rust-bors · Sep 20, 2026 · Sep 19, 2026
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
@@ -96,7 +96,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             // Do not suggest the other syntax if we are in trait impl:
             // the desugaring would contain an associated type constraint.
             if !is_impl {
-                err.span_suggestion(
+                err.span_suggestion_verbose(
                     span,
                     "use parenthetical notation instead",
                     fn_trait_to_string(self.tcx(), trait_segment, true),
@@ -2036,7 +2036,7 @@ impl<'a, 'tcx> rustc_errors::Diagnostic<'a, ()> for AmbiguityBetweenVariantAndAs
         could_refer_to(DefKind::Variant, variant_def_id, "");
         could_refer_to(mode.def_kind_for_diagnostics(), item_def_id, " also");
 
-        lint.span_suggestion(
+        lint.span_suggestion_verbose(
             span,
             "use fully-qualified syntax",
             format!("<{} as {}>::{}", self_ty, tcx.item_name(bound_def_id), segment_ident),

diff --git a/tests/ui/cast/dyn-any-to-fn-with-missing-generics.stderr b/tests/ui/cast/dyn-any-to-fn-with-missing-generics.stderr
@@ -2,11 +2,15 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/dyn-any-to-fn-with-missing-generics.rs:9:39
    |
 LL |     println!("{:?}",(vfnfer[0] as dyn Fn)(3));
-   |                                       ^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                                       ^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL |     println!("{:?}",(vfnfer[0] as dyn Fn() -> ())(3));
+   |                                         ++++++++
 
 error[E0107]: missing generics for trait `Fn`
   --> $DIR/dyn-any-to-fn-with-missing-generics.rs:9:39

diff --git a/tests/ui/impl-trait/opaque-used-in-extraneous-argument.stderr b/tests/ui/impl-trait/opaque-used-in-extraneous-argument.stderr
@@ -37,22 +37,32 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/opaque-used-in-extraneous-argument.rs:5:19
    |
 LL | fn frob() -> impl Fn<P, Output = T> + '_ {}
-   |                   ^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(P) -> T`
+   |                   ^^^^^^^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL - fn frob() -> impl Fn<P, Output = T> + '_ {}
+LL + fn frob() -> impl Fn(P) -> T + '_ {}
+   |
 
 error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
   --> $DIR/opaque-used-in-extraneous-argument.rs:5:19
    |
 LL | fn frob() -> impl Fn<P, Output = T> + '_ {}
-   |                   ^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(P) -> T`
+   |                   ^^^^^^^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use parenthetical notation instead
+   |
+LL - fn frob() -> impl Fn<P, Output = T> + '_ {}
+LL + fn frob() -> impl Fn(P) -> T + '_ {}
+   |
 
 error[E0061]: this function takes 0 arguments but 1 argument was supplied
   --> $DIR/opaque-used-in-extraneous-argument.rs:17:20

diff --git a/tests/ui/lint/future-incompatible-lint-group.stderr b/tests/ui/lint/future-incompatible-lint-group.stderr
@@ -12,7 +12,7 @@ error: ambiguous associated item
   --> $DIR/future-incompatible-lint-group.rs:19:17
    |
 LL |     fn foo() -> Self::V { 0 }
-   |                 ^^^^^^^ help: use fully-qualified syntax: `<E as Tr1>::V`
+   |                 ^^^^^^^
    |
 note: `V` could refer to the variant defined here
   --> $DIR/future-incompatible-lint-group.rs:8:10
@@ -32,6 +32,11 @@ note: the lint level is defined here
 LL | #![deny(future_incompatible)]
    |         ^^^^^^^^^^^^^^^^^^^
    = note: `#[deny(ambiguous_associated_items)]` implied by `#[deny(future_incompatible)]`
+help: use fully-qualified syntax
+   |
+LL -     fn foo() -> Self::V { 0 }
+LL +     fn foo() -> <E as Tr1>::V { 0 }
+   |
 
 error: aborting due to 1 previous error; 1 warning emitted
 
diff --git a/tests/ui/suggestions/fn-trait-notation.stderr b/tests/ui/suggestions/fn-trait-notation.stderr
@@ -2,31 +2,46 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/fn-trait-notation.rs:4:8
    |
 LL |     F: Fn<i32, Output = i32>,
-   |        ^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(i32) -> i32`
+   |        ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL -     F: Fn<i32, Output = i32>,
+LL +     F: Fn(i32) -> i32,
+   |
 
 error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
   --> $DIR/fn-trait-notation.rs:6:8
    |
 LL |     G: Fn<(i32, i32, ), Output = (i32, i32)>,
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(i32, i32) -> (i32, i32)`
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL -     G: Fn<(i32, i32, ), Output = (i32, i32)>,
+LL +     G: Fn(i32, i32) -> (i32, i32),
+   |
 
 error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
   --> $DIR/fn-trait-notation.rs:7:8
    |
 LL |     H: Fn<(i32,), Output = i32>,
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(i32) -> i32`
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL -     H: Fn<(i32,), Output = i32>,
+LL +     H: Fn(i32) -> i32,
+   |
 
 error[E0059]: type parameter to bare `Fn` trait must be a tuple
   --> $DIR/fn-trait-notation.rs:4:8

diff --git a/tests/ui/traits/next-solver/assertion-left-right-goal.stderr b/tests/ui/traits/next-solver/assertion-left-right-goal.stderr
@@ -2,44 +2,64 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/assertion-left-right-goal.rs:6:28
    |
 LL |     async fn new() -> impl Fn<()> {
-   |                            ^^^^^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                            ^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL -     async fn new() -> impl Fn<()> {
+LL +     async fn new() -> impl Fn() -> () {
+   |
 
 error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
   --> $DIR/assertion-left-right-goal.rs:6:28
    |
 LL |     async fn new() -> impl Fn<()> {
-   |                            ^^^^^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                            ^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use parenthetical notation instead
+   |
+LL -     async fn new() -> impl Fn<()> {
+LL +     async fn new() -> impl Fn() -> () {
+   |
 
 error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
   --> $DIR/assertion-left-right-goal.rs:6:28
    |
 LL |     async fn new() -> impl Fn<()> {
-   |                            ^^^^^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                            ^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use parenthetical notation instead
+   |
+LL -     async fn new() -> impl Fn<()> {
+LL +     async fn new() -> impl Fn() -> () {
+   |
 
 error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
   --> $DIR/assertion-left-right-goal.rs:6:28
    |
 LL |     async fn new() -> impl Fn<()> {
-   |                            ^^^^^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                            ^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: use parenthetical notation instead
+   |
+LL -     async fn new() -> impl Fn<()> {
+LL +     async fn new() -> impl Fn() -> () {
+   |
 
 error[E0308]: mismatched types
   --> $DIR/assertion-left-right-goal.rs:6:35

diff --git a/.../ui/type-alias-enum-variants/enum-variant-priority-lint-ambiguous_associated_items.stderr b/.../ui/type-alias-enum-variants/enum-variant-priority-lint-ambiguous_associated_items.stderr
@@ -2,7 +2,7 @@ error: ambiguous associated item
   --> $DIR/enum-variant-priority-lint-ambiguous_associated_items.rs:32:15
    |
 LL |     fn f() -> Self::V { 0 }
-   |               ^^^^^^^ help: use fully-qualified syntax: `<E as Tr>::V`
+   |               ^^^^^^^
    |
 note: `V` could refer to the variant defined here
   --> $DIR/enum-variant-priority-lint-ambiguous_associated_items.rs:22:5
@@ -17,6 +17,11 @@ LL |     type V;
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #57644 <https://github.com/rust-lang/rust/issues/57644>
    = note: `#[deny(ambiguous_associated_items)]` (part of `#[deny(future_incompatible)]`) on by default
+help: use fully-qualified syntax
+   |
+LL -     fn f() -> Self::V { 0 }
+LL +     fn f() -> <E as Tr>::V { 0 }
+   |
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/unboxed-closures/missing-fn-trait-args-issue-136407.feature_disabled.stderr b/tests/ui/unboxed-closures/missing-fn-trait-args-issue-136407.feature_disabled.stderr
@@ -2,11 +2,15 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/missing-fn-trait-args-issue-136407.rs:9:18
    |
 LL | pub fn shared<T: Fn>() {}
-   |                  ^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                  ^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL | pub fn shared<T: Fn() -> ()>() {}
+   |                    ++++++++
 
 error[E0107]: missing generics for trait `Fn`
   --> $DIR/missing-fn-trait-args-issue-136407.rs:9:18
@@ -18,11 +22,15 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/missing-fn-trait-args-issue-136407.rs:13:19
    |
 LL | pub fn mutable<T: FnMut>() {}
-   |                   ^^^^^ help: use parenthetical notation instead: `FnMut() -> ()`
+   |                   ^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL | pub fn mutable<T: FnMut() -> ()>() {}
+   |                        ++++++++
 
 error[E0107]: missing generics for trait `FnMut`
   --> $DIR/missing-fn-trait-args-issue-136407.rs:13:19
@@ -34,11 +42,15 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/missing-fn-trait-args-issue-136407.rs:17:16
    |
 LL | pub fn once<T: FnOnce>() {}
-   |                ^^^^^^ help: use parenthetical notation instead: `FnOnce() -> ()`
+   |                ^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL | pub fn once<T: FnOnce() -> ()>() {}
+   |                      ++++++++
 
 error[E0107]: missing generics for trait `FnOnce`
   --> $DIR/missing-fn-trait-args-issue-136407.rs:17:16
@@ -50,11 +62,15 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/missing-fn-trait-args-issue-136407.rs:21:24
    |
 LL | pub fn async_shared<T: AsyncFn>() {}
-   |                        ^^^^^^^ help: use parenthetical notation instead: `AsyncFn() -> ()`
+   |                        ^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL | pub fn async_shared<T: AsyncFn() -> ()>() {}
+   |                               ++++++++
 
 error[E0107]: missing generics for trait `AsyncFn`
   --> $DIR/missing-fn-trait-args-issue-136407.rs:21:24
@@ -66,11 +82,15 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/missing-fn-trait-args-issue-136407.rs:25:25
    |
 LL | pub fn async_mutable<T: AsyncFnMut>() {}
-   |                         ^^^^^^^^^^ help: use parenthetical notation instead: `AsyncFnMut() -> ()`
+   |                         ^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL | pub fn async_mutable<T: AsyncFnMut() -> ()>() {}
+   |                                   ++++++++
 
 error[E0107]: missing generics for trait `AsyncFnMut`
   --> $DIR/missing-fn-trait-args-issue-136407.rs:25:25
@@ -82,11 +102,15 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/missing-fn-trait-args-issue-136407.rs:29:22
    |
 LL | pub fn async_once<T: AsyncFnOnce>() {}
-   |                      ^^^^^^^^^^^ help: use parenthetical notation instead: `AsyncFnOnce() -> ()`
+   |                      ^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL | pub fn async_once<T: AsyncFnOnce() -> ()>() {}
+   |                                 ++++++++
 
 error[E0107]: missing generics for trait `AsyncFnOnce`
   --> $DIR/missing-fn-trait-args-issue-136407.rs:29:22
@@ -98,11 +122,16 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/missing-fn-trait-args-issue-136407.rs:33:23
    |
 LL | pub fn with_output<T: Fn<Output = ()>>() {}
-   |                       ^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                       ^^^^^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL - pub fn with_output<T: Fn<Output = ()>>() {}
+LL + pub fn with_output<T: Fn() -> ()>() {}
+   |
 
 error[E0107]: trait takes 1 generic argument but 0 generic arguments were supplied
   --> $DIR/missing-fn-trait-args-issue-136407.rs:33:23

diff --git a/tests/ui/unboxed-closures/unboxed-closure-sugar-not-used-on-fn.stderr b/tests/ui/unboxed-closures/unboxed-closure-sugar-not-used-on-fn.stderr
@@ -2,21 +2,31 @@ error[E0658]: the precise format of `Fn`-family traits' type parameters is subje
   --> $DIR/unboxed-closure-sugar-not-used-on-fn.rs:3:17
    |
 LL | fn bar1(x: &dyn Fn<(), Output=()>) {
-   |                 ^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                 ^^^^^^^^^^^^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL - fn bar1(x: &dyn Fn<(), Output=()>) {
+LL + fn bar1(x: &dyn Fn() -> ()) {
+   |
 
 error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
   --> $DIR/unboxed-closure-sugar-not-used-on-fn.rs:7:28
    |
 LL | fn bar2<T>(x: &T) where T: Fn<()> {
-   |                            ^^^^^^ help: use parenthetical notation instead: `Fn() -> ()`
+   |                            ^^^^^^
    |
    = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: use parenthetical notation instead
+   |
+LL - fn bar2<T>(x: &T) where T: Fn<()> {
+LL + fn bar2<T>(x: &T) where T: Fn() -> () {
+   |
 
 error: aborting due to 2 previous errors