instantly_dangling_pointer: add some FIXMEs

Author: GrigorenkoPV

compiler/rustc_lint/src/dangling.rs

@@ -112,7 +112,7 @@ fn is_temporary_rvalue(expr: &Expr<'_>) -> bool {
         ExprKind::Struct(_, _, _) => true,
         // These are 'static
         ExprKind::Lit(_) => false,
-        // False negatives are possible, but arrays get promoted to 'static way too often.
+        // FIXME: False negatives are possible, but arrays get promoted to 'static way too often.
         ExprKind::Array(_) => false,
 
         // These typecheck to `!`

compiler/rustc_lint/src/lints.rs

@@ -1129,6 +1129,7 @@ pub struct IgnoredUnlessCrateSpecified<'a> {
 #[diag(lint_instantly_dangling)]
 #[note]
 #[help]
+// FIXME: use #[primary_span]
 pub struct InstantlyDangling {
     pub callee: Symbol,
     pub ty: String,