Add a suggestion to cast target_feature fn items to fn pointers.

See
rust-lang#134090 (comment)
for the motivation behind this suggestion.

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -462,6 +462,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             err.note(
                                 "`#[target_feature]` functions do not implement the `Fn` traits",
                             );
+                            err.note(
+                                "try casting the function to a `fn` pointer or wrapping it in a closure",
+                            );
                         }
 
                         self.try_to_add_help_message(

tests/ui/rfcs/rfc-2396-target_feature-11/fn-traits.rs

@@ -5,6 +5,9 @@
 #[target_feature(enable = "avx")]
 fn foo() {}
 
+#[target_feature(enable = "avx")]
+fn bar(arg: i32) {}
+
 #[target_feature(enable = "avx")]
 unsafe fn foo_unsafe() {}
 
@@ -20,10 +23,15 @@ fn call_once(f: impl FnOnce()) {
     f()
 }
 
+fn call_once_i32(f: impl FnOnce(i32)) {
+    f(0)
+}
+
 fn main() {
     call(foo); //~ ERROR expected a `Fn()` closure, found `#[target_features] fn() {foo}`
     call_mut(foo); //~ ERROR expected a `FnMut()` closure, found `#[target_features] fn() {foo}`
     call_once(foo); //~ ERROR expected a `FnOnce()` closure, found `#[target_features] fn() {foo}`
+    call_once_i32(bar); //~ ERROR expected a `FnOnce(i32)` closure, found `#[target_features] fn(i32) {bar}`
 
     call(foo_unsafe);
     //~^ ERROR expected a `Fn()` closure, found `unsafe fn() {foo_unsafe}`

tests/ui/rfcs/rfc-2396-target_feature-11/fn-traits.stderr

@@ -1,5 +1,5 @@
 error[E0277]: expected a `Fn()` closure, found `#[target_features] fn() {foo}`
-  --> $DIR/fn-traits.rs:24:10
+  --> $DIR/fn-traits.rs:31:10
    |
 LL |     call(foo);
    |     ---- ^^^ expected an `Fn()` closure, found `#[target_features] fn() {foo}`
@@ -9,14 +9,15 @@ LL |     call(foo);
    = help: the trait `Fn()` is not implemented for fn item `#[target_features] fn() {foo}`
    = note: wrap the `#[target_features] fn() {foo}` in a closure with no arguments: `|| { /* code */ }`
    = note: `#[target_feature]` functions do not implement the `Fn` traits
+   = note: try casting the function to a `fn` pointer or wrapping it in a closure
 note: required by a bound in `call`
-  --> $DIR/fn-traits.rs:11:17
+  --> $DIR/fn-traits.rs:14:17
    |
 LL | fn call(f: impl Fn()) {
    |                 ^^^^ required by this bound in `call`
 
 error[E0277]: expected a `FnMut()` closure, found `#[target_features] fn() {foo}`
-  --> $DIR/fn-traits.rs:25:14
+  --> $DIR/fn-traits.rs:32:14
    |
 LL |     call_mut(foo);
    |     -------- ^^^ expected an `FnMut()` closure, found `#[target_features] fn() {foo}`
@@ -26,14 +27,15 @@ LL |     call_mut(foo);
    = help: the trait `FnMut()` is not implemented for fn item `#[target_features] fn() {foo}`
    = note: wrap the `#[target_features] fn() {foo}` in a closure with no arguments: `|| { /* code */ }`
    = note: `#[target_feature]` functions do not implement the `Fn` traits
+   = note: try casting the function to a `fn` pointer or wrapping it in a closure
 note: required by a bound in `call_mut`
-  --> $DIR/fn-traits.rs:15:25
+  --> $DIR/fn-traits.rs:18:25
    |
 LL | fn call_mut(mut f: impl FnMut()) {
    |                         ^^^^^^^ required by this bound in `call_mut`
 
 error[E0277]: expected a `FnOnce()` closure, found `#[target_features] fn() {foo}`
-  --> $DIR/fn-traits.rs:26:15
+  --> $DIR/fn-traits.rs:33:15
    |
 LL |     call_once(foo);
    |     --------- ^^^ expected an `FnOnce()` closure, found `#[target_features] fn() {foo}`
@@ -43,14 +45,32 @@ LL |     call_once(foo);
    = help: the trait `FnOnce()` is not implemented for fn item `#[target_features] fn() {foo}`
    = note: wrap the `#[target_features] fn() {foo}` in a closure with no arguments: `|| { /* code */ }`
    = note: `#[target_feature]` functions do not implement the `Fn` traits
+   = note: try casting the function to a `fn` pointer or wrapping it in a closure
 note: required by a bound in `call_once`
-  --> $DIR/fn-traits.rs:19:22
+  --> $DIR/fn-traits.rs:22:22
    |
 LL | fn call_once(f: impl FnOnce()) {
    |                      ^^^^^^^^ required by this bound in `call_once`
 
+error[E0277]: expected a `FnOnce(i32)` closure, found `#[target_features] fn(i32) {bar}`
+  --> $DIR/fn-traits.rs:34:19
+   |
+LL |     call_once_i32(bar);
+   |     ------------- ^^^ expected an `FnOnce(i32)` closure, found `#[target_features] fn(i32) {bar}`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `FnOnce(i32)` is not implemented for fn item `#[target_features] fn(i32) {bar}`
+   = note: `#[target_feature]` functions do not implement the `Fn` traits
+   = note: try casting the function to a `fn` pointer or wrapping it in a closure
+note: required by a bound in `call_once_i32`
+  --> $DIR/fn-traits.rs:26:26
+   |
+LL | fn call_once_i32(f: impl FnOnce(i32)) {
+   |                          ^^^^^^^^^^^ required by this bound in `call_once_i32`
+
 error[E0277]: expected a `Fn()` closure, found `unsafe fn() {foo_unsafe}`
-  --> $DIR/fn-traits.rs:28:10
+  --> $DIR/fn-traits.rs:36:10
    |
 LL |     call(foo_unsafe);
    |     ---- ^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
@@ -61,14 +81,15 @@ LL |     call(foo_unsafe);
    = note: unsafe function cannot be called generically without an unsafe block
    = note: wrap the `unsafe fn() {foo_unsafe}` in a closure with no arguments: `|| { /* code */ }`
    = note: `#[target_feature]` functions do not implement the `Fn` traits
+   = note: try casting the function to a `fn` pointer or wrapping it in a closure
 note: required by a bound in `call`
-  --> $DIR/fn-traits.rs:11:17
+  --> $DIR/fn-traits.rs:14:17
    |
 LL | fn call(f: impl Fn()) {
    |                 ^^^^ required by this bound in `call`
 
 error[E0277]: expected a `FnMut()` closure, found `unsafe fn() {foo_unsafe}`
-  --> $DIR/fn-traits.rs:30:14
+  --> $DIR/fn-traits.rs:38:14
    |
 LL |     call_mut(foo_unsafe);
    |     -------- ^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
@@ -79,14 +100,15 @@ LL |     call_mut(foo_unsafe);
    = note: unsafe function cannot be called generically without an unsafe block
    = note: wrap the `unsafe fn() {foo_unsafe}` in a closure with no arguments: `|| { /* code */ }`
    = note: `#[target_feature]` functions do not implement the `Fn` traits
+   = note: try casting the function to a `fn` pointer or wrapping it in a closure
 note: required by a bound in `call_mut`
-  --> $DIR/fn-traits.rs:15:25
+  --> $DIR/fn-traits.rs:18:25
    |
 LL | fn call_mut(mut f: impl FnMut()) {
    |                         ^^^^^^^ required by this bound in `call_mut`
 
 error[E0277]: expected a `FnOnce()` closure, found `unsafe fn() {foo_unsafe}`
-  --> $DIR/fn-traits.rs:32:15
+  --> $DIR/fn-traits.rs:40:15
    |
 LL |     call_once(foo_unsafe);
    |     --------- ^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
@@ -97,12 +119,13 @@ LL |     call_once(foo_unsafe);
    = note: unsafe function cannot be called generically without an unsafe block
    = note: wrap the `unsafe fn() {foo_unsafe}` in a closure with no arguments: `|| { /* code */ }`
    = note: `#[target_feature]` functions do not implement the `Fn` traits
+   = note: try casting the function to a `fn` pointer or wrapping it in a closure
 note: required by a bound in `call_once`
-  --> $DIR/fn-traits.rs:19:22
+  --> $DIR/fn-traits.rs:22:22
    |
 LL | fn call_once(f: impl FnOnce()) {
    |                      ^^^^^^^^ required by this bound in `call_once`
 
-error: aborting due to 6 previous errors
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0277`.