Actually implement the feature in the compiler

Including all the bootstrapping tweaks in the library.

compiler/rustc_ast_lowering/src/expr.rs

@@ -562,8 +562,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
         )
     }
 
-    /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_ok(<expr>) }`,
-    /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_ok(()) }`
+    /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
+    /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
     /// and save the block id to use it as a break target for desugaring of the `?` operator.
     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
         self.with_catch_scope(body.id, |this| {
@@ -592,9 +592,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
             let ok_wrapped_span =
                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 
-            // `::std::ops::Try::from_ok($tail_expr)`
+            // `::std::ops::Try::from_output($tail_expr)`
             block.expr = Some(this.wrap_in_try_constructor(
-                hir::LangItem::TryFromOk,
+                hir::LangItem::TryTraitFromOutput,
                 try_span,
                 tail_expr,
                 ok_wrapped_span,
@@ -1896,14 +1896,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
             self.allow_try_trait.clone(),
         );
 
-        // `Try::into_result(<expr>)`
+        // `Try::branch(<expr>)`
         let scrutinee = {
             // expand <expr>
             let sub_expr = self.lower_expr_mut(sub_expr);
 
             self.expr_call_lang_item_fn(
                 unstable_span,
-                hir::LangItem::TryIntoResult,
+                hir::LangItem::TryTraitBranch,
                 arena_vec![self; sub_expr],
             )
         };
@@ -1921,8 +1921,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
         };
         let attrs = vec![attr];
 
-        // `Ok(val) => #[allow(unreachable_code)] val,`
-        let ok_arm = {
+        // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
+        let continue_arm = {
             let val_ident = Ident::with_dummy_span(sym::val);
             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
@@ -1931,27 +1931,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 val_pat_nid,
                 ThinVec::from(attrs.clone()),
             ));
-            let ok_pat = self.pat_ok(span, val_pat);
-            self.arm(ok_pat, val_expr)
+            let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
+            self.arm(continue_pat, val_expr)
         };
 
-        // `Err(err) => #[allow(unreachable_code)]
-        //              return Try::from_error(From::from(err)),`
-        let err_arm = {
-            let err_ident = Ident::with_dummy_span(sym::err);
-            let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
-            let from_expr = {
-                let err_expr = self.expr_ident_mut(try_span, err_ident, err_local_nid);
-                self.expr_call_lang_item_fn(
-                    try_span,
-                    hir::LangItem::FromFrom,
-                    arena_vec![self; err_expr],
-                )
-            };
-            let from_err_expr = self.wrap_in_try_constructor(
-                hir::LangItem::TryFromError,
-                unstable_span,
-                from_expr,
+        // `ControlFlow::Break(residual) =>
+        //     #[allow(unreachable_code)]
+        //     return Try::from_residual(residual),`
+        let break_arm = {
+            let residual_ident = Ident::with_dummy_span(sym::residual);
+            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,
+                try_span,
+                self.arena.alloc(residual_expr),
                 unstable_span,
             );
             let thin_attrs = ThinVec::from(attrs);
@@ -1962,25 +1956,25 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     try_span,
                     hir::ExprKind::Break(
                         hir::Destination { label: None, target_id },
-                        Some(from_err_expr),
+                        Some(from_residual_expr),
                     ),
                     thin_attrs,
                 ))
             } else {
                 self.arena.alloc(self.expr(
                     try_span,
-                    hir::ExprKind::Ret(Some(from_err_expr)),
+                    hir::ExprKind::Ret(Some(from_residual_expr)),
                     thin_attrs,
                 ))
             };
 
-            let err_pat = self.pat_err(try_span, err_local);
-            self.arm(err_pat, ret_expr)
+            let break_pat = self.pat_cf_break(try_span, residual_local);
+            self.arm(break_pat, ret_expr)
         };
 
         hir::ExprKind::Match(
             scrutinee,
-            arena_vec![self; err_arm, ok_arm],
+            arena_vec![self; break_arm, continue_arm],
             hir::MatchSource::TryDesugar,
         )
     }

compiler/rustc_ast_lowering/src/lib.rs

@@ -331,7 +331,7 @@ pub fn lower_crate<'a, 'hir>(
         lifetimes_to_define: Vec::new(),
         is_collecting_in_band_lifetimes: false,
         in_scope_lifetimes: Vec::new(),
-        allow_try_trait: Some([sym::try_trait][..].into()),
+        allow_try_trait: Some([sym::control_flow_enum, sym::try_trait_v2][..].into()),
         allow_gen_future: Some([sym::gen_future][..].into()),
     }
     .lower_crate(krate)
@@ -2479,14 +2479,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.pat(span, hir::PatKind::Lit(expr))
     }
 
-    fn pat_ok(&mut self, span: Span, pat: &'hir hir::Pat<'hir>) -> &'hir hir::Pat<'hir> {
+    fn pat_cf_continue(&mut self, span: Span, pat: &'hir hir::Pat<'hir>) -> &'hir hir::Pat<'hir> {
         let field = self.single_pat_field(span, pat);
-        self.pat_lang_item_variant(span, hir::LangItem::ResultOk, field)
+        self.pat_lang_item_variant(span, hir::LangItem::ControlFlowContinue, field)
     }
 
-    fn pat_err(&mut self, span: Span, pat: &'hir hir::Pat<'hir>) -> &'hir hir::Pat<'hir> {
+    fn pat_cf_break(&mut self, span: Span, pat: &'hir hir::Pat<'hir>) -> &'hir hir::Pat<'hir> {
         let field = self.single_pat_field(span, pat);
-        self.pat_lang_item_variant(span, hir::LangItem::ResultErr, field)
+        self.pat_lang_item_variant(span, hir::LangItem::ControlFlowBreak, field)
     }
 
     fn pat_some(&mut self, span: Span, pat: &'hir hir::Pat<'hir>) -> &'hir hir::Pat<'hir> {

compiler/rustc_hir/src/lang_items.rs

@@ -308,12 +308,12 @@ language_item_table! {
 
     Termination,             sym::termination,         termination,                Target::Trait;
 
-    Try,                     kw::Try,                  try_trait,                  Target::Trait;
+    Try,                     sym::Try,                 try_trait,                  Target::Trait;
 
     // Language items from AST lowering
-    TryFromError,            sym::from_error,          from_error_fn,              Target::Method(MethodKind::Trait { body: false });
-    TryFromOk,               sym::from_ok,             from_ok_fn,                 Target::Method(MethodKind::Trait { body: false });
-    TryIntoResult,           sym::into_result,         into_result_fn,             Target::Method(MethodKind::Trait { body: false });
+    TryTraitFromResidual,    sym::from_residual,       from_residual_fn,           Target::Method(MethodKind::Trait { body: false });
+    TryTraitFromOutput,      sym::from_output,         from_output_fn,             Target::Method(MethodKind::Trait { body: false });
+    TryTraitBranch,          sym::branch,              branch_fn,                  Target::Method(MethodKind::Trait { body: false });
 
     PollReady,               sym::Ready,               poll_ready_variant,         Target::Variant;
     PollPending,             sym::Pending,             poll_pending_variant,       Target::Variant;
@@ -331,6 +331,9 @@ language_item_table! {
     ResultOk,                sym::Ok,                  result_ok_variant,          Target::Variant;
     ResultErr,               sym::Err,                 result_err_variant,         Target::Variant;
 
+    ControlFlowContinue,     sym::Continue,            cf_continue_variant,        Target::Variant;
+    ControlFlowBreak,        sym::Break,               cf_break_variant,           Target::Variant;
+
     IntoIterIntoIter,        sym::into_iter,           into_iter_fn,               Target::Method(MethodKind::Trait { body: false });
     IteratorNext,            sym::next,                next_fn,                    Target::Method(MethodKind::Trait { body: false});
 

compiler/rustc_span/src/symbol.rs

@@ -130,10 +130,12 @@ symbols! {
         BTreeSet,
         BinaryHeap,
         Borrow,
+        Break,
         C,
         CString,
         Center,
         Clone,
+        Continue,
         Copy,
         Count,
         Debug,
@@ -326,6 +328,7 @@ symbols! {
         box_patterns,
         box_syntax,
         braced_empty_structs,
+        branch,
         breakpoint,
         bridge,
         bswap,
@@ -410,6 +413,7 @@ symbols! {
         constructor,
         contents,
         context,
+        control_flow_enum,
         convert,
         copy,
         copy_closures,
@@ -510,7 +514,6 @@ symbols! {
         env,
         eq,
         ermsb_target_feature,
-        err,
         exact_div,
         except,
         exchange_malloc,
@@ -580,10 +583,10 @@ symbols! {
         frem_fast,
         from,
         from_desugaring,
-        from_error,
         from_generator,
         from_method,
-        from_ok,
+        from_output,
+        from_residual,
         from_size_align_unchecked,
         from_trait,
         from_usize,
@@ -652,7 +655,6 @@ symbols! {
         instruction_set,
         intel,
         into_iter,
-        into_result,
         into_trait,
         intra_doc_pointers,
         intrinsics,
@@ -962,6 +964,7 @@ symbols! {
         repr_packed,
         repr_simd,
         repr_transparent,
+        residual,
         result,
         result_type,
         rhs,
@@ -1227,7 +1230,7 @@ symbols! {
         try_blocks,
         try_from_trait,
         try_into_trait,
-        try_trait,
+        try_trait_v2,
         tt,
         tuple,
         tuple_from_req,

library/core/src/iter/adapters/peekable.rs

@@ -1,5 +1,5 @@
 use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
-use crate::ops::TryWhereOutputEquals;
+use crate::ops::{ControlFlow, TryWhereOutputEquals};
 
 /// An iterator with a `peek()` that returns an optional reference to the next
 /// element.
@@ -130,12 +130,35 @@ where
     }
 
     #[inline]
+    #[cfg(not(bootstrap))]
     fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
     where
         Self: Sized,
         F: FnMut(B, Self::Item) -> R,
         R: TryWhereOutputEquals<B>,
     {
+        match self.peeked.take() {
+            Some(None) => try { init },
+            Some(Some(v)) => match self.iter.try_rfold(init, &mut f).branch() {
+                ControlFlow::Continue(acc) => f(acc, v),
+                ControlFlow::Break(r) => {
+                    self.peeked = Some(Some(v));
+                    R::from_residual(r)
+                }
+            },
+            None => self.iter.try_rfold(init, f),
+        }
+    }
+
+    #[inline]
+    #[cfg(bootstrap)]
+    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: TryWhereOutputEquals<B>,
+    {
+        let _use_the_import: ControlFlow<()>;
         match self.peeked.take() {
             Some(None) => try { init },
             Some(Some(v)) => match self.iter.try_rfold(init, &mut f).into_result() {

library/core/src/iter/traits/iterator.rs

@@ -2412,6 +2412,36 @@ pub trait Iterator {
     /// ```
     #[inline]
     #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
+    #[cfg(not(bootstrap))]
+    fn try_find<F, R, E>(&mut self, f: F) -> Result<Option<Self::Item>, E>
+    where
+        Self: Sized,
+        F: FnMut(&Self::Item) -> R,
+        R: TryWhereOutputEquals<bool>,
+        // FIXME: This is a weird bound; the API should change
+        R: crate::ops::TryV2<Residual = Result<crate::convert::Infallible, E>>,
+    {
+        #[inline]
+        fn check<F, T, R, E>(mut f: F) -> impl FnMut((), T) -> ControlFlow<Result<T, E>>
+        where
+            F: FnMut(&T) -> R,
+            R: TryWhereOutputEquals<bool>,
+            R: crate::ops::TryV2<Residual = Result<crate::convert::Infallible, E>>,
+        {
+            move |(), x| match f(&x).branch() {
+                ControlFlow::Continue(false) => ControlFlow::CONTINUE,
+                ControlFlow::Continue(true) => ControlFlow::Break(Ok(x)),
+                ControlFlow::Break(Err(x)) => ControlFlow::Break(Err(x)),
+            }
+        }
+
+        self.try_fold((), check(f)).break_value().transpose()
+    }
+
+    /// We're bootstrapping.
+    #[inline]
+    #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
+    #[cfg(bootstrap)]
     fn try_find<F, R>(&mut self, f: F) -> Result<Option<Self::Item>, R::Error>
     where
         Self: Sized,

library/core/src/ops/control_flow.rs

@@ -52,8 +52,10 @@ use crate::{convert, ops};
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum ControlFlow<B, C = ()> {
     /// Move on to the next phase of the operation as normal.
+    #[cfg_attr(not(bootstrap), lang = "Continue")]
     Continue(C),
     /// Exit the operation without running subsequent phases.
+    #[cfg_attr(not(bootstrap), lang = "Break")]
     Break(B),
     // Yes, the order of the variants doesn't match the type parameters.
     // They're in this order so that `ControlFlow<A, B>` <-> `Result<B, A>`
@@ -181,6 +183,7 @@ impl<B, C> ControlFlow<B, C> {
     }
 }
 
+#[cfg(bootstrap)]
 impl<R: ops::TryV1> ControlFlow<R, R::Ok> {
     /// Create a `ControlFlow` from any type implementing `Try`.
     #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
@@ -203,6 +206,29 @@ impl<R: ops::TryV1> ControlFlow<R, R::Ok> {
     }
 }
 
+#[cfg(not(bootstrap))]
+impl<R: ops::TryV2> ControlFlow<R, R::Output> {
+    /// Create a `ControlFlow` from any type implementing `Try`.
+    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
+    #[inline]
+    pub fn from_try(r: R) -> Self {
+        match R::branch(r) {
+            ControlFlow::Continue(v) => ControlFlow::Continue(v),
+            ControlFlow::Break(v) => ControlFlow::Break(R::from_residual(v)),
+        }
+    }
+
+    /// Convert a `ControlFlow` into any type implementing `Try`;
+    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
+    #[inline]
+    pub fn into_try(self) -> R {
+        match self {
+            ControlFlow::Continue(v) => R::from_output(v),
+            ControlFlow::Break(v) => v,
+        }
+    }
+}
+
 impl<B> ControlFlow<B, ()> {
     /// It's frequently the case that there's no value needed with `Continue`,
     /// so this provides a way to avoid typing `(())`, if you prefer it.