Rollup merge of rust-lang#148725 - scottmcm:experiment-new-try-block-v3, r=petrochenkov

Implement the alternative `try` block desugaring

As discussed in rust-lang/rfcs#3721 (comment), update the `try` in nightly to match the RFC as a way to experiment.

This addresses the following unresolved issue from rust-lang#31436

>  Address issues with type inference (`try { expr? }?` currently requires an explicit type annotation somewhere).

Author: Zalathar

core/src/ops/try_trait.rs

@@ -359,11 +359,24 @@ 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>;
+}
+
+/// Used in `try {}` blocks so the type produced in the `?` desugaring
+/// depends on the residual type `R` and the output type of the block `O`,
+/// but importantly not on the contextual type the way it would be if
+/// we called `<_ as FromResidual>::from_residual(r)` directly.
+#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
+// needs to be `pub` to avoid `private type` errors
+#[expect(unreachable_pub)]
+#[inline] // FIXME: force would be nice, but fails -- see #148915
+#[lang = "into_try_type"]
+pub fn residual_into_try_type<R: Residual<O>, O>(r: R) -> <R as Residual<O>>::TryType {
+    FromResidual::from_residual(r)
 }
 
 #[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)?;