core: avoid extern types in formatting infrastructure

library/core/src/fmt/rt.rs

@@ -5,6 +5,7 @@
 
 use super::*;
 use crate::hint::unreachable_unchecked;
+use crate::ptr::NonNull;
 
 #[lang = "format_placeholder"]
 #[derive(Copy, Clone)]
@@ -65,8 +66,11 @@ pub(super) enum Flag {
 }
 
 #[derive(Copy, Clone)]
-enum ArgumentType<'a> {
-    Placeholder { value: &'a Opaque, formatter: fn(&Opaque, &mut Formatter<'_>) -> Result },
+enum ArgumentType {
+    Placeholder {
+        value: NonNull<()>,
+        formatter: unsafe fn(NonNull<()>, &mut Formatter<'_>) -> Result,
+    },
     Count(usize),
 }
 
@@ -83,7 +87,8 @@ enum ArgumentType<'a> {
 #[lang = "format_argument"]
 #[derive(Copy, Clone)]
 pub struct Argument<'a> {
-    ty: ArgumentType<'a>,
+    ty: ArgumentType,
+    _lifetime: PhantomData<&'a ()>,
 }
 
 #[rustc_diagnostic_item = "ArgumentMethods"]
@@ -98,13 +103,13 @@ impl<'a> Argument<'a> {
         // `mem::transmute(f)` is safe since `fn(&T, &mut Formatter<'_>) -> Result`
         // and `fn(&Opaque, &mut Formatter<'_>) -> Result` have the same ABI
         // (as long as `T` is `Sized`)
-        unsafe {
-            Argument {
-                ty: ArgumentType::Placeholder {
-                    formatter: mem::transmute(f),
-                    value: mem::transmute(x),
-                },
-            }
+        Argument {
+            ty: ArgumentType::Placeholder {
+                value: NonNull::from(x).cast(),
+                // SAFETY: function pointers always have the same layout.
+                formatter: unsafe { mem::transmute(f) },
+            },
+            _lifetime: PhantomData,
         }
     }
 
@@ -146,7 +151,7 @@ impl<'a> Argument<'a> {
     }
     #[inline(always)]
     pub fn from_usize(x: &usize) -> Argument<'_> {
-        Argument { ty: ArgumentType::Count(*x) }
+        Argument { ty: ArgumentType::Count(*x), _lifetime: PhantomData }
     }
 
     /// Format this placeholder argument.
@@ -162,7 +167,14 @@ impl<'a> Argument<'a> {
     #[inline(always)]
     pub(super) unsafe fn fmt(&self, f: &mut Formatter<'_>) -> Result {
         match self.ty {
-            ArgumentType::Placeholder { formatter, value } => formatter(value, f),
+            // SAFETY:
+            // `Argument` is constructed so that if `formatter` originally had
+            // the type `fn(&T, ...)` then `value` has type `&T`. Since we use
+            // `value` within the lifetime 'a of the reference and references
+            // and `NonNull` are ABI-compatible, this is completely equivalent
+            // to calling the original function passed to `new` with the original
+            // reference, which is always sound.
+            ArgumentType::Placeholder { formatter, value } => unsafe { formatter(value, f) },
             // SAFETY: the caller promised this.
             ArgumentType::Count(_) => unsafe { unreachable_unchecked() },
         }
@@ -208,7 +220,3 @@ impl UnsafeArg {
         Self { _private: () }
     }
 }
-
-extern "C" {
-    type Opaque;
-}