eddyb autoref trick written by dtolnay and tweaked by danielhenrymantilla

Fix stability annotations

`autoref!` cleanup: improve comments, and move it under `core::ops`

Author: dtolnay

library/core/src/macros/mod.rs

@@ -515,9 +515,15 @@ macro_rules! r#try {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "write_macro")]
+#[allow_internal_unstable(autoref)]
 macro_rules! write {
     ($dst:expr, $($arg:tt)*) => {
-        $dst.write_fmt($crate::format_args!($($arg)*))
+        match $crate::ops::autoref::autoref_mut!($dst) {
+            mut _dst => {
+                let result = _dst.write_fmt($crate::format_args!($($arg)*));
+                result
+            }
+        }
     };
 }
 
@@ -549,13 +555,18 @@ macro_rules! write {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "writeln_macro")]
-#[allow_internal_unstable(format_args_nl)]
+#[allow_internal_unstable(autoref, format_args_nl)]
 macro_rules! writeln {
     ($dst:expr $(,)?) => {
         $crate::write!($dst, "\n")
     };
     ($dst:expr, $($arg:tt)*) => {
-        $dst.write_fmt($crate::format_args_nl!($($arg)*))
+        match $crate::ops::autoref::autoref_mut!($dst) {
+            mut _dst => {
+                let result = _dst.write_fmt($crate::format_args_nl!($($arg)*));
+                result
+            }
+        }
     };
 }
 

library/core/src/ops/autoref.rs

@@ -0,0 +1,46 @@
+#![allow(missing_docs, missing_debug_implementations)]
+// Poor man's `k#autoref` operator.
+//
+// See https://github.com/rust-lang/rust/issues/99684
+// and https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Desired.20behavior.20of.20.60write!.60.20is.20unimplementable
+// for some more context about this idea.
+//
+// Right now we polyfill this idea, by reducing support to `&mut`-autoref-only
+// (effectively "just" preventing an unnecessary `&mut` level of indirection
+// from being applied for a thing already behind a `&mut …`), which happens to
+// work for `&mut`-based `.write_fmt()` methods, and some cases of `&`-based
+// `.write_fmt()` —the whole duck-typed design / API of `write!` is asking for
+// trouble—, but won't work for a `self`-based `.write_fmt()`, as pointed out
+// here: https://github.com/rust-lang/rust/pull/100202#pullrequestreview-1064499226
+//
+// Finally, in order to reduce the chances of name conflicts as much as
+// possible, the method name is a bit mangled, and to prevent usage of this
+// method in stable-rust, an unstable const generic parameter that needs to be
+// turbofished is added to it as well.
+
+/// The unstable const generic parameter achieving the "unstable seal" effect.
+#[unstable(feature = "autoref", issue = "none")]
+#[derive(Eq, PartialEq)]
+pub struct UnstableMethodSeal;
+
+#[unstable(feature = "autoref", issue = "none")]
+pub trait AutoRef {
+    #[unstable(feature = "autoref", issue = "none")]
+    #[inline(always)]
+    fn __rustc_unstable_auto_ref_mut_helper<const _SEAL: UnstableMethodSeal>(
+        &mut self,
+    ) -> &mut Self {
+        self
+    }
+}
+
+#[unstable(feature = "autoref", issue = "none")]
+impl<T: ?Sized> AutoRef for T {}
+
+#[unstable(feature = "autoref", issue = "none")]
+#[allow_internal_unstable(autoref)]
+#[rustc_macro_transparency = "semitransparent"]
+pub macro autoref_mut($x:expr) {{
+    use $crate::ops::autoref::AutoRef as _;
+    $x.__rustc_unstable_auto_ref_mut_helper::<{ $crate::ops::autoref::UnstableMethodSeal }>()
+}}

library/core/src/ops/mod.rs

@@ -139,6 +139,9 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 mod arith;
+#[doc(hidden)]
+#[unstable(feature = "autoref", issue = "none")]
+pub mod autoref;
 mod bit;
 mod control_flow;
 mod deref;