Add hard error and migration lint for unsafe attrs

compiler/rustc_lint/src/context/diagnostics.rs

@@ -319,6 +319,16 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
         BuiltinLintDiag::UnusedQualifications { removal_span } => {
             lints::UnusedQualifications { removal_span }.decorate_lint(diag);
         }
+        BuiltinLintDiag::UnsafeAttrOutsideUnsafe {
+            attribute_name_span,
+            sugg_spans: (left, right),
+        } => {
+            lints::UnsafeAttrOutsideUnsafe {
+                span: attribute_name_span,
+                suggestion: lints::UnsafeAttrOutsideUnsafeSuggestion { left, right },
+            }
+            .decorate_lint(diag);
+        }
         BuiltinLintDiag::AssociatedConstElidedLifetime {
             elided,
             span: lt_span,

compiler/rustc_lint/src/lints.rs

@@ -2890,3 +2890,24 @@ pub struct RedundantImportVisibility {
     pub import_vis: String,
     pub max_vis: String,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_unsafe_attr_outside_unsafe)]
+pub struct UnsafeAttrOutsideUnsafe {
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub suggestion: UnsafeAttrOutsideUnsafeSuggestion,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    lint_unsafe_attr_outside_unsafe_suggestion,
+    applicability = "machine-applicable"
+)]
+pub struct UnsafeAttrOutsideUnsafeSuggestion {
+    #[suggestion_part(code = "unsafe(")]
+    pub left: Span,
+    #[suggestion_part(code = ")")]
+    pub right: Span,
+}