DEMO: Implement the alternative try desugaring

compiler/rustc_ast_lowering/src/expr.rs

@@ -1505,7 +1505,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ///     ControlFlow::Break(residual) =>
     ///         #[allow(unreachable_code)]
     ///         // If there is an enclosing `try {...}`:
-    ///         break 'catch_target Try::from_residual(residual),
+    ///         break 'catch_target Residual::into_try_type(residual),
     ///         // Otherwise:
     ///         return Try::from_residual(residual),
     /// }
@@ -1571,7 +1571,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
             let from_residual_expr = self.wrap_in_try_constructor(
-                hir::LangItem::TryTraitFromResidual,
+                if self.catch_scope.is_some() {
+                    hir::LangItem::ResidualIntoTryType
+                } else {
+                    hir::LangItem::TryTraitFromResidual
+                },
                 try_span,
                 self.arena.alloc(residual_expr),
                 unstable_span,

compiler/rustc_hir/src/lang_items.rs

@@ -294,6 +294,7 @@ language_item_table! {
     TryTraitFromOutput,      sym::from_output,         from_output_fn,             Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     TryTraitBranch,          sym::branch,              branch_fn,                  Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     TryTraitFromYeet,        sym::from_yeet,           from_yeet_fn,               Target::Fn,             GenericRequirement::None;
+    ResidualIntoTryType,     sym::into_try_type,       into_try_type_fn,           Target::Method(MethodKind::Trait { body: true }), GenericRequirement::None;
 
     PollReady,               sym::Ready,               poll_ready_variant,         Target::Variant,        GenericRequirement::None;
     PollPending,             sym::Pending,             poll_pending_variant,       Target::Variant,        GenericRequirement::None;

compiler/rustc_span/src/symbol.rs

@@ -806,6 +806,7 @@ symbols! {
         intel,
         into_future,
         into_iter,
+        into_try_type,
         intra_doc_pointers,
         intrinsics,
         irrefutable_let_patterns,

library/core/src/ops/try_trait.rs

@@ -356,10 +356,18 @@ where
 /// and in the other direction,
 /// `<Result<Infallible, E> as Residual<T>>::TryType = Result<T, E>`.
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
-pub trait Residual<O> {
+pub 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>;
+
+    /// Here for convenience in the `?` desugaring.
+    /// Probably should not be stabilized, as it should never be overridden.
+    #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
+    #[cfg_attr(not(bootstrap), 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")]

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

@@ -52,7 +52,7 @@ 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)?;

src/test/ui/try-block/try-block-bad-type.rs

@@ -4,7 +4,7 @@
 
 pub fn main() {
     let res: Result<u32, std::array::TryFromSliceError> = try {
-        Err("")?; //~ ERROR `?` couldn't convert the error
+        Err("")?; //~ ERROR mismatched types
         5
     };
 

src/test/ui/try-block/try-block-bad-type.stderr

@@ -1,12 +1,11 @@
-error[E0277]: `?` couldn't convert the error to `TryFromSliceError`
-  --> $DIR/try-block-bad-type.rs:7:16
+error[E0308]: mismatched types
+  --> $DIR/try-block-bad-type.rs:7:9
    |
 LL |         Err("")?;
-   |                ^ the trait `From<&str>` is not implemented for `TryFromSliceError`
+   |         ^^^^^^^^ expected struct `TryFromSliceError`, found `&str`
    |
-   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
-   = help: the trait `From<Infallible>` is implemented for `TryFromSliceError`
-   = note: required because of the requirements on the impl of `FromResidual<Result<Infallible, &str>>` for `Result<u32, TryFromSliceError>`
+   = note: expected enum `Result<u32, TryFromSliceError>`
+              found enum `Result<_, &str>`
 
 error[E0271]: type mismatch resolving `<Result<i32, i32> as Try>::Output == &str`
   --> $DIR/try-block-bad-type.rs:12:9
@@ -38,5 +37,5 @@ LL |     let res: i32 = try { 5 };
 
 error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0271, E0277.
+Some errors have detailed explanations: E0271, E0277, E0308.
 For more information about an error, try `rustc --explain E0271`.