Auto merge of #125778 - estebank:issue-67100, r=compiler-errors

Use parenthetical notation for `Fn` traits Always use the `Fn(T) -> R` format when printing closure traits instead of `Fn<(T,), Output = R>`. Address #67100: ``` error[E0277]: expected a `Fn()` closure, found `F` --> file.rs:6:13 | 6 | call_fn(f) | ------- ^ expected an `Fn()` closure, found `F` | | | required by a bound introduced by this call | = note: wrap the `F` in a closure with no arguments: `|| { /* code */ }` note: required by a bound in `call_fn` --> file.rs:1:15 | 1 | fn call_fn<F: Fn() -> ()>(f: &F) { | ^^^^^^^^^^ required by this bound in `call_fn` help: consider further restricting this bound | 5 | fn call_any<F: std::any::Any + Fn()>(f: &F) { | ++++++ ```
rust-lang · Jun 3, 2024 · 1d52972 · 1d52972
2 parents 621e957 + e6bd6c2
commit 1d52972
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diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -1361,7 +1361,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     match *predicate.self_ty().kind() {
                         ty::Param(param_ty) => Ok((
                             generics.type_param(param_ty, tcx),
-                            predicate.trait_ref.print_only_trait_path().to_string(),
+                            predicate.trait_ref.print_trait_sugared().to_string(),
                         )),
                         _ => Err(()),
                     }

diff --git a/compiler/rustc_const_eval/src/check_consts/ops.rs b/compiler/rustc_const_eval/src/check_consts/ops.rs
@@ -113,7 +113,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     if let Some(generics) = tcx.hir_node_by_def_id(caller).generics() {
                         let constraint = with_no_trimmed_paths!(format!(
                             "~const {}",
-                            trait_ref.print_only_trait_path()
+                            trait_ref.print_trait_sugared(),
                         ));
                         suggest_constraining_type_param(
                             tcx,

diff --git a/compiler/rustc_hir_analysis/src/coherence/builtin.rs b/compiler/rustc_hir_analysis/src/coherence/builtin.rs
@@ -554,7 +554,7 @@ fn infringing_fields_error(
                         if let ty::Param(_) = ty.kind() {
                             bounds.push((
                                 format!("{ty}"),
-                                trait_ref.print_only_trait_path().to_string(),
+                                trait_ref.print_trait_sugared().to_string(),
                                 Some(trait_ref.def_id),
                             ));
                         }

diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -3202,7 +3202,7 @@ define_print_and_forward_display! {
         if let ty::PredicatePolarity::Negative = self.0.polarity {
             p!("!")
         }
-        p!(print(self.0.trait_ref.print_only_trait_path()));
+        p!(print(self.0.trait_ref.print_trait_sugared()));
     }
 
     PrintClosureAsImpl<'tcx> {

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -29,10 +29,11 @@ use rustc_macros::extension;
 use rustc_middle::hir::map;
 use rustc_middle::traits::IsConstable;
 use rustc_middle::ty::error::TypeError::{self, Sorts};
+use rustc_middle::ty::print::PrintPolyTraitRefExt;
 use rustc_middle::ty::{
     self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, GenericArgs,
-    InferTy, IsSuggestable, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
-    TypeVisitableExt, TypeckResults, Upcast,
+    InferTy, IsSuggestable, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable, TypeFolder,
+    TypeSuperFoldable, TypeVisitableExt, TypeckResults, Upcast,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_span::def_id::LocalDefId;
@@ -219,15 +220,15 @@ pub fn suggest_restriction<'tcx, G: EmissionGuarantee>(
             (_, None) => predicate_constraint(hir_generics, trait_pred.upcast(tcx)),
             (None, Some((ident, []))) => (
                 ident.span.shrink_to_hi(),
-                format!(": {}", trait_pred.print_modifiers_and_trait_path()),
+                format!(": {}", trait_pred.to_poly_trait_ref().print_trait_sugared()),
             ),
             (_, Some((_, [.., bounds]))) => (
                 bounds.span().shrink_to_hi(),
-                format!(" + {}", trait_pred.print_modifiers_and_trait_path()),
+                format!(" + {}", trait_pred.to_poly_trait_ref().print_trait_sugared()),
             ),
             (Some(_), Some((_, []))) => (
                 hir_generics.span.shrink_to_hi(),
-                format!(": {}", trait_pred.print_modifiers_and_trait_path()),
+                format!(": {}", trait_pred.to_poly_trait_ref().print_trait_sugared()),
             ),
         };
 

diff --git a/tests/ui/async-await/async-fn/impl-header.stderr b/tests/ui/async-await/async-fn/impl-header.stderr
@@ -28,7 +28,7 @@ error[E0277]: expected a `FnMut()` closure, found `F`
 LL | impl async Fn<()> for F {}
    |                       ^ expected an `FnMut()` closure, found `F`
    |
-   = help: the trait `FnMut<()>` is not implemented for `F`
+   = help: the trait `FnMut()` is not implemented for `F`
    = note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Fn`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL

diff --git a/tests/ui/closures/closure-expected.stderr b/tests/ui/closures/closure-expected.stderr
@@ -6,7 +6,7 @@ LL |     let y = x.or_else(4);
    |               |
    |               required by a bound introduced by this call
    |
-   = help: the trait `FnOnce<()>` is not implemented for `{integer}`
+   = help: the trait `FnOnce()` is not implemented for `{integer}`
    = note: wrap the `{integer}` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Option::<T>::or_else`
   --> $SRC_DIR/core/src/option.rs:LL:COL

diff --git a/tests/ui/closures/coerce-unsafe-to-closure.stderr b/tests/ui/closures/coerce-unsafe-to-closure.stderr
@@ -6,7 +6,7 @@ LL |     let x: Option<&[u8]> = Some("foo").map(std::mem::transmute);
    |                                        |
    |                                        required by a bound introduced by this call
    |
-   = help: the trait `FnOnce<(&str,)>` is not implemented for fn item `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}`
+   = help: the trait `FnOnce(&str)` is not implemented for fn item `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}`
    = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `Option::<T>::map`
   --> $SRC_DIR/core/src/option.rs:LL:COL

diff --git a/tests/ui/consts/fn_trait_refs.stderr b/tests/ui/consts/fn_trait_refs.stderr
@@ -107,8 +107,8 @@ LL |     f()
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 help: consider further restricting this bound
    |
-LL |     T: ~const Fn<()> + ~const Destruct + ~const std::ops::Fn<()>,
-   |                                        +++++++++++++++++++++++++
+LL |     T: ~const Fn<()> + ~const Destruct + ~const Fn(),
+   |                                        +++++++++++++
 help: add `#![feature(effects)]` to the crate attributes to enable
    |
 LL + #![feature(effects)]
@@ -132,8 +132,8 @@ LL |     f()
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 help: consider further restricting this bound
    |
-LL |     T: ~const FnMut<()> + ~const Destruct + ~const std::ops::FnMut<()>,
-   |                                           ++++++++++++++++++++++++++++
+LL |     T: ~const FnMut<()> + ~const Destruct + ~const FnMut(),
+   |                                           ++++++++++++++++
 help: add `#![feature(effects)]` to the crate attributes to enable
    |
 LL + #![feature(effects)]
@@ -157,8 +157,8 @@ LL |     f()
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 help: consider further restricting this bound
    |
-LL |     T: ~const FnOnce<()> + ~const std::ops::FnOnce<()>,
-   |                          +++++++++++++++++++++++++++++
+LL |     T: ~const FnOnce<()> + ~const FnOnce(),
+   |                          +++++++++++++++++
 help: add `#![feature(effects)]` to the crate attributes to enable
    |
 LL + #![feature(effects)]

diff --git a/tests/ui/consts/unstable-const-fn-in-libcore.stderr b/tests/ui/consts/unstable-const-fn-in-libcore.stderr
@@ -13,8 +13,8 @@ LL |             Opt::None => f(),
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 help: consider further restricting this bound
    |
-LL |     const fn unwrap_or_else<F: ~const FnOnce() -> T + ~const std::ops::FnOnce<()>>(self, f: F) -> T {
-   |                                                     +++++++++++++++++++++++++++++
+LL |     const fn unwrap_or_else<F: ~const FnOnce() -> T + ~const FnOnce()>(self, f: F) -> T {
+   |                                                     +++++++++++++++++
 help: add `#![feature(effects)]` to the crate attributes to enable
    |
 LL + #![feature(effects)]

diff --git a/tests/ui/expr/malformed_closure/block_instead_of_closure_in_arg.stderr b/tests/ui/expr/malformed_closure/block_instead_of_closure_in_arg.stderr
@@ -14,7 +14,7 @@ LL | ||         }
 LL | |      });
    | |______^ expected an `FnOnce(&bool)` closure, found `bool`
    |
-   = help: the trait `for<'a> FnOnce<(&'a bool,)>` is not implemented for `bool`
+   = help: the trait `for<'a> FnOnce(&'a bool)` is not implemented for `bool`
 note: required by a bound in `Option::<T>::filter`
   --> $SRC_DIR/core/src/option.rs:LL:COL
 help: you might have meant to create the closure instead of a block

diff --git a/tests/ui/expr/malformed_closure/ruby_style_closure_successful_parse.stderr b/tests/ui/expr/malformed_closure/ruby_style_closure_successful_parse.stderr
@@ -11,7 +11,7 @@ LL | |         Some(x * 2)
 LL | |     });
    | |_____^ expected an `FnOnce({integer})` closure, found `Option<usize>`
    |
-   = help: the trait `FnOnce<({integer},)>` is not implemented for `Option<usize>`
+   = help: the trait `FnOnce({integer})` is not implemented for `Option<usize>`
 note: required by a bound in `Option::<T>::and_then`
   --> $SRC_DIR/core/src/option.rs:LL:COL
 help: you might have meant to open the closure body instead of placing a closure within a block

diff --git a/tests/ui/extern/extern-wrong-value-type.stderr b/tests/ui/extern/extern-wrong-value-type.stderr
@@ -6,7 +6,7 @@ LL |     is_fn(f);
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `Fn<()>` is not implemented for fn item `extern "C" fn() {f}`
+   = help: the trait `Fn()` is not implemented for fn item `extern "C" fn() {f}`
    = note: wrap the `extern "C" fn() {f}` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `is_fn`
   --> $DIR/extern-wrong-value-type.rs:4:28

diff --git a/tests/ui/feature-gates/feature-gate-unboxed-closures-manual-impls.stderr b/tests/ui/feature-gates/feature-gate-unboxed-closures-manual-impls.stderr
@@ -80,7 +80,7 @@ error[E0277]: expected a `FnMut()` closure, found `Foo`
 LL | impl Fn<()> for Foo {
    |                 ^^^ expected an `FnMut()` closure, found `Foo`
    |
-   = help: the trait `FnMut<()>` is not implemented for `Foo`
+   = help: the trait `FnMut()` is not implemented for `Foo`
    = note: wrap the `Foo` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `Fn`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
@@ -149,7 +149,7 @@ error[E0277]: expected a `FnOnce()` closure, found `Bar`
 LL | impl FnMut<()> for Bar {
    |                    ^^^ expected an `FnOnce()` closure, found `Bar`
    |
-   = help: the trait `FnOnce<()>` is not implemented for `Bar`
+   = help: the trait `FnOnce()` is not implemented for `Bar`
    = note: wrap the `Bar` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `FnMut`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL

diff --git a/tests/ui/fn/fn-trait-formatting.stderr b/tests/ui/fn/fn-trait-formatting.stderr
@@ -47,7 +47,7 @@ LL |     needs_fn(1);
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `Fn<(isize,)>` is not implemented for `{integer}`
+   = help: the trait `Fn(isize)` is not implemented for `{integer}`
 note: required by a bound in `needs_fn`
   --> $DIR/fn-trait-formatting.rs:1:31
    |

diff --git a/tests/ui/fn/issue-39259.stderr b/tests/ui/fn/issue-39259.stderr
@@ -16,7 +16,7 @@ error[E0277]: expected a `FnMut(u32)` closure, found `S`
 LL | impl Fn(u32) -> u32 for S {
    |                         ^ expected an `FnMut(u32)` closure, found `S`
    |
-   = help: the trait `FnMut<(u32,)>` is not implemented for `S`
+   = help: the trait `FnMut(u32)` is not implemented for `S`
 note: required by a bound in `Fn`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
 

diff --git a/tests/ui/generic-associated-types/issue-68642-broken-llvm-ir.stderr b/tests/ui/generic-associated-types/issue-68642-broken-llvm-ir.stderr
@@ -12,8 +12,8 @@ LL |     type F<'a>: Fn() -> u32;
    |                 ^^^^^^^^^^^ required by this bound in `Fun::F`
 help: consider restricting type parameter `T`
    |
-LL | impl<T: std::ops::Fn<()>> Fun for T {
-   |       ++++++++++++++++++
+LL | impl<T: Fn()> Fun for T {
+   |       ++++++
 
 error: aborting due to 1 previous error
 

diff --git a/tests/ui/generic-associated-types/issue-68643-broken-mir.stderr b/tests/ui/generic-associated-types/issue-68643-broken-mir.stderr
@@ -12,8 +12,8 @@ LL |     type F<'a>: Fn() -> u32;
    |                 ^^^^^^^^^^^ required by this bound in `Fun::F`
 help: consider restricting type parameter `T`
    |
-LL | impl<T: std::ops::Fn<()>> Fun for T {
-   |       ++++++++++++++++++
+LL | impl<T: Fn()> Fun for T {
+   |       ++++++
 
 error: aborting due to 1 previous error
 

diff --git a/tests/ui/generic-associated-types/issue-68644-codegen-selection.stderr b/tests/ui/generic-associated-types/issue-68644-codegen-selection.stderr
@@ -12,8 +12,8 @@ LL |     type F<'a>: Fn() -> u32;
    |                 ^^^^^^^^^^^ required by this bound in `Fun::F`
 help: consider restricting type parameter `T`
    |
-LL | impl<T: std::ops::Fn<()>> Fun for T {
-   |       ++++++++++++++++++
+LL | impl<T: Fn()> Fun for T {
+   |       ++++++
 
 error: aborting due to 1 previous error
 

diff --git a/tests/ui/generic-associated-types/issue-68645-codegen-fulfillment.stderr b/tests/ui/generic-associated-types/issue-68645-codegen-fulfillment.stderr
@@ -12,8 +12,8 @@ LL |     type F<'a>: Fn() -> u32;
    |                 ^^^^^^^^^^^ required by this bound in `Fun::F`
 help: consider restricting type parameter `T`
    |
-LL | impl<T: std::ops::Fn<()>> Fun for T {
-   |       ++++++++++++++++++
+LL | impl<T: Fn()> Fun for T {
+   |       ++++++
 
 error: aborting due to 1 previous error
 

diff --git a/tests/ui/impl-trait/normalize-tait-in-const.stderr b/tests/ui/impl-trait/normalize-tait-in-const.stderr
@@ -13,8 +13,8 @@ LL |     fun(filter_positive());
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 help: consider further restricting this bound
    |
-LL | const fn with_positive<F: ~const for<'a> Fn(&'a Alias<'a>) + ~const Destruct + ~const std::ops::Fn<(&Alias<'_>,)>>(fun: F) {
-   |                                                                              ++++++++++++++++++++++++++++++++++++
+LL | const fn with_positive<F: ~const for<'a> Fn(&'a Alias<'a>) + ~const Destruct + ~const Fn(&Alias<'_>)>(fun: F) {
+   |                                                                              +++++++++++++++++++++++
 help: add `#![feature(effects)]` to the crate attributes to enable
    |
 LL + #![feature(effects)]

diff --git a/tests/ui/intrinsics/const-eval-select-bad.stderr b/tests/ui/intrinsics/const-eval-select-bad.stderr
@@ -24,7 +24,7 @@ LL |     const_eval_select((), 42, 0xDEADBEEF);
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `FnOnce<()>` is not implemented for `{integer}`
+   = help: the trait `FnOnce()` is not implemented for `{integer}`
    = note: wrap the `{integer}` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `const_eval_select`
   --> $SRC_DIR/core/src/intrinsics.rs:LL:COL
@@ -37,7 +37,7 @@ LL |     const_eval_select((), 42, 0xDEADBEEF);
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `FnOnce<()>` is not implemented for `{integer}`
+   = help: the trait `FnOnce()` is not implemented for `{integer}`
    = note: wrap the `{integer}` in a closure with no arguments: `|| { /* code */ }`
 note: required by a bound in `const_eval_select`
   --> $SRC_DIR/core/src/intrinsics.rs:LL:COL

diff --git a/tests/ui/issues/issue-22034.stderr b/tests/ui/issues/issue-22034.stderr
@@ -4,7 +4,7 @@ error[E0277]: expected a `Fn()` closure, found `()`
 LL |         &mut *(ptr as *mut dyn Fn())
    |                ^^^ expected an `Fn()` closure, found `()`
    |
-   = help: the trait `Fn<()>` is not implemented for `()`
+   = help: the trait `Fn()` is not implemented for `()`
    = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`
    = note: required for the cast from `*mut ()` to `*mut dyn Fn()`
 

diff --git a/tests/ui/issues/issue-23966.stderr b/tests/ui/issues/issue-23966.stderr
@@ -6,7 +6,7 @@ LL |     "".chars().fold(|_, _| (), ());
    |                |
    |                required by a bound introduced by this call
    |
-   = help: the trait `FnMut<(_, char)>` is not implemented for `()`
+   = help: the trait `FnMut(_, char)` is not implemented for `()`
 note: required by a bound in `fold`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 

diff --git a/tests/ui/lifetimes/issue-76168-hr-outlives-3.stderr b/tests/ui/lifetimes/issue-76168-hr-outlives-3.stderr
@@ -10,15 +10,15 @@ LL | | F:,
 LL | | for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: Future<Output = ()> + 'a,
    | |__________________________________________________________________________^ expected an `FnOnce(&'a mut i32)` closure, found `i32`
    |
-   = help: the trait `for<'a> FnOnce<(&'a mut i32,)>` is not implemented for `i32`
+   = help: the trait `for<'a> FnOnce(&'a mut i32)` is not implemented for `i32`
 
 error[E0277]: expected a `FnOnce(&'a mut i32)` closure, found `i32`
   --> $DIR/issue-76168-hr-outlives-3.rs:6:10
    |
 LL | async fn wrapper<F>(f: F)
    |          ^^^^^^^ expected an `FnOnce(&'a mut i32)` closure, found `i32`
    |
-   = help: the trait `for<'a> FnOnce<(&'a mut i32,)>` is not implemented for `i32`
+   = help: the trait `for<'a> FnOnce(&'a mut i32)` is not implemented for `i32`
 
 error[E0277]: expected a `FnOnce(&'a mut i32)` closure, found `i32`
   --> $DIR/issue-76168-hr-outlives-3.rs:6:1
@@ -32,7 +32,7 @@ LL | | F:,
 LL | | for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: Future<Output = ()> + 'a,
    | |__________________________________________________________________________^ expected an `FnOnce(&'a mut i32)` closure, found `i32`
    |
-   = help: the trait `for<'a> FnOnce<(&'a mut i32,)>` is not implemented for `i32`
+   = help: the trait `for<'a> FnOnce(&'a mut i32)` is not implemented for `i32`
 
 error[E0277]: expected a `FnOnce(&'a mut i32)` closure, found `i32`
   --> $DIR/issue-76168-hr-outlives-3.rs:6:1
@@ -46,7 +46,7 @@ LL | | F:,
 LL | | for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: Future<Output = ()> + 'a,
    | |__________________________________________________________________________^ expected an `FnOnce(&'a mut i32)` closure, found `i32`
    |
-   = help: the trait `for<'a> FnOnce<(&'a mut i32,)>` is not implemented for `i32`
+   = help: the trait `for<'a> FnOnce(&'a mut i32)` is not implemented for `i32`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0277]: expected a `FnOnce(&'a mut i32)` closure, found `i32`
@@ -59,7 +59,7 @@ LL | |     &mut i;
 LL | | }
    | |_^ expected an `FnOnce(&'a mut i32)` closure, found `i32`
    |
-   = help: the trait `for<'a> FnOnce<(&'a mut i32,)>` is not implemented for `i32`
+   = help: the trait `for<'a> FnOnce(&'a mut i32)` is not implemented for `i32`
 
 error: aborting due to 5 previous errors
 

diff --git a/tests/ui/lifetimes/issue-95023.stderr b/tests/ui/lifetimes/issue-95023.stderr
@@ -38,7 +38,7 @@ error[E0277]: expected a `FnMut(&isize)` closure, found `Error`
 LL | impl Fn(&isize) for Error {
    |                     ^^^^^ expected an `FnMut(&isize)` closure, found `Error`
    |
-   = help: the trait `FnMut<(&isize,)>` is not implemented for `Error`
+   = help: the trait `FnMut(&isize)` is not implemented for `Error`
 note: required by a bound in `Fn`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
 

diff --git a/tests/ui/mismatched_types/suggest-option-asderef-unfixable.stderr b/tests/ui/mismatched_types/suggest-option-asderef-unfixable.stderr
@@ -26,7 +26,7 @@ LL |     let _ = produces_string().and_then(takes_str_but_wrong_abi);
    |                               |
    |                               required by a bound introduced by this call
    |
-   = help: the trait `FnOnce<(String,)>` is not implemented for fn item `for<'a> extern "C" fn(&'a str) -> Option<()> {takes_str_but_wrong_abi}`
+   = help: the trait `FnOnce(String)` is not implemented for fn item `for<'a> extern "C" fn(&'a str) -> Option<()> {takes_str_but_wrong_abi}`
 note: required by a bound in `Option::<T>::and_then`
   --> $SRC_DIR/core/src/option.rs:LL:COL
 
@@ -38,7 +38,7 @@ LL |     let _ = produces_string().and_then(takes_str_but_unsafe);
    |                               |
    |                               required by a bound introduced by this call
    |
-   = help: the trait `FnOnce<(String,)>` is not implemented for fn item `for<'a> unsafe fn(&'a str) -> Option<()> {takes_str_but_unsafe}`
+   = help: the trait `FnOnce(String)` is not implemented for fn item `for<'a> unsafe fn(&'a str) -> Option<()> {takes_str_but_unsafe}`
    = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `Option::<T>::and_then`
   --> $SRC_DIR/core/src/option.rs:LL:COL