Remove .. from return type notation

compiler/rustc_ast/src/ast.rs

@@ -167,9 +167,6 @@ pub enum GenericArgs {
     AngleBracketed(AngleBracketedArgs),
     /// The `(A, B)` and `C` in `Foo(A, B) -> C`.
     Parenthesized(ParenthesizedArgs),
-    /// Associated return type bounds, like `T: Trait<method(..): Send>`
-    /// which applies the `Send` bound to the return-type of `method`.
-    ReturnTypeNotation(Span),
 }
 
 impl GenericArgs {
@@ -181,7 +178,6 @@ pub fn span(&self) -> Span {
         match self {
             AngleBracketed(data) => data.span,
             Parenthesized(data) => data.span,
-            ReturnTypeNotation(span) => *span,
         }
     }
 }

compiler/rustc_ast/src/mut_visit.rs

@@ -561,7 +561,6 @@ pub fn noop_visit_generic_args<T: MutVisitor>(generic_args: &mut GenericArgs, vi
     match generic_args {
         GenericArgs::AngleBracketed(data) => vis.visit_angle_bracketed_parameter_data(data),
         GenericArgs::Parenthesized(data) => vis.visit_parenthesized_parameter_data(data),
-        GenericArgs::ReturnTypeNotation(_span) => {}
     }
 }
 

compiler/rustc_ast/src/visit.rs

@@ -482,7 +482,6 @@ pub fn walk_generic_args<'a, V>(visitor: &mut V, generic_args: &'a GenericArgs)
             walk_list!(visitor, visit_ty, &data.inputs);
             walk_fn_ret_ty(visitor, &data.output);
         }
-        GenericArgs::ReturnTypeNotation(_span) => {}
     }
 }
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -353,13 +353,7 @@ pub enum BadReturnTypeNotation {
     #[diag(ast_lowering_bad_return_type_notation_inputs)]
     Inputs {
         #[primary_span]
-        #[suggestion(code = "(..)", applicability = "maybe-incorrect")]
+        #[suggestion(code = "()", applicability = "maybe-incorrect")]
-        span: Span,
-    },
-    #[diag(ast_lowering_bad_return_type_notation_needs_dots)]
-    NeedsDots {
-        #[primary_span]
-        #[suggestion(code = "(..)", applicability = "maybe-incorrect")]
         span: Span,
     },
     #[diag(ast_lowering_bad_return_type_notation_output)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -987,15 +987,22 @@ fn lower_assoc_ty_constraint(
                 GenericArgs::AngleBracketed(data) => {
                     self.lower_angle_bracketed_parameter_data(data, ParamMode::Explicit, itctx).0
                 }
-                &GenericArgs::ReturnTypeNotation(span) => GenericArgsCtor {
-                    args: Default::default(),
-                    bindings: &[],
-                    parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
-                    span,
-                },
                 GenericArgs::Parenthesized(data) => {
-                    if let Some(start_char) = constraint.ident.as_str().chars().next()
+                    if data.inputs.is_empty() && matches!(data.output, FnRetTy::Default(..)) {
-                        && start_char.is_ascii_lowercase()
+                        let parenthesized = if self.tcx.features().return_type_notation {
+                            hir::GenericArgsParentheses::ReturnTypeNotation
+                        } else {
+                            self.emit_bad_parenthesized_trait_in_assoc_ty(data);
+                            hir::GenericArgsParentheses::No
+                        };
+                        GenericArgsCtor {
+                            args: Default::default(),
+                            bindings: &[],
+                            parenthesized,
+                            span: data.inputs_span,
+                        }
+                    } else if let Some(first_char) = constraint.ident.as_str().chars().next()
+                        && first_char.is_ascii_lowercase()
                     {
                         let mut err = if !data.inputs.is_empty() {
                             self.tcx.sess.create_err(errors::BadReturnTypeNotation::Inputs {
@@ -1006,9 +1013,7 @@ fn lower_assoc_ty_constraint(
                                 span: data.inputs_span.shrink_to_hi().to(ty.span),
                             })
                         } else {
-                            self.tcx.sess.create_err(errors::BadReturnTypeNotation::NeedsDots {
+                            unreachable!("inputs are empty and return type is not provided")
-                                span: data.inputs_span,
-                            })
                         };
                         if !self.tcx.features().return_type_notation
                             && self.tcx.sess.is_nightly_build()

compiler/rustc_ast_lowering/src/path.rs

@@ -13,7 +13,6 @@
 use rustc_span::{BytePos, Span, DUMMY_SP};
 
 use smallvec::{smallvec, SmallVec};
-use thin_vec::ThinVec;
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
     #[instrument(level = "trace", skip(self))]
@@ -219,18 +218,6 @@ pub(crate) fn lower_path_segment(
                         )
                     }
                 },
-                &GenericArgs::ReturnTypeNotation(span) => {
-                    self.tcx.sess.emit_err(GenericTypeWithParentheses { span, sub: None });
-                    (
-                        self.lower_angle_bracketed_parameter_data(
-                            &AngleBracketedArgs { span, args: ThinVec::default() },
-                            param_mode,
-                            itctx,
-                        )
-                        .0,
-                        false,
-                    )
-                }
             }
         } else {
             (

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1075,7 +1075,6 @@ fn visit_generic_args(&mut self, generic_args: &'a GenericArgs) {
                     self.with_impl_trait(None, |this| this.visit_ty(ty));
                 }
             }
-            GenericArgs::ReturnTypeNotation(_span) => {}
         }
     }
 
@@ -1386,7 +1385,6 @@ fn deny_equality_constraints(
                                     match &mut assoc_path.segments[len].args {
                                         Some(args) => match args.deref_mut() {
                                             GenericArgs::Parenthesized(_) => continue,
-                                            GenericArgs::ReturnTypeNotation(_span) => continue,
                                             GenericArgs::AngleBracketed(args) => {
                                                 args.args.push(arg);
                                             }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -485,20 +485,23 @@ fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
 
     fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
         if let AssocConstraintKind::Bound { .. } = constraint.kind {
-            if let Some(args) = constraint.gen_args.as_ref()
+            if let Some(ast::GenericArgs::Parenthesized(args)) = constraint.gen_args.as_ref()
-                && matches!(
+                && args.inputs.is_empty()
-                    args,
+                && matches!(args.output, ast::FnRetTy::Default(..))
-                    ast::GenericArgs::ReturnTypeNotation(..)
-                )
             {
-                // RTN is gated below with a `gate_all`.
+                gate_feature_post!(
+                    &self,
+                    return_type_notation,
+                    constraint.span,
+                    "return type notation is experimental"
+                );
             } else {
                 gate_feature_post!(
                     &self,
                     associated_type_bounds,
                     constraint.span,
                     "associated type bounds are unstable"
-                )
+                );
             }
         }
         visit::walk_assoc_constraint(self, constraint)
@@ -589,7 +592,6 @@ macro_rules! gate_all {
     gate_all!(yeet_expr, "`do yeet` expression is experimental");
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
-    gate_all!(return_type_notation, "return type notation is experimental");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).
@@ -605,6 +607,7 @@ macro_rules! gate_all {
 
     gate_all!(trait_alias, "trait aliases are experimental");
     gate_all!(associated_type_bounds, "associated type bounds are unstable");
+    gate_all!(return_type_notation, "return type notation is experimental");
     gate_all!(decl_macro, "`macro` is experimental");
     gate_all!(box_patterns, "box pattern syntax is experimental");
     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -936,10 +936,6 @@ fn print_generic_args(&mut self, args: &ast::GenericArgs, colons_before_params:
                 self.word(")");
                 self.print_fn_ret_ty(&data.output);
             }
-
-            ast::GenericArgs::ReturnTypeNotation(_span) => {
-                self.word("(..)");
-            }
         }
     }
 }

compiler/rustc_parse/messages.ftl

@@ -738,3 +738,7 @@ parse_box_syntax_removed = `box_syntax` has been removed
 parse_bad_return_type_notation_output =
     return type not allowed with return type notation
     .suggestion = remove the return type
+
+parse_bad_return_type_notation_dotdot =
+    return type notation uses `()` instead of `(..)` for elided arguments
+    .suggestion = remove the `..`

compiler/rustc_parse/src/errors.rs

@@ -2324,3 +2324,11 @@ pub(crate) struct BadReturnTypeNotationOutput {
     #[suggestion(code = "", applicability = "maybe-incorrect")]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_bad_return_type_notation_dotdot)]
+pub(crate) struct BadReturnTypeNotationDotDot {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub span: Span,
+}

compiler/rustc_parse/src/parser/path.rs

@@ -290,16 +290,17 @@ pub(super) fn parse_path_segment(
                     })?;
                     let span = lo.to(self.prev_token.span);
                     AngleBracketedArgs { args, span }.into()
-                } else if self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
+                } else if self.may_recover()
+                    && self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
                     // FIXME(return_type_notation): Could also recover `...` here.
                     && self.look_ahead(1, |tok| tok.kind == token::DotDot)
                 {
-                    let lo = self.token.span;
                     self.bump();
+                    self.sess
+                        .emit_err(errors::BadReturnTypeNotationDotDot { span: self.token.span });
                     self.bump();
                     self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
                     let span = lo.to(self.prev_token.span);
-                    self.sess.gated_spans.gate(sym::return_type_notation, span);
 
                     if self.eat_noexpect(&token::RArrow) {
                         let lo = self.prev_token.span;
@@ -308,7 +309,13 @@ pub(super) fn parse_path_segment(
                             .emit_err(errors::BadReturnTypeNotationOutput { span: lo.to(ty.span) });
                     }
 
-                    P(GenericArgs::ReturnTypeNotation(span))
+                    ParenthesizedArgs {
+                        span,
+                        inputs: ThinVec::new(),
+                        inputs_span: span,
+                        output: ast::FnRetTy::Default(self.prev_token.span.shrink_to_hi()),
+                    }
+                    .into()
                 } else {
                     // `(T, U) -> R`
                     let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
@@ -566,13 +573,13 @@ fn parse_angle_arg(
                     };
 
                     let span = lo.to(self.prev_token.span);
-
                     // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
                     if let AssocConstraintKind::Bound { .. } = kind {
-                        if gen_args.as_ref().map_or(false, |args| {
+                        if let Some(ast::GenericArgs::Parenthesized(args)) = &gen_args
-                            matches!(args, GenericArgs::ReturnTypeNotation(..))
+                            && args.inputs.is_empty()
-                        }) {
+                            && matches!(args.output, ast::FnRetTy::Default(..))
-                            // This is already gated in `parse_path_segment`
+                        {
+                            self.sess.gated_spans.gate(sym::return_type_notation, span);
                         } else {
                             self.sess.gated_spans.gate(sym::associated_type_bounds, span);
                         }

compiler/rustc_passes/src/hir_stats.rs

@@ -666,7 +666,7 @@ fn visit_path_segment(&mut self, path_segment: &'v ast::PathSegment) {
     fn visit_generic_args(&mut self, g: &'v ast::GenericArgs) {
         record_variants!(
             (self, g, g, Id::None, ast, GenericArgs, GenericArgs),
-            [AngleBracketed, Parenthesized, ReturnTypeNotation]
+            [AngleBracketed, Parenthesized]
         );
         ast_visit::walk_generic_args(self, g)
     }

compiler/rustc_resolve/src/late.rs

@@ -1116,7 +1116,6 @@ fn visit_path_segment(&mut self, path_segment: &'ast PathSegment) {
                         }
                     }
                 }
-                GenericArgs::ReturnTypeNotation(_span) => {}
             }
         }
     }

compiler/rustc_resolve/src/lib.rs

@@ -312,7 +312,6 @@ fn from(seg: &'a ast::PathSegment) -> Segment {
                     (args.span, found_lifetimes)
                 }
                 GenericArgs::Parenthesized(args) => (args.span, true),
-                GenericArgs::ReturnTypeNotation(span) => (*span, false),
             }
         } else {
             (DUMMY_SP, false)

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.rs

@@ -12,11 +12,11 @@ fn foo<T: Trait<method(i32): Send>>() {}
 //~^ ERROR argument types not allowed with return type notation
 //~| ERROR associated type bounds are unstable
 
-fn bar<T: Trait<method(..) -> (): Send>>() {}
+fn bar<T: Trait<method() -> (): Send>>() {}
 //~^ ERROR return type not allowed with return type notation
-
-fn baz<T: Trait<method(): Send>>() {}
-//~^ ERROR return type notation arguments must be elided with `..`
 //~| ERROR associated type bounds are unstable
 
+fn baz<T: Trait<method(..): Send>>() {}
+//~^ ERROR return type notation uses `()` instead of `(..)` for elided arguments
+
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.stderr

@@ -1,8 +1,8 @@
-error: return type not allowed with return type notation
+error: return type notation uses `()` instead of `(..)` for elided arguments
-  --> $DIR/bad-inputs-and-output.rs:15:28
+  --> $DIR/bad-inputs-and-output.rs:19:24
    |
-LL | fn bar<T: Trait<method(..) -> (): Send>>() {}
+LL | fn baz<T: Trait<method(..): Send>>() {}
-   |                            ^^^^^ help: remove the return type
+   |                        ^^ help: remove the `..`
 
 error[E0658]: associated type bounds are unstable
   --> $DIR/bad-inputs-and-output.rs:11:17
@@ -14,10 +14,10 @@ LL | fn foo<T: Trait<method(i32): Send>>() {}
    = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
 
 error[E0658]: associated type bounds are unstable
-  --> $DIR/bad-inputs-and-output.rs:18:17
+  --> $DIR/bad-inputs-and-output.rs:15:17
    |
-LL | fn baz<T: Trait<method(): Send>>() {}
+LL | fn bar<T: Trait<method() -> (): Send>>() {}
-   |                 ^^^^^^^^^^^^^^
+   |                 ^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
    = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
@@ -43,13 +43,13 @@ error: argument types not allowed with return type notation
   --> $DIR/bad-inputs-and-output.rs:11:23
    |
 LL | fn foo<T: Trait<method(i32): Send>>() {}
-   |                       ^^^^^ help: remove the input types: `(..)`
+   |                       ^^^^^ help: remove the input types: `()`
 
-error: return type notation arguments must be elided with `..`
+error: return type not allowed with return type notation
-  --> $DIR/bad-inputs-and-output.rs:18:23
+  --> $DIR/bad-inputs-and-output.rs:15:25
    |
-LL | fn baz<T: Trait<method(): Send>>() {}
+LL | fn bar<T: Trait<method() -> (): Send>>() {}
-   |                       ^^ help: add `..`: `(..)`
+   |                         ^^^^^^ help: remove the return type
 
 error: aborting due to 5 previous errors; 2 warnings emitted
 

tests/ui/associated-type-bounds/return-type-notation/basic.rs

@@ -18,7 +18,7 @@ async fn foo<T: Foo>() -> Result<(), ()> {
 fn is_send(_: impl Send) {}
 
 fn test<
-    #[cfg(with)] T: Foo<method(..): Send>,
+    #[cfg(with)] T: Foo<method(): Send>,
     #[cfg(without)] T: Foo,
 >() {
     is_send(foo::<T>());

tests/ui/associated-type-bounds/return-type-notation/equality.rs

@@ -10,7 +10,7 @@ trait Trait {
     async fn method() {}
 }
 
-fn test<T: Trait<method(..) = Box<dyn Future<Output = ()>>>>() {}
+fn test<T: Trait<method() = Box<dyn Future<Output = ()>>>>() {}
 //~^ ERROR return type notation is not allowed to use type equality
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/equality.stderr

@@ -18,8 +18,8 @@ LL | #![feature(return_type_notation, async_fn_in_trait)]
 error: return type notation is not allowed to use type equality
   --> $DIR/equality.rs:13:18
    |
-LL | fn test<T: Trait<method(..) = Box<dyn Future<Output = ()>>>>() {}
+LL | fn test<T: Trait<method() = Box<dyn Future<Output = ()>>>>() {}
-   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error; 2 warnings emitted
 

tests/ui/associated-type-bounds/return-type-notation/missing.rs

@@ -8,7 +8,7 @@ trait Trait {
     async fn method() {}
 }
 
-fn bar<T: Trait<methid(..): Send>>() {}
+fn bar<T: Trait<methid(): Send>>() {}
 //~^ ERROR cannot find associated function `methid` in trait `Trait`
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/missing.stderr

@@ -18,8 +18,8 @@ LL | #![feature(return_type_notation, async_fn_in_trait)]
 error: cannot find associated function `methid` in trait `Trait`
   --> $DIR/missing.rs:11:17
    |
-LL | fn bar<T: Trait<methid(..): Send>>() {}
+LL | fn bar<T: Trait<methid(): Send>>() {}
-   |                 ^^^^^^^^^^^^^^^^
+   |                 ^^^^^^^^^^^^^^
 
 error: aborting due to previous error; 2 warnings emitted
 

tests/ui/associated-type-bounds/return-type-notation/non-rpitit.rs

@@ -5,7 +5,7 @@ trait Trait {
     fn method() {}
 }
 
-fn test<T: Trait<method(..): Send>>() {}
+fn test<T: Trait<method(): Send>>() {}
 //~^ ERROR  return type notation used on function that is not `async` and does not return `impl Trait`
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/non-rpitit.stderr

@@ -13,8 +13,8 @@ error: return type notation used on function that is not `async` and does not re
 LL |     fn method() {}
    |     ----------- this function must be `async` or return `impl Trait`
 ...
-LL | fn test<T: Trait<method(..): Send>>() {}
+LL | fn test<T: Trait<method(): Send>>() {}
-   |                  ^^^^^^^^^^^^^^^^
+   |                  ^^^^^^^^^^^^^^
    |
    = note: function returns `()`, which is not compatible with associated type return bounds
 

tests/ui/feature-gates/feature-gate-return_type_notation.cfg.stderr

@@ -1,14 +1,14 @@
 error[E0658]: return type notation is experimental
-  --> $DIR/feature-gate-return_type_notation.rs:12:18
+  --> $DIR/feature-gate-return_type_notation.rs:15:17
    |
-LL | fn foo<T: Trait<m(..): Send>>() {}
+LL | fn foo<T: Trait<m(): Send>>() {}
-   |                  ^^^^
+   |                 ^^^^^^^^^
    |
    = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
    = help: add `#![feature(return_type_notation)]` to the crate attributes to enable
 
 warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/feature-gate-return_type_notation.rs:4:12
+  --> $DIR/feature-gate-return_type_notation.rs:7:12
    |
 LL | #![feature(async_fn_in_trait)]
    |            ^^^^^^^^^^^^^^^^^
@@ -16,6 +16,21 @@ LL | #![feature(async_fn_in_trait)]
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error: aborting due to previous error; 1 warning emitted
+error: parenthesized generic arguments cannot be used in associated type constraints
+  --> $DIR/feature-gate-return_type_notation.rs:15:17
+   |
+LL | fn foo<T: Trait<m(): Send>>() {}
+   |                 ^--
+   |                  |
+   |                  help: remove these parentheses
 
-For more information about this error, try `rustc --explain E0658`.
+error[E0220]: associated type `m` not found for `Trait`
+  --> $DIR/feature-gate-return_type_notation.rs:15:17
+   |
+LL | fn foo<T: Trait<m(): Send>>() {}
+   |                 ^ associated type `m` not found
+
+error: aborting due to 3 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0220, E0658.
+For more information about an error, try `rustc --explain E0220`.

tests/ui/feature-gates/feature-gate-return_type_notation.no.stderr

@@ -1,14 +1,5 @@
-error[E0658]: return type notation is experimental
-  --> $DIR/feature-gate-return_type_notation.rs:12:18
-   |
-LL | fn foo<T: Trait<m(..): Send>>() {}
-   |                  ^^^^
-   |
-   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
-   = help: add `#![feature(return_type_notation)]` to the crate attributes to enable
-
 warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/feature-gate-return_type_notation.rs:4:12
+  --> $DIR/feature-gate-return_type_notation.rs:7:12
    |
 LL | #![feature(async_fn_in_trait)]
    |            ^^^^^^^^^^^^^^^^^
@@ -16,6 +7,16 @@ LL | #![feature(async_fn_in_trait)]
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error: aborting due to previous error; 1 warning emitted
+warning: return type notation is experimental
+  --> $DIR/feature-gate-return_type_notation.rs:15:17
+   |
+LL | fn foo<T: Trait<m(): Send>>() {}
+   |                 ^^^^^^^^^
+   |
+   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
+   = help: add `#![feature(return_type_notation)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 2 warnings emitted
 
-For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/feature-gate-return_type_notation.rs

@@ -1,6 +1,9 @@
 // edition: 2021
 // revisions: cfg no
 
+//[no] check-pass
+// Since we're not adding new syntax, `cfg`'d out RTN must pass.
+
 #![feature(async_fn_in_trait)]
 //~^ WARN the feature `async_fn_in_trait` is incomplete
 
@@ -9,7 +12,11 @@ trait Trait {
 }
 
 #[cfg(cfg)]
-fn foo<T: Trait<m(..): Send>>() {}
+fn foo<T: Trait<m(): Send>>() {}
-//~^ ERROR return type notation is experimental
+//[cfg]~^ ERROR return type notation is experimental
+//[cfg]~| ERROR parenthesized generic arguments cannot be used in associated type constraints
+//[cfg]~| ERROR associated type `m` not found for `Trait`
+//[no]~^^^^ WARN return type notation is experimental
+//[no]~| WARN unstable syntax can change at any point in the future, causing a hard error!
 
 fn main() {}