Code change:
<ins> error[E0277]: expected a `std::ops::Fn<()>` closure, found `fn() {foo}` --> $DIR/fn-traits.rs:24:10 | LL | fn call(f: impl Fn()) { |                 ---- required by this bound in `call` ... LL |     call(foo); |          ^^^ expected an `Fn<()>` closure, found `fn() {foo}` | = help: the trait `std::ops::Fn<()>` is not implemented for `fn() {foo}` = note: wrap the `fn() {foo}` in a closure with no arguments: `|| { /* code */ } = note: `#[target_feature]` functions do not implement the `Fn` traits error[E0277]: expected a `std::ops::FnMut<()>` closure, found `fn() {foo}` --> $DIR/fn-traits.rs:25:14 | LL | fn call_mut(f: impl FnMut()) { |                     ------- required by this bound in `call_mut` ... LL |     call_mut(foo); |              ^^^ expected an `FnMut<()>` closure, found `fn() {foo}` | = help: the trait `std::ops::FnMut<()>` is not implemented for `fn() {foo}` = note: wrap the `fn() {foo}` in a closure with no arguments: `|| { /* code */ } = note: `#[target_feature]` functions do not implement the `Fn` traits error[E0277]: expected a `std::ops::FnOnce<()>` closure, found `fn() {foo}` --> $DIR/fn-traits.rs:26:15 | LL | fn call_once(f: impl FnOnce()) { |                      -------- required by this bound in `call_once` ... LL |     call_once(foo); |               ^^^ expected an `FnOnce<()>` closure, found `fn() {foo}` | = help: the trait `std::ops::FnOnce<()>` is not implemented for `fn() {foo}` = note: wrap the `fn() {foo}` in a closure with no arguments: `|| { /* code */ } = note: `#[target_feature]` functions do not implement the `Fn` traits error[E0277]: expected a `std::ops::Fn<()>` closure, found `unsafe fn() {foo_unsafe}` --> $DIR/fn-traits.rs:28:10 | LL | fn call(f: impl Fn()) { |                 ---- required by this bound in `call` ... LL |     call(foo_unsafe); |          ^^^^^^^^^^ expected an `Fn<()>` closure, found `unsafe fn() {foo_unsafe}` | = help: the trait `std::ops::Fn<()>` is not implemented for `unsafe fn() {foo_unsafe}` = note: wrap the `unsafe fn() {foo_unsafe}` in a closure with no arguments: `|| { /* code */ } = note: `#[target_feature]` functions do not implement the `Fn` traits error[E0277]: expected a `std::ops::FnMut<()>` closure, found `unsafe fn() {foo_unsafe}` --> $DIR/fn-traits.rs:30:14 | LL | fn call_mut(f: impl FnMut()) { |                     ------- required by this bound in `call_mut` ... LL |     call_mut(foo_unsafe); |              ^^^^^^^^^^ expected an `FnMut<()>` closure, found `unsafe fn() {foo_unsafe}` | = help: the trait `std::ops::FnMut<()>` is not implemented for `unsafe fn() {foo_unsafe}` = note: wrap the `unsafe fn() {foo_unsafe}` in a closure with no arguments: `|| { /* code */ } = note: `#[target_feature]` functions do not implement the `Fn` traits error[E0277]: expected a `std::ops::FnOnce<()>` closure, found `unsafe fn() {foo_unsafe}` --> $DIR/fn-traits.rs:32:15 | LL | fn call_once(f: impl FnOnce()) { |                      -------- required by this bound in `call_once` ... LL |     call_once(foo_unsafe); |               ^^^^^^^^^^ expected an `FnOnce<()>` closure, found `unsafe fn() {foo_unsafe}` | = help: the trait `std::ops::FnOnce<()>` is not implemented for `unsafe fn() {foo_unsafe}` = note: wrap the `unsafe fn() {foo_unsafe}` in a closure with no arguments: `|| { /* code */ } = note: `#[target_feature]` functions do not implement the `Fn` traits error: aborting due to 6 previous errors For more information about this error, try `rustc --explain E0277`. </ins>

Related discussion:
text: (This would be like a clause.) To that end, I was thinking that you could model this last point using closures, but I found that it doesn't work. In particular, gives an error, even if it appears inside of a function annotated with . This seems like something we should discuss, but it's a separate issue. We did discuss briefly whether target features should be part of the type of function pointers but I think the conclusion was 'well maybe but there is already stable code out there that lets you coerce, and it seems rather niche'. We also discussed whether we should permit one to annotate closures with target-feature annotations. In any case, those are things we can address separately from this issue, but the point is that adopting more accepting solutions can be done in the future.
I'm going to move the question of whether closures should "inherit" target features to the main tracking issue -- I'm not sure if that is something that was discussed as part of the original RFC discussion?
Opened to discuss closures.