Rollup merge of #62639 - immunant:invariant_valistimpl, r=eddyb

Make VaListImpl<'f> invariant over the 'f lifetime

After doing some research on variance and going back to look at `VaList` and `VaListImpl`, I realized that `VaList<'a, 'f>` is invariant over the `'f` lifetime (and covariant over `'a`), but `VaListImpl<'f>` isn't invariant but probably should be. This patch makes `VaListImpl<'f>` invariant over `'f`.

r? @eddyb
cc @dlrobertson

1 files changed, 8 insertions, 5 deletions

diff --git a/src/libcore/ffi.rs b/src/libcore/ffi.rs
index 4f87cc506ef..eda0e7c518c 100644
--- a/src/libcore/ffi.rs
+++ b/src/libcore/ffi.rs
@@ -60,7 +60,10 @@ impl fmt::Debug for c_void {
 #[lang = "va_list"]
 pub struct VaListImpl<'f> {
     ptr: *mut c_void,
-    _marker: PhantomData<&'f c_void>,
+
+    // Invariant over `'f`, so each `VaListImpl<'f>` object is tied to
+    // the region of the function it's defined in
+    _marker: PhantomData<&'f mut &'f c_void>,
 }
 
 #[cfg(any(all(not(target_arch = "aarch64"), not(target_arch = "powerpc"),
@@ -96,7 +99,7 @@ pub struct VaListImpl<'f> {
     vr_top: *mut c_void,
     gr_offs: i32,
     vr_offs: i32,
-    _marker: PhantomData<&'f c_void>,
+    _marker: PhantomData<&'f mut &'f c_void>,
 }
 
 /// PowerPC ABI implementation of a `va_list`.
@@ -114,7 +117,7 @@ pub struct VaListImpl<'f> {
     reserved: u16,
     overflow_arg_area: *mut c_void,
     reg_save_area: *mut c_void,
-    _marker: PhantomData<&'f c_void>,
+    _marker: PhantomData<&'f mut &'f c_void>,
 }
 
 /// x86_64 ABI implementation of a `va_list`.
@@ -131,7 +134,7 @@ pub struct VaListImpl<'f> {
     fp_offset: i32,
     overflow_arg_area: *mut c_void,
     reg_save_area: *mut c_void,
-    _marker: PhantomData<&'f c_void>,
+    _marker: PhantomData<&'f mut &'f c_void>,
 }
 
 /// asm.js ABI implementation of a `va_list`.
@@ -148,7 +151,7 @@ pub struct VaListImpl<'f> {
 #[lang = "va_list"]
 pub struct VaListImpl<'f> {
     inner: [crate::mem::MaybeUninit<i32>; 4],
-    _marker: PhantomData<&'f c_void>,
+    _marker: PhantomData<&'f mut &'f c_void>,
 }
 
 #[cfg(all(target_arch = "asmjs", not(windows)))]