add dropck_outlives comments

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -161,8 +161,12 @@ impl<'me, 'typeck, 'flow, 'tcx> LivenessResults<'me, 'typeck, 'flow, 'tcx> {
         }
     }
 
-    // Runs dropck for locals whose liveness isn't relevant. This is
-    // necessary to eagerly detect unbound recursion during drop glue computation.
+    /// Runs dropck for locals whose liveness isn't relevant. This is
+    /// necessary to eagerly detect unbound recursion during drop glue computation.
+    ///
+    /// These are all the locals which do not potentially reference a region local
+    /// to this body. Locals which only reference free regions are always drop-live
+    /// and can therefore safely be dropped.
     fn dropck_boring_locals(&mut self, boring_locals: Vec<Local>) {
         for local in boring_locals {
             let local_ty = self.cx.body.local_decls[local].ty;

compiler/rustc_trait_selection/src/traits/query/type_op/outlives.rs

@@ -31,16 +31,6 @@ impl<'tcx> super::QueryTypeOp<'tcx> for DropckOutlives<'tcx> {
         tcx: TyCtxt<'tcx>,
         canonicalized: Canonical<'tcx, ParamEnvAnd<'tcx, Self>>,
     ) -> Result<CanonicalQueryResponse<'tcx, Self::QueryResponse>, NoSolution> {
-        // Subtle: note that we are not invoking
-        // `infcx.at(...).dropck_outlives(...)` here, but rather the
-        // underlying `dropck_outlives` query. This same underlying
-        // query is also used by the
-        // `infcx.at(...).dropck_outlives(...)` fn. Avoiding the
-        // wrapper means we don't need an infcx in this code, which is
-        // good because the interface doesn't give us one (so that we
-        // know we are not registering any subregion relations or
-        // other things).
-
         // FIXME convert to the type expected by the `dropck_outlives`
         // query. This should eventually be fixed by changing the
         // *underlying query*.