Rollup merge of #155311 - cyrgani:expand-clean, r=Kivooeo,petrochenkov

various small `rustc_expand` cleanups

Each commit should be reviewable on its own.

Author: JonathanBrouwer

crates/proc-macro-srv/src/dylib/proc_macros.rs

@@ -7,7 +7,7 @@ pub(crate) struct ProcMacros([bridge::client::ProcMacro]);
 
 impl From<bridge::PanicMessage> for crate::PanicMessage {
     fn from(p: bridge::PanicMessage) -> Self {
-        Self { message: p.as_str().map(|s| s.to_owned()) }
+        Self { message: p.into_string() }
     }
 }