privacy: port "field is private" diag

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_privacy/src/errors.rs

@@ -0,0 +1,29 @@
+use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
+use rustc_span::{Span, Symbol};
+
+#[derive(SessionDiagnostic)]
+#[error(privacy::field_is_private, code = "E0451")]
+pub struct FieldIsPrivate {
+    #[primary_span]
+    pub span: Span,
+    pub field_name: Symbol,
+    pub variant_descr: &'static str,
+    pub def_path_str: String,
+    #[subdiagnostic]
+    pub label: FieldIsPrivateLabel,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum FieldIsPrivateLabel {
+    #[label(privacy::field_is_private_is_update_syntax_label)]
+    IsUpdateSyntax {
+        #[primary_span]
+        span: Span,
+        field_name: Symbol,
+    },
+    #[label(privacy::field_is_private_label)]
+    Other {
+        #[primary_span]
+        span: Span,
+    },
+}