Auto merge of #49878 - dlrobertson:va_list_pt0, r=eddyb

libcore: Add VaList and variadic arg handling intrinsics

## Summary

 - Add intrinsics for `va_start`, `va_end`, `va_copy`, and `va_arg`.
 - Add `core::va_list::VaList` to `libcore`.

Part 1 of (at least) 3 for #44930

Comments and critiques are very much welcomed 😄

1 files changed, 185 insertions, 0 deletions

diff --git a/src/libcore/ffi.rs b/src/libcore/ffi.rs
index a03756f9c22..edeb3b0d368 100644
--- a/src/libcore/ffi.rs
+++ b/src/libcore/ffi.rs
@@ -1,6 +1,7 @@
 #![stable(feature = "", since = "1.30.0")]
 
 #![allow(non_camel_case_types)]
+#![cfg_attr(stage0, allow(dead_code))]
 
 //! Utilities related to FFI bindings.
 
@@ -40,3 +41,187 @@ impl fmt::Debug for c_void {
         f.pad("c_void")
     }
 }
+
+/// Basic implementation of a `va_list`.
+#[cfg(any(all(not(target_arch = "aarch64"), not(target_arch = "powerpc"),
+              not(target_arch = "x86_64")),
+          windows))]
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+extern {
+    type VaListImpl;
+}
+
+#[cfg(any(all(not(target_arch = "aarch64"), not(target_arch = "powerpc"),
+              not(target_arch = "x86_64")),
+          windows))]
+impl fmt::Debug for VaListImpl {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "va_list* {:p}", self)
+    }
+}
+
+/// AArch64 ABI implementation of a `va_list`. See the
+/// [Aarch64 Procedure Call Standard] for more details.
+///
+/// [AArch64 Procedure Call Standard]:
+/// http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf
+#[cfg(all(target_arch = "aarch64", not(windows)))]
+#[repr(C)]
+#[derive(Debug)]
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+struct VaListImpl {
+    stack: *mut (),
+    gr_top: *mut (),
+    vr_top: *mut (),
+    gr_offs: i32,
+    vr_offs: i32,
+}
+
+/// PowerPC ABI implementation of a `va_list`.
+#[cfg(all(target_arch = "powerpc", not(windows)))]
+#[repr(C)]
+#[derive(Debug)]
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+struct VaListImpl {
+    gpr: u8,
+    fpr: u8,
+    reserved: u16,
+    overflow_arg_area: *mut (),
+    reg_save_area: *mut (),
+}
+
+/// x86_64 ABI implementation of a `va_list`.
+#[cfg(all(target_arch = "x86_64", not(windows)))]
+#[repr(C)]
+#[derive(Debug)]
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+struct VaListImpl {
+    gp_offset: i32,
+    fp_offset: i32,
+    overflow_arg_area: *mut (),
+    reg_save_area: *mut (),
+}
+
+/// A wrapper for a `va_list`
+#[lang = "va_list"]
+#[derive(Debug)]
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+#[repr(transparent)]
+#[cfg(not(stage0))]
+pub struct VaList<'a>(&'a mut VaListImpl);
+
+// The VaArgSafe trait needs to be used in public interfaces, however, the trait
+// itself must not be allowed to be used outside this module. Allowing users to
+// implement the trait for a new type (thereby allowing the va_arg intrinsic to
+// be used on a new type) is likely to cause undefined behavior.
+//
+// FIXME(dlrobertson): In order to use the VaArgSafe trait in a public interface
+// but also ensure it cannot be used elsewhere, the trait needs to be public
+// within a private module. Once RFC 2145 has been implemented look into
+// improving this.
+mod sealed_trait {
+    /// Trait which whitelists the allowed types to be used with [VaList::arg]
+    ///
+    /// [VaList::va_arg]: struct.VaList.html#method.arg
+    #[unstable(feature = "c_variadic",
+               reason = "the `c_variadic` feature has not been properly tested on \
+                         all supported platforms",
+               issue = "27745")]
+    pub trait VaArgSafe {}
+}
+
+macro_rules! impl_va_arg_safe {
+    ($($t:ty),+) => {
+        $(
+            #[unstable(feature = "c_variadic",
+                       reason = "the `c_variadic` feature has not been properly tested on \
+                                 all supported platforms",
+                       issue = "27745")]
+            impl sealed_trait::VaArgSafe for $t {}
+        )+
+    }
+}
+
+impl_va_arg_safe!{i8, i16, i32, i64, usize}
+impl_va_arg_safe!{u8, u16, u32, u64, isize}
+impl_va_arg_safe!{f64}
+
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+impl<T> sealed_trait::VaArgSafe for *mut T {}
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+impl<T> sealed_trait::VaArgSafe for *const T {}
+
+#[cfg(not(stage0))]
+impl<'a> VaList<'a> {
+    /// Advance to the next arg.
+    #[unstable(feature = "c_variadic",
+               reason = "the `c_variadic` feature has not been properly tested on \
+                         all supported platforms",
+               issue = "27745")]
+    pub unsafe fn arg<T: sealed_trait::VaArgSafe>(&mut self) -> T {
+        va_arg(self)
+    }
+
+    /// Copy the `va_list` at the current location.
+    #[unstable(feature = "c_variadic",
+               reason = "the `c_variadic` feature has not been properly tested on \
+                         all supported platforms",
+               issue = "27745")]
+    pub unsafe fn copy<F, R>(&mut self, f: F) -> R
+            where F: for<'copy> FnOnce(VaList<'copy>) -> R {
+        #[cfg(any(all(not(target_arch = "aarch64"), not(target_arch = "powerpc"),
+                      not(target_arch = "x86_64")),
+                  windows))]
+        let mut ap = va_copy(self);
+        #[cfg(all(any(target_arch = "aarch64", target_arch = "powerpc", target_arch = "x86_64"),
+                  not(windows)))]
+        let mut ap_inner = va_copy(self);
+        #[cfg(all(any(target_arch = "aarch64", target_arch = "powerpc", target_arch = "x86_64"),
+                  not(windows)))]
+        let mut ap = VaList(&mut ap_inner);
+        let ret = f(VaList(ap.0));
+        va_end(&mut ap);
+        ret
+    }
+}
+
+#[cfg(not(stage0))]
+extern "rust-intrinsic" {
+    /// Destroy the arglist `ap` after initialization with `va_start` or
+    /// `va_copy`.
+    fn va_end(ap: &mut VaList);
+
+    /// Copy the current location of arglist `src` to the arglist `dst`.
+    #[cfg(any(all(not(target_arch = "aarch64"), not(target_arch = "powerpc"),
+                  not(target_arch = "x86_64")),
+              windows))]
+    fn va_copy<'a>(src: &VaList<'a>) -> VaList<'a>;
+    #[cfg(all(any(target_arch = "aarch64", target_arch = "powerpc", target_arch = "x86_64"),
+              not(windows)))]
+    fn va_copy(src: &VaList) -> VaListImpl;
+
+    /// Loads an argument of type `T` from the `va_list` `ap` and increment the
+    /// argument `ap` points to.
+    fn va_arg<T: sealed_trait::VaArgSafe>(ap: &mut VaList) -> T;
+}