Make #[target_feature] Fn trait error message less confusing

Author: calebzulawski

src/librustc_trait_selection/traits/error_reporting/mod.rs

@@ -287,18 +287,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             .starts_with("std::convert::From<");
                         let is_unsize =
                             { Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait() };
-                        let is_fn_trait = [
-                            self.tcx.lang_items().fn_trait(),
-                            self.tcx.lang_items().fn_mut_trait(),
-                            self.tcx.lang_items().fn_once_trait(),
-                        ]
-                        .contains(&Some(trait_ref.def_id()));
-                        let is_target_feature_fn =
-                            if let ty::FnDef(def_id, _) = trait_ref.skip_binder().self_ty().kind {
-                                !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
-                            } else {
-                                false
-                            };
                         let (message, note) = if is_try && is_from {
                             (
                                 Some(format!(
@@ -441,11 +429,22 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             );
                         }
 
+                        let is_fn_trait = [
+                            self.tcx.lang_items().fn_trait(),
+                            self.tcx.lang_items().fn_mut_trait(),
+                            self.tcx.lang_items().fn_once_trait(),
+                        ]
+                        .contains(&Some(trait_ref.def_id()));
+                        let is_target_feature_fn =
+                            if let ty::FnDef(def_id, _) = trait_ref.skip_binder().self_ty().kind {
+                                !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
+                            } else {
+                                false
+                            };
                         if is_fn_trait && is_target_feature_fn {
-                            err.note(&format!(
-                                "`{}` has `#[target_feature]` and is unsafe to call",
-                                trait_ref.skip_binder().self_ty(),
-                            ));
+                            err.note(
+                                "`#[target_feature]` functions do not implement the `Fn` traits",
+                            );
                         }
 
                         // Try to report a help message

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

@@ -9,7 +9,7 @@ LL |     call(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: `fn() {foo}` has `#[target_feature]` and is unsafe to call
+   = 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
@@ -22,7 +22,7 @@ LL |     call_mut(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: `fn() {foo}` has `#[target_feature]` and is unsafe to call
+   = 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
@@ -35,7 +35,7 @@ LL |     call_once(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: `fn() {foo}` has `#[target_feature]` and is unsafe to call
+   = 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
@@ -48,7 +48,7 @@ LL |     call(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: `unsafe fn() {foo_unsafe}` has `#[target_feature]` and is unsafe to call
+   = 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
@@ -61,7 +61,7 @@ LL |     call_mut(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: `unsafe fn() {foo_unsafe}` has `#[target_feature]` and is unsafe to call
+   = 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
@@ -74,7 +74,7 @@ LL |     call_once(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: `unsafe fn() {foo_unsafe}` has `#[target_feature]` and is unsafe to call
+   = note: `#[target_feature]` functions do not implement the `Fn` traits
 
 error: aborting due to 6 previous errors