Rollup merge of rust-lang#82773 - mgacek8:feature/add_diagnostic_item_to_Default_trait, r=oli-obk

Add diagnostic item to `Default` trait

This PR adds diagnostic item to `Default` trait to be used by rust-lang/rust-clippy#6562 issue.
Also fixes the obsolete path to the `symbols.rs` file in the comment.

Author: m-ou-se

core/src/default.rs

@@ -80,6 +80,7 @@
 ///     bar: f32,
 /// }
 /// ```
+#[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.