Auto merge of #118125 - nnethercote:custom_encodable, r=compiler-errors

Make some `newtype_index!` derived impls opt-in instead of opt-out

Opt-in is the standard Rust way of doing things, and avoids some unnecessary dependencies on the `rustc_serialize` crate.

r? `@lcnr`

compiler/rustc_lint/src/levels.rs

@@ -56,7 +56,6 @@ struct LintLevelSets {
 }
 
 rustc_index::newtype_index! {
-    #[custom_encodable] // we don't need encoding
     struct LintStackIndex {
         const COMMAND_LINE = 0;
     }