Add suggestions for function pointers

- On compiler-error's suggestion of moving this lower down the stack,
along the path of `report_mismatched_types()`, which is used
by `rustc_hir_analysis` and `rustc_hir_typeck`.
- update ui tests, add test
- add suggestions for references to fn pointers
- modify `TypeErrCtxt::same_type_modulo_infer` to take `T: relate::Relate` instead of `Ty`

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1613,12 +1613,14 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
                     self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
                 }
+
                 let reported = err.emit_unless(unsized_return);
 
                 self.final_ty = Some(fcx.tcx.ty_error_with_guaranteed(reported));
             }
         }
     }
+
     fn note_unreachable_loop_return(
         &self,
         err: &mut Diagnostic,

compiler/rustc_hir_typeck/src/demand.rs

@@ -81,7 +81,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.annotate_expected_due_to_let_ty(err, expr, error);
         self.emit_type_mismatch_suggestions(err, expr, expr_ty, expected, expected_ty_expr, error);
         self.note_type_is_not_clone(err, expected, expr_ty, expr);
-        self.note_need_for_fn_pointer(err, expected, expr_ty);
         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
         self.check_for_range_as_method_call(err, expr, expr_ty, expected);
         self.check_for_binding_assigned_block_without_tail_expression(err, expr, expr_ty, expected);

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -926,43 +926,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    pub(in super::super) fn note_need_for_fn_pointer(
-        &self,
-        err: &mut Diagnostic,
-        expected: Ty<'tcx>,
-        found: Ty<'tcx>,
-    ) {
-        let (sig, did, substs) = match (&expected.kind(), &found.kind()) {
-            (ty::FnDef(did1, substs1), ty::FnDef(did2, substs2)) => {
-                let sig1 = self.tcx.bound_fn_sig(*did1).subst(self.tcx, substs1);
-                let sig2 = self.tcx.bound_fn_sig(*did2).subst(self.tcx, substs2);
-                if sig1 != sig2 {
-                    return;
-                }
-                err.note(
-                    "different `fn` items always have unique types, even if their signatures are \
-                     the same",
-                );
-                (sig1, *did1, substs1)
-            }
-            (ty::FnDef(did, substs), ty::FnPtr(sig2)) => {
-                let sig1 = self.tcx.bound_fn_sig(*did).subst(self.tcx, substs);
-                if sig1 != *sig2 {
-                    return;
-                }
-                (sig1, *did, substs)
-            }
-            _ => return,
-        };
-        err.help(&format!("change the expected type to be function pointer `{}`", sig));
-        err.help(&format!(
-            "if the expected type is due to type inference, cast the expected `fn` to a function \
-             pointer: `{} as {}`",
-            self.tcx.def_path_str_with_substs(did, substs),
-            sig
-        ));
-    }
-
     // Instantiates the given path, which must refer to an item with the given
     // number of type parameters and type.
     #[instrument(skip(self, span), level = "debug")]

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1841,6 +1841,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
                 self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
                 self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
+                self.suggest_function_pointers(cause, span, &exp_found, diag);
             }
         }
 
@@ -2585,7 +2586,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     /// with the other type. A TyVar inference type is compatible with any type, and an IntVar or
     /// FloatVar inference type are compatible with themselves or their concrete types (Int and
     /// Float types, respectively). When comparing two ADTs, these rules apply recursively.
-    pub fn same_type_modulo_infer(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
+    pub fn same_type_modulo_infer<T: relate::Relate<'tcx>>(&self, a: T, b: T) -> bool {
         let (a, b) = self.resolve_vars_if_possible((a, b));
         SameTypeModuloInfer(self).relate(a, b).is_ok()
     }

compiler/rustc_infer/src/infer/error_reporting/suggest.rs

@@ -8,7 +8,7 @@ use rustc_middle::traits::{
     StatementAsExpression,
 };
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self as ty, Ty, TypeVisitable};
+use rustc_middle::ty::{self as ty, IsSuggestable, Ty, TypeVisitable};
 use rustc_span::{sym, BytePos, Span};
 
 use crate::errors::SuggAddLetForLetChains;
@@ -351,6 +351,82 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
+    pub(super) fn suggest_function_pointers(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        span: Span,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut Diagnostic,
+    ) {
+        debug!("suggest_function_pointers(cause={:?}, exp_found={:?})", cause, exp_found);
+        let ty::error::ExpectedFound { expected, found } = exp_found;
+        let expected_inner = expected.peel_refs();
+        let found_inner = found.peel_refs();
+        if !expected_inner.is_fn() || !found_inner.is_fn() {
+            return;
+        }
+        match (&expected_inner.kind(), &found_inner.kind()) {
+            (ty::FnPtr(sig), ty::FnDef(did, substs)) => {
+                let expected_sig = &(self.normalize_fn_sig)(*sig);
+                let found_sig =
+                    &(self.normalize_fn_sig)(self.tcx.bound_fn_sig(*did).subst(self.tcx, substs));
+
+                let fn_name = self.tcx.def_path_str_with_substs(*did, substs);
+
+                if !self.same_type_modulo_infer(*found_sig, *expected_sig)
+                    || !sig.is_suggestable(self.tcx, true)
+                    || ty::util::is_intrinsic(self.tcx, *did)
+                {
+                    return;
+                }
+
+                let (msg, sugg) = match (expected.is_ref(), found.is_ref()) {
+                    (true, false) => {
+                        let msg = "consider using a reference";
+                        let sug = format!("&{fn_name}");
+                        (msg, sug)
+                    }
+                    (false, true) => {
+                        let msg = "consider removing the reference";
+                        let sug = format!("{fn_name}");
+                        (msg, sug)
+                    }
+                    (true, true) => {
+                        diag.note("fn items are distinct from fn pointers");
+                        let msg = "consider casting to a fn pointer";
+                        let sug = format!("&({fn_name} as {sig})");
+                        (msg, sug)
+                    }
+                    (false, false) => {
+                        diag.note("fn items are distinct from fn pointers");
+                        let msg = "consider casting to a fn pointer";
+                        let sug = format!("{fn_name} as {sig}");
+                        (msg, sug)
+                    }
+                };
+                diag.span_suggestion(span, msg, &sugg, Applicability::MaybeIncorrect);
+            }
+            (ty::FnDef(did1, substs1), ty::FnDef(did2, substs2)) => {
+                let expected_sig =
+                    &(self.normalize_fn_sig)(self.tcx.bound_fn_sig(*did1).subst(self.tcx, substs1));
+                let found_sig =
+                    &(self.normalize_fn_sig)(self.tcx.bound_fn_sig(*did2).subst(self.tcx, substs2));
+
+                if self.same_type_modulo_infer(*found_sig, *expected_sig) {
+                    diag.note(
+                    "different fn items have unique types, even if their signatures are the same",
+                    );
+                }
+            }
+            (ty::FnDef(_, _), ty::FnPtr(_)) => {
+                diag.note("fn items are distinct from fn pointers");
+            }
+            _ => {
+                return;
+            }
+        };
+    }
+
     pub fn should_suggest_as_ref(&self, expected: Ty<'tcx>, found: Ty<'tcx>) -> Option<&str> {
         if let (ty::Adt(exp_def, exp_substs), ty::Ref(_, found_ty, _)) =
             (expected.kind(), found.kind())