-unsafe-does.html
     #[inline]
-    #[rustc_const_unstable(feature = "c_variadic_const", issue = "none")]
+    #[rustc_const_unstable(feature = "const_c_variadic", issue = "151787")]
     pub const unsafe fn arg<T: VaArgSafe>(&mut self) -> T {
         // SAFETY: the caller must uphold the safety contract for `va_arg`.
         unsafe { va_arg(self) }

tests/ui/consts/const-eval/c-variadic-fail.rs

@@ -1,7 +1,7 @@
 //@ build-fail
 
 #![feature(c_variadic)]
-#![feature(c_variadic_const)]
+#![feature(const_c_variadic)]
 #![feature(const_trait_impl)]
 #![feature(const_destruct)]
 #![feature(const_clone)]

tests/ui/consts/const-eval/c-variadic.rs

@@ -3,8 +3,8 @@
 //@ ignore-backends: gcc
 
 #![feature(c_variadic)]
+#![feature(const_c_variadic)]
 #![feature(const_destruct)]
-#![feature(c_variadic_const)]
 #![feature(const_cmp)]
 #![feature(const_trait_impl)]
 

tests/ui/feature-gates/feature-gate-const-c-variadic.rs

@@ -0,0 +1,11 @@
+#![feature(c_variadic)]
+
+fn main() {
+    const unsafe extern "C" fn foo(ap: ...) {
+        //~^ ERROR c-variadic const function definitions are unstable
+        core::mem::forget(ap);
+    }
+
+    const { unsafe { foo() } }
+    //~^ ERROR calling const c-variadic functions is unstable in constants
+}

tests/ui/feature-gates/feature-gate-const-c-variadic.stderr

@@ -0,0 +1,24 @@
+error[E0658]: c-variadic const function definitions are unstable
+  --> $DIR/feature-gate-const-c-variadic.rs:4:5
+   |
+LL |     const unsafe extern "C" fn foo(ap: ...) {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0015]: calling const c-variadic functions is unstable in constants
+  --> $DIR/feature-gate-const-c-variadic.rs:9:22
+   |
+LL |     const { unsafe { foo() } }
+   |                      ^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0015, E0658.
+For more information about an error, try `rustc --explain E0015`.