From 87dc85d3222743aeb82848537ad77c2ece746ef6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Tue, 17 Oct 2023 19:29:43 +0000
Subject: [PATCH] Suggest assoc fn `new` when trying to build tuple struct with
 private fields

Fix #22488.
---
 .../rustc_resolve/src/late/diagnostics.rs     | 21 ++++++++++++++++++-
 tests/ui/privacy/suggest-box-new.fixed        | 15 +++++++++++++
 tests/ui/privacy/suggest-box-new.rs           | 15 +++++++++++++
 tests/ui/privacy/suggest-box-new.stderr       | 20 ++++++++++++++++++
 4 files changed, 70 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/privacy/suggest-box-new.fixed
 create mode 100644 tests/ui/privacy/suggest-box-new.rs
 create mode 100644 tests/ui/privacy/suggest-box-new.stderr

diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index a5f8f61f3db50..fd5d6fabf021d 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -1570,7 +1570,26 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                         err.set_primary_message(
                             "cannot initialize a tuple struct which contains private fields",
                         );
-
+                        if !def_id.is_local()
+                            && self
+                                .r
+                                .tcx
+                                .inherent_impls(def_id)
+                                .iter()
+                                .flat_map(|impl_def_id| {
+                                    self.r.tcx.provided_trait_methods(*impl_def_id)
+                                })
+                                .any(|assoc| !assoc.fn_has_self_parameter && assoc.name == sym::new)
+                        {
+                            // FIXME: look for associated functions with Self return type,
+                            // instead of relying only on the name and lack of self receiver.
+                            err.span_suggestion_verbose(
+                                span.shrink_to_hi(),
+                                "you might have meant to use the `new` associated function",
+                                "::new".to_string(),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
                         // Use spans of the tuple struct definition.
                         self.r.field_def_ids(def_id).map(|field_ids| {
                             field_ids
diff --git a/tests/ui/privacy/suggest-box-new.fixed b/tests/ui/privacy/suggest-box-new.fixed
new file mode 100644
index 0000000000000..f5ae5c2abfd99
--- /dev/null
+++ b/tests/ui/privacy/suggest-box-new.fixed
@@ -0,0 +1,15 @@
+// run-rustfix
+#![allow(dead_code)]
+struct U <T> {
+    wtf: Option<Box<U<T>>>,
+    x: T,
+}
+fn main() {
+    U {
+        wtf: Some(Box::new(U { //~ ERROR cannot initialize a tuple struct which contains private fields
+            wtf: None,
+            x: (),
+        })),
+        x: ()
+    };
+}
diff --git a/tests/ui/privacy/suggest-box-new.rs b/tests/ui/privacy/suggest-box-new.rs
new file mode 100644
index 0000000000000..2e18dba8b9fb2
--- /dev/null
+++ b/tests/ui/privacy/suggest-box-new.rs
@@ -0,0 +1,15 @@
+// run-rustfix
+#![allow(dead_code)]
+struct U <T> {
+    wtf: Option<Box<U<T>>>,
+    x: T,
+}
+fn main() {
+    U {
+        wtf: Some(Box(U { //~ ERROR cannot initialize a tuple struct which contains private fields
+            wtf: None,
+            x: (),
+        })),
+        x: ()
+    };
+}
diff --git a/tests/ui/privacy/suggest-box-new.stderr b/tests/ui/privacy/suggest-box-new.stderr
new file mode 100644
index 0000000000000..ed7fa036408a4
--- /dev/null
+++ b/tests/ui/privacy/suggest-box-new.stderr
@@ -0,0 +1,20 @@
+error[E0423]: cannot initialize a tuple struct which contains private fields
+  --> $DIR/suggest-box-new.rs:9:19
+   |
+LL |         wtf: Some(Box(U {
+   |                   ^^^
+   |
+note: constructor is not visible here due to private fields
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
+   |
+   = note: private field
+   |
+   = note: private field
+help: you might have meant to use the `new` associated function
+   |
+LL |         wtf: Some(Box::new(U {
+   |                      +++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0423`.