Add sysroot-abi feature, copy 1.64 ABI fo rnow

crates/proc-macro-srv/src/lib.rs

@@ -11,6 +11,10 @@
 //!   rustc rather than `unstable`. (Although in general ABI compatibility is still an issue)…
 
 #![warn(rust_2018_idioms, unused_lifetimes, semicolon_in_expressions_from_macros)]
+#![cfg_attr(
+    feature = "sysroot-abi",
+    feature(proc_macro_internals, proc_macro_diagnostic, proc_macro_span)
+)]
 #![allow(unreachable_pub)]
 
 mod dylib;