Add emit_span_lint_lazy to lazily create LintDiagnostic structs

compiler/rustc_lint/src/context.rs

@@ -524,6 +524,20 @@ pub trait LintContext {
         });
     }
 
+    /// Emit a lint at `span` from a lazily-constructed lint struct (some type that implements
+    /// `LintDiagnostic`, typically generated by `#[derive(LintDiagnostic)]`).
+    fn emit_span_lint_lazy<S: Into<MultiSpan>, L: for<'a> LintDiagnostic<'a, ()>>(
+        &self,
+        lint: &'static Lint,
+        span: S,
+        decorator: impl FnOnce() -> L,
+    ) {
+        self.opt_span_lint(lint, Some(span), |lint| {
+            let decorator = decorator();
+            decorator.decorate_lint(lint);
+        });
+    }
+
     /// Emit a lint at the appropriate level, with an associated span.
     ///
     /// [`lint_level`]: rustc_middle::lint::lint_level#decorate-signature