lint: add diagnostic translation migration lints

Introduce allow-by-default lints for checking whether diagnostics are
written in `SessionDiagnostic`/`AddSubdiagnostic` impls and whether
diagnostics are translatable. These lints can be denied for modules once
they are fully migrated to impls and translation.

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

compiler/rustc_session/src/lib.rs

@@ -5,6 +5,7 @@
 #![feature(never_type)]
 #![feature(once_cell)]
 #![feature(option_get_or_insert_default)]
+#![feature(rustc_attrs)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 

compiler/rustc_session/src/parse.rs

@@ -311,6 +311,7 @@ impl ParseSess {
         self.create_warning(warning).emit()
     }
 
+    #[cfg_attr(not(bootstrap), rustc_lint_diagnostics)]
     pub fn struct_err(
         &self,
         msg: impl Into<DiagnosticMessage>,
@@ -318,6 +319,7 @@ impl ParseSess {
         self.span_diagnostic.struct_err(msg)
     }
 
+    #[cfg_attr(not(bootstrap), rustc_lint_diagnostics)]
     pub fn struct_warn(&self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'_, ()> {
         self.span_diagnostic.struct_warn(msg)
     }

compiler/rustc_session/src/session.rs

@@ -209,6 +209,7 @@ pub struct PerfStats {
 
 /// Trait implemented by error types. This should not be implemented manually. Instead, use
 /// `#[derive(SessionDiagnostic)]` -- see [rustc_macros::SessionDiagnostic].
+#[rustc_diagnostic_item = "SessionDiagnostic"]
 pub trait SessionDiagnostic<'a, T: EmissionGuarantee = ErrorGuaranteed> {
     /// Write out as a diagnostic out of `sess`.
     #[must_use]