Handle fieldless tuple structs in diagnostic code

Fixes #75062

Author: Aaron1011

src/librustc_resolve/diagnostics.rs

@@ -1075,10 +1075,9 @@ impl<'a> Resolver<'a> {
         ) = binding.kind
         {
             let def_id = (&*self).parent(ctor_def_id).expect("no parent for a constructor");
-            if let Some(fields) = self.field_names.get(&def_id) {
-                let first_field = fields.first().expect("empty field list in the map");
-                return Some(fields.iter().fold(first_field.span, |acc, field| acc.to(field.span)));
-            }
+            let fields = self.field_names.get(&def_id)?;
+            let first_field = fields.first()?; // Handle `struct Foo()`
+            return Some(fields.iter().fold(first_field.span, |acc, field| acc.to(field.span)));
         }
         None
     }

src/test/ui/privacy/issue-75062-fieldless-tuple-struct.rs

@@ -0,0 +1,10 @@
+// Regression test for issue #75062
+// Tests that we don't ICE on a privacy error for a fieldless tuple struct.
+
+mod foo {
+    struct Bar();
+}
+
+fn main() {
+    foo::Bar(); //~ ERROR tuple struct
+}

src/test/ui/privacy/issue-75062-fieldless-tuple-struct.stderr

@@ -0,0 +1,15 @@
+error[E0603]: tuple struct `Bar` is private
+  --> $DIR/issue-75062-fieldless-tuple-struct.rs:9:10
+   |
+LL |     foo::Bar();
+   |          ^^^ private tuple struct
+   |
+note: the tuple struct `Bar` is defined here
+  --> $DIR/issue-75062-fieldless-tuple-struct.rs:5:5
+   |
+LL |     struct Bar();
+   |     ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0603`.