Try to render shorthand differently

Author: oli-obk

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

@@ -205,9 +205,15 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
             if self_ty.is_fn() {
                 let fn_sig = self_ty.fn_sig(self.tcx);
-                let shortname = match fn_sig.safety() {
-                    hir::Safety::Safe => "fn",
-                    hir::Safety::Unsafe => "unsafe fn",
+                let shortname = if let ty::FnDef(def_id, _) = self_ty.kind()
+                    && self.tcx.codegen_fn_attrs(def_id).safe_target_features
+                {
+                    "#[target_feature] fn"
+                } else {
+                    match fn_sig.safety() {
+                        hir::Safety::Safe => "fn",
+                        hir::Safety::Unsafe => "unsafe fn",
+                    }
                 };
                 flags.push((sym::_Self, Some(shortname.to_owned())));
             }

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

@@ -2,12 +2,11 @@ error[E0277]: expected a `Fn()` closure, found `#[target_features] fn() {foo}`
   --> $DIR/fn-traits.rs:24:10
    |
 LL |     call(foo);
-   |     ---- ^^^ call the function in a closure: `|| unsafe { /* code */ }`
+   |     ---- ^^^ expected an `Fn()` closure, found `#[target_features] fn() {foo}`
    |     |
    |     required by a bound introduced by this call
    |
    = help: the trait `Fn()` is not implemented for fn item `#[target_features] fn() {foo}`
-   = note: unsafe function cannot be called generically without an unsafe block
    = 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: required by a bound in `call`
@@ -20,12 +19,11 @@ error[E0277]: expected a `FnMut()` closure, found `#[target_features] fn() {foo}
   --> $DIR/fn-traits.rs:25:14
    |
 LL |     call_mut(foo);
-   |     -------- ^^^ call the function in a closure: `|| unsafe { /* code */ }`
+   |     -------- ^^^ expected an `FnMut()` closure, found `#[target_features] fn() {foo}`
    |     |
    |     required by a bound introduced by this call
    |
    = help: the trait `FnMut()` is not implemented for fn item `#[target_features] fn() {foo}`
-   = note: unsafe function cannot be called generically without an unsafe block
    = 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: required by a bound in `call_mut`
@@ -38,12 +36,11 @@ error[E0277]: expected a `FnOnce()` closure, found `#[target_features] fn() {foo
   --> $DIR/fn-traits.rs:26:15
    |
 LL |     call_once(foo);
-   |     --------- ^^^ call the function in a closure: `|| unsafe { /* code */ }`
+   |     --------- ^^^ expected an `FnOnce()` closure, found `#[target_features] fn() {foo}`
    |     |
    |     required by a bound introduced by this call
    |
    = help: the trait `FnOnce()` is not implemented for fn item `#[target_features] fn() {foo}`
-   = note: unsafe function cannot be called generically without an unsafe block
    = 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: required by a bound in `call_once`