Add a warning to LateContext::get_def_path

Author: blyxyas

compiler/rustc_lint/src/context.rs

@@ -709,6 +709,12 @@ impl<'tcx> LateContext<'tcx> {
     }
 
     /// Gets the absolute path of `def_id` as a vector of `Symbol`.
+    /// Note that this is kinda expensive because it has to
+    /// travel the tree and pretty-print. Use sparingly (or find a
+    /// way to only get the segments you're interested in, e.g. via
+    /// [`TyCtxt::crate_name`]).
+    ///
+    /// FIXME: It would be great if this could be optimized.
     ///
     /// # Examples
     ///