diagnostics: give a special note for unsafe fn / Fn/FnOnce/FnMut

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

@@ -208,6 +208,15 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 flags.push((sym::_Self, Some("&[]".to_owned())));
             }
 
+            if self_ty.is_fn() {
+                let fn_sig = self_ty.fn_sig(self.tcx);
+                let shortname = match fn_sig.unsafety() {
+                    hir::Unsafety::Normal => "fn",
+                    hir::Unsafety::Unsafe => "unsafe fn",
+                };
+                flags.push((sym::_Self, Some(shortname.to_owned())));
+            }
+
             if let ty::Array(aty, len) = self_ty.kind() {
                 flags.push((sym::_Self, Some("[]".to_owned())));
                 flags.push((sym::_Self, Some(format!("[{}]", aty))));

library/core/src/ops/function.rs

@@ -59,6 +59,12 @@
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
     ),
+    on(
+        _Self = "unsafe fn",
+        note = "unsafe function cannot be called generically without an unsafe block",
+        // SAFETY: tidy is not smart enough to tell that the below unsafe block is a string
+        label = "call the function in a closure: `|| unsafe {{ /* code */ }}`"
+    ),
     message = "expected a `{Fn}<{Args}>` closure, found `{Self}`",
     label = "expected an `Fn<{Args}>` closure, found `{Self}`"
 )]
@@ -139,6 +145,12 @@ pub trait Fn<Args>: FnMut<Args> {
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
     ),
+    on(
+        _Self = "unsafe fn",
+        note = "unsafe function cannot be called generically without an unsafe block",
+        // SAFETY: tidy is not smart enough to tell that the below unsafe block is a string
+        label = "call the function in a closure: `|| unsafe {{ /* code */ }}`"
+    ),
     message = "expected a `{FnMut}<{Args}>` closure, found `{Self}`",
     label = "expected an `FnMut<{Args}>` closure, found `{Self}`"
 )]
@@ -211,6 +223,12 @@ pub trait FnMut<Args>: FnOnce<Args> {
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
     ),
+    on(
+        _Self = "unsafe fn",
+        note = "unsafe function cannot be called generically without an unsafe block",
+        // SAFETY: tidy is not smart enough to tell that the below unsafe block is a string
+        label = "call the function in a closure: `|| unsafe {{ /* code */ }}`"
+    ),
     message = "expected a `{FnOnce}<{Args}>` closure, found `{Self}`",
     label = "expected an `FnOnce<{Args}>` closure, found `{Self}`"
 )]

src/test/ui/closures/coerce-unsafe-to-closure.stderr

@@ -2,11 +2,12 @@ error[E0277]: expected a `FnOnce<(&str,)>` closure, found `unsafe extern "rust-i
   --> $DIR/coerce-unsafe-to-closure.rs:2:44
    |
 LL |     let x: Option<&[u8]> = Some("foo").map(std::mem::transmute);
-   |                                        --- ^^^^^^^^^^^^^^^^^^^ expected an `FnOnce<(&str,)>` closure, found `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}`
+   |                                        --- ^^^^^^^^^^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |                                        |
    |                                        required by a bound introduced by this call
    |
    = help: the trait `FnOnce<(&str,)>` is not implemented for `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}`
+   = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `Option::<T>::map`
   --> $SRC_DIR/core/src/option.rs:LL:COL
    |

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

@@ -53,7 +53,7 @@ error[E0277]: expected a `Fn<()>` closure, found `unsafe fn() {foo_unsafe}`
   --> $DIR/fn-traits.rs:28:10
    |
 LL |     call(foo_unsafe);
-   |     ---- ^^^^^^^^^^ expected an `Fn<()>` closure, found `unsafe fn() {foo_unsafe}`
+   |     ---- ^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |     |
    |     required by a bound introduced by this call
    |
@@ -70,7 +70,7 @@ error[E0277]: expected a `FnMut<()>` closure, found `unsafe fn() {foo_unsafe}`
   --> $DIR/fn-traits.rs:30:14
    |
 LL |     call_mut(foo_unsafe);
-   |     -------- ^^^^^^^^^^ expected an `FnMut<()>` closure, found `unsafe fn() {foo_unsafe}`
+   |     -------- ^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |     |
    |     required by a bound introduced by this call
    |
@@ -87,7 +87,7 @@ error[E0277]: expected a `FnOnce<()>` closure, found `unsafe fn() {foo_unsafe}`
   --> $DIR/fn-traits.rs:32:15
    |
 LL |     call_once(foo_unsafe);
-   |     --------- ^^^^^^^^^^ expected an `FnOnce<()>` closure, found `unsafe fn() {foo_unsafe}`
+   |     --------- ^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |     |
    |     required by a bound introduced by this call
    |

src/test/ui/unboxed-closures/unboxed-closures-unsafe-extern-fn.stderr

@@ -2,11 +2,12 @@ error[E0277]: expected a `Fn<(&isize,)>` closure, found `for<'r> unsafe fn(&'r i
   --> $DIR/unboxed-closures-unsafe-extern-fn.rs:20:21
    |
 LL |     let x = call_it(&square, 22);
-   |             ------- ^^^^^^^ expected an `Fn<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
+   |             ------- ^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the trait `for<'r> Fn<(&'r isize,)>` is not implemented for `for<'r> unsafe fn(&'r isize) -> isize {square}`
+   = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `call_it`
   --> $DIR/unboxed-closures-unsafe-extern-fn.rs:9:15
    |
@@ -17,11 +18,12 @@ error[E0277]: expected a `FnMut<(&isize,)>` closure, found `for<'r> unsafe fn(&'
   --> $DIR/unboxed-closures-unsafe-extern-fn.rs:25:25
    |
 LL |     let y = call_it_mut(&mut square, 22);
-   |             ----------- ^^^^^^^^^^^ expected an `FnMut<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
+   |             ----------- ^^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the trait `for<'r> FnMut<(&'r isize,)>` is not implemented for `for<'r> unsafe fn(&'r isize) -> isize {square}`
+   = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `call_it_mut`
   --> $DIR/unboxed-closures-unsafe-extern-fn.rs:12:19
    |
@@ -32,11 +34,12 @@ error[E0277]: expected a `FnOnce<(&isize,)>` closure, found `for<'r> unsafe fn(&
   --> $DIR/unboxed-closures-unsafe-extern-fn.rs:30:26
    |
 LL |     let z = call_it_once(square, 22);
-   |             ------------ ^^^^^^ expected an `FnOnce<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
+   |             ------------ ^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the trait `for<'r> FnOnce<(&'r isize,)>` is not implemented for `for<'r> unsafe fn(&'r isize) -> isize {square}`
+   = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `call_it_once`
   --> $DIR/unboxed-closures-unsafe-extern-fn.rs:15:20
    |

src/test/ui/unboxed-closures/unboxed-closures-wrong-arg-type-extern-fn.stderr

@@ -2,11 +2,12 @@ error[E0277]: expected a `Fn<(&isize,)>` closure, found `unsafe fn(isize) -> isi
   --> $DIR/unboxed-closures-wrong-arg-type-extern-fn.rs:21:21
    |
 LL |     let x = call_it(&square, 22);
-   |             ------- ^^^^^^^ expected an `Fn<(&isize,)>` closure, found `unsafe fn(isize) -> isize {square}`
+   |             ------- ^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the trait `for<'r> Fn<(&'r isize,)>` is not implemented for `unsafe fn(isize) -> isize {square}`
+   = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `call_it`
   --> $DIR/unboxed-closures-wrong-arg-type-extern-fn.rs:10:15
    |
@@ -17,11 +18,12 @@ error[E0277]: expected a `FnMut<(&isize,)>` closure, found `unsafe fn(isize) ->
   --> $DIR/unboxed-closures-wrong-arg-type-extern-fn.rs:26:25
    |
 LL |     let y = call_it_mut(&mut square, 22);
-   |             ----------- ^^^^^^^^^^^ expected an `FnMut<(&isize,)>` closure, found `unsafe fn(isize) -> isize {square}`
+   |             ----------- ^^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the trait `for<'r> FnMut<(&'r isize,)>` is not implemented for `unsafe fn(isize) -> isize {square}`
+   = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `call_it_mut`
   --> $DIR/unboxed-closures-wrong-arg-type-extern-fn.rs:13:19
    |
@@ -32,11 +34,12 @@ error[E0277]: expected a `FnOnce<(&isize,)>` closure, found `unsafe fn(isize) ->
   --> $DIR/unboxed-closures-wrong-arg-type-extern-fn.rs:31:26
    |
 LL |     let z = call_it_once(square, 22);
-   |             ------------ ^^^^^^ expected an `FnOnce<(&isize,)>` closure, found `unsafe fn(isize) -> isize {square}`
+   |             ------------ ^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the trait `for<'r> FnOnce<(&'r isize,)>` is not implemented for `unsafe fn(isize) -> isize {square}`
+   = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `call_it_once`
   --> $DIR/unboxed-closures-wrong-arg-type-extern-fn.rs:16:20
    |