make Va::arg and VaList::drop const fns

Author: folkertdev

library/core/src/ffi/va_list.rs

@@ -247,7 +247,8 @@ impl<'f> VaList<'f> {
     ///
     /// [valid]: https://doc.rust-lang.org/nightly/nomicon/what-unsafe-does.html
     #[inline]
-    pub unsafe fn arg<T: VaArgSafe>(&mut self) -> T {
+    #[rustc_const_unstable(feature = "c_variadic_const", issue = "none")]
+    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) }
     }
@@ -265,7 +266,8 @@ impl<'f> Clone for VaList<'f> {
     }
 }
 
-impl<'f> Drop for VaList<'f> {
+#[rustc_const_unstable(feature = "c_variadic_const", issue = "none")]
+impl<'f> const Drop for VaList<'f> {
     fn drop(&mut self) {
         // Rust requires that not calling `va_end` on a `va_list` does not cause undefined behaviour
         // (as it is safe to leak values). As `va_end` is a no-op on all current LLVM targets, this

library/core/src/intrinsics/mod.rs

@@ -3474,7 +3474,7 @@ pub unsafe fn va_copy<'f>(dest: *mut VaList<'f>, src: &VaList<'f>);
 ///
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaList<'_>) -> T;
+pub const unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaList<'_>) -> T;
 
 /// Destroy the arglist `ap` after initialization with `va_start` or `va_copy`.
 ///

tests/ui/parser/variadic-ffi-semantic-restrictions.stderr

@@ -217,6 +217,10 @@ LL | const unsafe extern "C" fn f4_1(x: isize, _: ...) {}
    |                                           ^        - value is dropped here
    |                                           |
    |                                           the destructor for this type cannot be evaluated in constant functions
+   |
+   = note: see issue #133214 <https://github.com/rust-lang/rust/issues/133214> for more information
+   = help: add `#![feature(const_destruct)]` 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+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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[E0493]: destructor of `VaList<'_>` cannot be evaluated at compile-time
   --> $DIR/variadic-ffi-semantic-restrictions.rs:36:36
@@ -225,6 +229,10 @@ LL | const extern "C" fn f4_2(x: isize, _: ...) {}
    |                                    ^        - value is dropped here
    |                                    |
    |                                    the destructor for this type cannot be evaluated in constant functions
+   |
+   = note: see issue #133214 <https://github.com/rust-lang/rust/issues/133214> for more information
+   = help: add `#![feature(const_destruct)]` 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[E0493]: destructor of `VaList<'_>` cannot be evaluated at compile-time
   --> $DIR/variadic-ffi-semantic-restrictions.rs:62:29
@@ -233,6 +241,10 @@ LL |     const fn i_f5(x: isize, _: ...) {}
    |                             ^        - value is dropped here
    |                             |
    |                             the destructor for this type cannot be evaluated in constant functions
+   |
+   = note: see issue #133214 <https://github.com/rust-lang/rust/issues/133214> for more information
+   = help: add `#![feature(const_destruct)]` 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 29 previous errors