Suggest assoc fn new when trying to build tuple struct with private fields

Fix #22488.

Author: estebank

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1570,7 +1570,23 @@ 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 without self receiver and Self
+                            // return type, instead of relying only on the name.
+                            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

tests/ui/privacy/suggest-box-new.fixed

@@ -0,0 +1,16 @@
+// 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: ()
+    };
+}
+

tests/ui/privacy/suggest-box-new.rs

@@ -0,0 +1,16 @@
+// 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: ()
+    };
+}
+

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`.