put back the workarounds for rust-lang#60846

based on rust-lang#61754 (comment) I am adding `bootstrap` to the cfg-preconditions for the two manual `unsafe impls`'s of `Send` and `Sync` for `TokenTree`.

src/libsyntax/tokenstream.rs

@@ -59,6 +59,17 @@ where
     TokenStream: Send + Sync,
 {}
 
+// These are safe since we ensure that they hold for all fields in the `_dummy` function.
+//
+// These impls are only here because the compiler takes forever to compute the Send and Sync
+// bounds without them.
+// FIXME: Remove these impls when the compiler can compute the bounds quickly again.
+// See https://github.com/rust-lang/rust/issues/60846
+#[cfg(all(bootstrap, parallel_compiler))]
+unsafe impl Send for TokenTree {}
+#[cfg(all(bootstrap, parallel_compiler))]
+unsafe impl Sync for TokenTree {}
+
 impl TokenTree {
     /// Use this token tree as a matcher to parse given tts.
     pub fn parse(cx: &base::ExtCtxt<'_>, mtch: &[quoted::TokenTree], tts: TokenStream)