Improve the diagnostic of fn item in variadic fn

Author: Orion Gonzalez

compiler/rustc_hir_typeck/messages.ftl

@@ -79,6 +79,11 @@ hir_typeck_field_multiply_specified_in_initializer =
     .label = used more than once
     .previous_use_label = first use of `{$ident}`
 
+hir_typeck_fn_item_to_variadic_function = can't pass a function item to a variadic function
+    .suggestion = use a function pointer instead
+    .help = a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+    .note = for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html
+
 hir_typeck_fru_expr = this expression does not end in a comma...
 hir_typeck_fru_expr2 = ... so this is interpreted as a `..` range expression, instead of functional record update syntax
 hir_typeck_fru_note = this expression may have been misinterpreted as a `..` range expression

compiler/rustc_hir_typeck/src/errors.rs

@@ -797,3 +797,13 @@ pub(crate) struct PassToVariadicFunction<'a, 'tcx> {
     #[note(hir_typeck_teach_help)]
     pub(crate) teach: bool,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_typeck_fn_item_to_variadic_function, code = E0617)]
+pub(crate) struct PassFnItemToVariadicFunction {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = "{replace}", applicability = "machine-applicable")]
+    pub sugg_span: Option<Span>,
+    pub replace: String,
+}

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -472,9 +472,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         variadic_error(tcx.sess, arg.span, arg_ty, "c_uint");
                     }
                     ty::FnDef(..) => {
-                        let ptr_ty = Ty::new_fn_ptr(self.tcx, arg_ty.fn_sig(self.tcx));
-                        let ptr_ty = self.resolve_vars_if_possible(ptr_ty);
-                        variadic_error(tcx.sess, arg.span, arg_ty, &ptr_ty.to_string());
+                        let sess = tcx.sess;
+                        let fn_item_span = arg.span;
+                        let fn_ptr = Ty::new_fn_ptr(self.tcx, arg_ty.fn_sig(self.tcx));
+                        let fn_ptr = self.resolve_vars_if_possible(fn_ptr).to_string();
+
+                        let (sugg_span, replace) =
+                            if let Ok(fn_item) = sess.source_map().span_to_snippet(fn_item_span) {
+                                (Some(fn_item_span), format!("{fn_item} as {fn_ptr}"))
+                            } else {
+                                (None, "".to_string())
+                            };
+
+                        sess.dcx().emit_err(errors::PassFnItemToVariadicFunction {
+                            span: fn_item_span,
+                            replace,
+                            sugg_span,
+                        });
                     }
                     _ => {}
                 }