update use of feature flags

compiler/rustc_middle/src/traits/solve/cache.rs

@@ -5,7 +5,7 @@ use rustc_data_structures::sync::Lock;
 use rustc_query_system::cache::WithDepNode;
 use rustc_query_system::dep_graph::DepNodeIndex;
 use rustc_session::Limit;
-/// The trait solver cache used by `-Ztrait-solver=next`.
+/// The trait solver cache used by `-Znext-solver`.
 ///
 /// FIXME(@lcnr): link to some official documentation of how
 /// this works.

compiler/rustc_middle/src/ty/fast_reject.rs

@@ -62,7 +62,7 @@ pub enum TreatParams {
     ///
     /// N.B. during deep rejection, this acts identically to `ForLookup`.
     ///
-    /// FIXME(-Ztrait-solver=next): Remove this variant and cleanup
+    /// FIXME(-Znext-solver): Remove this variant and cleanup
     /// the code.
     NextSolverLookup,
 }