Also check function items' signatures for Fn* trait compatibility

compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs

@@ -189,11 +189,20 @@ pub(crate) fn extract_tupled_inputs_and_output_from_callable<'tcx>(
     goal_kind: ty::ClosureKind,
 ) -> Result<Option<ty::Binder<'tcx, (Ty<'tcx>, Ty<'tcx>)>>, NoSolution> {
     match *self_ty.kind() {
-        ty::FnDef(def_id, substs) => Ok(Some(
+        // keep this in sync with assemble_fn_pointer_candidates until the old solver is removed.
-            tcx.fn_sig(def_id)
+        ty::FnDef(def_id, substs) => {
-                .subst(tcx, substs)
+            let sig = tcx.fn_sig(def_id);
+            if sig.skip_binder().is_fn_trait_compatible()
+                && tcx.codegen_fn_attrs(def_id).target_features.is_empty()
+            {
+                Ok(Some(
+                    sig.subst(tcx, substs)
                         .map_bound(|sig| (tcx.mk_tup(sig.inputs()), sig.output())),
-        )),
+                ))
+            } else {
+                Err(NoSolution)
+            }
+        }
         // keep this in sync with assemble_fn_pointer_candidates until the old solver is removed.
         ty::FnPtr(sig) => {
             if sig.is_fn_trait_compatible() {

tests/ui/traits/new-solver/fn-trait.rs

@@ -21,8 +21,12 @@ fn main() {
     //~^ ERROR: expected a `Fn<()>` closure, found `unsafe fn() -> i32`
     //~| ERROR: type mismatch resolving `<unsafe fn() -> i32 as FnOnce<()>>::Output == i32`
     require_fn(g);
+    //~^ ERROR: expected a `Fn<()>` closure, found `extern "C" fn() -> i32 {g}`
+    //~| ERROR: type mismatch resolving `<extern "C" fn() -> i32 {g} as FnOnce<()>>::Output == i32`
     require_fn(g as extern "C" fn() -> i32);
     //~^ ERROR: expected a `Fn<()>` closure, found `extern "C" fn() -> i32`
     //~| ERROR: type mismatch resolving `<extern "C" fn() -> i32 as FnOnce<()>>::Output == i32`
     require_fn(h);
+    //~^ ERROR: expected a `Fn<()>` closure, found `unsafe fn() -> i32 {h}`
+    //~| ERROR: type mismatch resolving `<unsafe fn() -> i32 {h} as FnOnce<()>>::Output == i32`
 }

tests/ui/traits/new-solver/fn-trait.stderr

@@ -28,8 +28,38 @@ note: required by a bound in `require_fn`
 LL | fn require_fn(_: impl Fn() -> i32) {}
    |                               ^^^ required by this bound in `require_fn`
 
+error[E0277]: expected a `Fn<()>` closure, found `extern "C" fn() -> i32 {g}`
+  --> $DIR/fn-trait.rs:23:16
+   |
+LL |     require_fn(g);
+   |     ---------- ^ expected an `Fn<()>` closure, found `extern "C" fn() -> i32 {g}`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Fn<()>` is not implemented for fn item `extern "C" fn() -> i32 {g}`
+   = note: wrap the `extern "C" fn() -> i32 {g}` in a closure with no arguments: `|| { /* code */ }`
+note: required by a bound in `require_fn`
+  --> $DIR/fn-trait.rs:3:23
+   |
+LL | fn require_fn(_: impl Fn() -> i32) {}
+   |                       ^^^^^^^^^^^ required by this bound in `require_fn`
+
+error[E0271]: type mismatch resolving `<extern "C" fn() -> i32 {g} as FnOnce<()>>::Output == i32`
+  --> $DIR/fn-trait.rs:23:16
+   |
+LL |     require_fn(g);
+   |     ---------- ^ types differ
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `require_fn`
+  --> $DIR/fn-trait.rs:3:31
+   |
+LL | fn require_fn(_: impl Fn() -> i32) {}
+   |                               ^^^ required by this bound in `require_fn`
+
 error[E0277]: expected a `Fn<()>` closure, found `extern "C" fn() -> i32`
-  --> $DIR/fn-trait.rs:24:16
+  --> $DIR/fn-trait.rs:26:16
    |
 LL |     require_fn(g as extern "C" fn() -> i32);
    |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `Fn<()>` closure, found `extern "C" fn() -> i32`
@@ -45,7 +75,7 @@ LL | fn require_fn(_: impl Fn() -> i32) {}
    |                       ^^^^^^^^^^^ required by this bound in `require_fn`
 
 error[E0271]: type mismatch resolving `<extern "C" fn() -> i32 as FnOnce<()>>::Output == i32`
-  --> $DIR/fn-trait.rs:24:16
+  --> $DIR/fn-trait.rs:26:16
    |
 LL |     require_fn(g as extern "C" fn() -> i32);
    |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
@@ -58,7 +88,37 @@ note: required by a bound in `require_fn`
 LL | fn require_fn(_: impl Fn() -> i32) {}
    |                               ^^^ required by this bound in `require_fn`
 
-error: aborting due to 4 previous errors
+error[E0277]: expected a `Fn<()>` closure, found `unsafe fn() -> i32 {h}`
+  --> $DIR/fn-trait.rs:29:16
+   |
+LL |     require_fn(h);
+   |     ---------- ^ call the function in a closure: `|| unsafe { /* code */ }`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Fn<()>` is not implemented for fn item `unsafe fn() -> i32 {h}`
+   = note: wrap the `unsafe fn() -> i32 {h}` in a closure with no arguments: `|| { /* code */ }`
+note: required by a bound in `require_fn`
+  --> $DIR/fn-trait.rs:3:23
+   |
+LL | fn require_fn(_: impl Fn() -> i32) {}
+   |                       ^^^^^^^^^^^ required by this bound in `require_fn`
+
+error[E0271]: type mismatch resolving `<unsafe fn() -> i32 {h} as FnOnce<()>>::Output == i32`
+  --> $DIR/fn-trait.rs:29:16
+   |
+LL |     require_fn(h);
+   |     ---------- ^ types differ
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `require_fn`
+  --> $DIR/fn-trait.rs:3:31
+   |
+LL | fn require_fn(_: impl Fn() -> i32) {}
+   |                               ^^^ required by this bound in `require_fn`
+
+error: aborting due to 8 previous errors
 
 Some errors have detailed explanations: E0271, E0277.
 For more information about an error, try `rustc --explain E0271`.