Rollup merge of #152675 - folkertdev:va-list-docs, r=tgross35

Improve `VaList` stdlib docs

tracking issue: #44930

Some improvements to the `VaList` documentation, at least adding an example. We should link to the reference for c-variadic functions once stable. I've tried to call out explicitly that the type is meant for sending over the FFI boundary.

r? workingjubilee
cc @tgross35

Author: jdonszelmann

library/core/src/ffi/va_list.rs

@@ -183,7 +183,44 @@ crate::cfg_select! {
     }
 }
 
-/// A variable argument list, equivalent to `va_list` in C.
+/// A variable argument list, ABI-compatible with `va_list` in C.
+///
+/// This type is created in c-variadic functions when `...` is desugared. A `VaList`
+/// is automatically initialized (equivalent to calling `va_start` in C).
+///
+/// ```
+/// #![feature(c_variadic)]
+///
+/// use std::ffi::VaList;
+///
+/// /// # Safety
+/// /// Must be passed at least `count` arguments of type `i32`.
+/// unsafe extern "C" fn my_func(count: u32, ap: ...) -> i32 {
+///     unsafe { vmy_func(count, ap) }
+/// }
+///
+/// /// # Safety
+/// /// Must be passed at least `count` arguments of type `i32`.
+/// unsafe fn vmy_func(count: u32, mut ap: VaList<'_>) -> i32 {
+///     let mut sum = 0;
+///     for _ in 0..count {
+///         sum += unsafe { ap.arg::<i32>() };
+///     }
+///     sum
+/// }
+///
+/// assert_eq!(unsafe { my_func(1, 42i32) }, 42);
+/// assert_eq!(unsafe { my_func(3, 42i32, -7i32, 20i32) }, 55);
+/// ```
+///
+/// The [`VaList::arg`] method can be used to read an argument from the list. This method
+/// automatically advances the `VaList` to the next argument. The C equivalent is `va_arg`.
+///
+/// Cloning a `VaList` performs the equivalent of C `va_copy`, producing an independent cursor
+/// that arguments can be read from without affecting the original. Dropping a `VaList` performs
+/// the equivalent of C `va_end`.
+///
+/// This can be used across an FFI boundary, and fully matches the platform's `va_list`.
 #[repr(transparent)]
 #[lang = "va_list"]
 pub struct VaList<'a> {
@@ -278,20 +315,17 @@ unsafe impl<T> VaArgSafe for *mut T {}
 unsafe impl<T> VaArgSafe for *const T {}
 
 impl<'f> VaList<'f> {
-    /// Advance to and read the next variable argument.
+    /// Read an argument from the variable argument list, and advance to the next argument.
     ///
-    /// # Safety
+    /// Only types that implement [`VaArgSafe`] can be read from a variable argument list.
     ///
-    /// This function is only sound to call when:
+    /// # Safety
     ///
-    /// - there is a next variable argument available.
-    /// - the next argument's type must be ABI-compatible with the type `T`.
-    /// - the next argument must have a properly initialized value of type `T`.
+    /// This function is only sound to call when there is another argument to read, and that
+    /// argument is a properly initialized value of the type `T`.
     ///
     /// Calling this function with an incompatible type, an invalid value, or when there
     /// are no more variable arguments, is unsound.
-    ///
-    /// [valid]: https://doc.rust-lang.org/nightly/nomicon/what-unsafe-does.html
     #[inline]
     #[rustc_const_unstable(feature = "const_c_variadic", issue = "151787")]
     pub const unsafe fn arg<T: VaArgSafe>(&mut self) -> T {