Implement the alternative try desugaring

Author: scottmcm

core/src/ops/try_trait.rs

@@ -359,11 +359,20 @@ where
 /// and in the other direction,
 /// `<Result<Infallible, E> as Residual<T>>::TryType = Result<T, E>`.
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-#[rustc_const_unstable(feature = "const_try", issue = "74935")]
-pub const trait Residual<O> {
+#[rustc_const_unstable(feature = "const_try_residual", issue = "91285")]
+pub const trait Residual<O>: Sized {
     /// The "return" type of this meta-function.
     #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-    type TryType: Try<Output = O, Residual = Self>;
+    type TryType: [const] Try<Output = O, Residual = Self>;
+
+    /// Here for convenience in the `?` desugaring.
+    /// Probably should not be stabilized, as it should never be overridden.
+    /// (without a `final fn` of some form, cc RFC#3678)
+    #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
+    #[lang = "into_try_type"]
+    fn into_try_type(self) -> Self::TryType {
+        FromResidual::from_residual(self)
+    }
 }
 
 #[unstable(feature = "pub_crate_should_not_need_unstable_attr", issue = "none")]

std/src/sys/pal/unix/kernel_copy/tests.rs

@@ -50,7 +50,7 @@ fn copy_specialization() -> Result<()> {
             "inner Take allowed reading beyond end of file, some bytes should be left"
         );
 
-        let mut sink = sink.into_inner()?;
+        let mut sink = sink.into_inner().map_err(io::Error::from)?;
         sink.seek(SeekFrom::Start(0))?;
         let mut copied = Vec::new();
         sink.read_to_end(&mut copied)?;