Auto merge of #59625 - immunant:copy_variadics_typealias, r=eddyb

Refactor C FFI variadics to more closely match their C counterparts, and add Clone implementation

We had to make some changes to expose `va_copy` and `va_end` directly to users (mainly for C2Rust, but not exclusively):
- redefine the Rust variadic structures to more closely correspond to C: `VaList` now matches `va_list`, and `VaListImpl` matches `__va_list_tag`
- add `Clone` for `VaListImpl`
- add explicit `as_va_list()` conversion function from `VaListImpl` to `VaList`
- add deref coercion from `VaList` to `VaListImpl`
- add support for the `asmjs` target

All these changes were needed for use cases like:
```Rust
let mut ap2 = va_copy(ap);
vprintf(fmt, ap2);
va_end(&mut ap2);
```

2 files changed, 6 insertions, 6 deletions

diff --git a/src/test/ui/error-codes/E0617.rs b/src/test/ui/error-codes/E0617.rs
index 51f13c7dbd5..439c3db5768 100644
--- a/src/test/ui/error-codes/E0617.rs
+++ b/src/test/ui/error-codes/E0617.rs
@@ -22,7 +22,7 @@ fn main() {
         //~^ ERROR can't pass `u16` to variadic function
         //~| HELP cast the value to `c_uint`
         printf(::std::ptr::null(), printf);
-        //~^ ERROR can't pass `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaList<'r>, ...) {printf}` to variadic function
-        //~| HELP cast the value to `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaList<'r>, ...)`
+        //~^ ERROR can't pass `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaListImpl<'r>, ...) {printf}` to variadic function
+        //~| HELP cast the value to `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaListImpl<'r>, ...)`
     }
 }
diff --git a/src/test/ui/error-codes/E0617.stderr b/src/test/ui/error-codes/E0617.stderr
index 8387d5c7e93..d866320bbcd 100644
--- a/src/test/ui/error-codes/E0617.stderr
+++ b/src/test/ui/error-codes/E0617.stderr
@@ -28,15 +28,15 @@ error[E0617]: can't pass `u16` to variadic function
 LL |         printf(::std::ptr::null(), 0u16);
    |                                    ^^^^ help: cast the value to `c_uint`: `0u16 as c_uint`
 
-error[E0617]: can't pass `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaList<'r>, ...) {printf}` to variadic function
+error[E0617]: can't pass `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaListImpl<'r>, ...) {printf}` to variadic function
   --> $DIR/E0617.rs:24:36
    |
 LL |         printf(::std::ptr::null(), printf);
    |                                    ^^^^^^
-help: cast the value to `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaList<'r>, ...)`
+help: cast the value to `for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaListImpl<'r>, ...)`
    |
-LL |         printf(::std::ptr::null(), printf as for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaList<'r>, ...));
-   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |         printf(::std::ptr::null(), printf as for<'r> unsafe extern "C" fn(*const i8, std::ffi::VaListImpl<'r>, ...));
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 6 previous errors