Auto merge of #105880 - Nilstrieb:make-newtypes-less-not-rust, r=oli-obk

Improve syntax of `newtype_index`

This makes it more like proper Rust and also makes the implementation a lot simpler.

Mostly just turns weird flags in the body into proper attributes.

It should probably also be converted to an attribute macro instead of function-like, but that can be done in a future PR.

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -282,9 +282,9 @@ impl graph::WithPredecessors for CoverageGraph {
 
 rustc_index::newtype_index! {
     /// A node in the control-flow graph of CoverageGraph.
+    #[debug_format = "bcb{}"]
     pub(super) struct BasicCoverageBlock {
-        DEBUG_FORMAT = "bcb{}",
-        const START_BCB = 0,
+        const START_BCB = 0;
     }
 }