Add two comments relating to new-solver performance

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -21,7 +21,11 @@ use crate::traits::{FulfillmentError, ScrubbedTraitError};
 
 mod derive_errors;
 
-// FIXME: Do we need to use a `ThinVec` here?
+// `ThinVec` is important for performance, but not for the usual memory layout reasons.
+// `try_evaluate_obligations` is extremely hot and uses `retain_mut`. `ThinVec::retain_mut` is
+// simple and sub-optimal in terms of how it moves elements, but it can be inlined.
+// `Vec::retain_mut` is more sophisticated and minimizes element moves, but also contains more code
+// and doesn't get inlined in `try_evaluate_obligations`, giving worse performance overall.
 type PendingObligations<'tcx> =
     ThinVec<(PredicateObligation<'tcx>, Option<GoalStalledOn<TyCtxt<'tcx>>>)>;
 
@@ -453,7 +457,9 @@ mod size_asserts {
     use super::*;
     // tidy-alphabetical-start
     // Before #160005 this pair was greater than 128 bytes, which triggered the use of (slow)
-    // `memcpy` for moving elements of `PendingObligations`.
+    // `memcpy` for moving elements of `PendingObligations`. Then #160479 greatly reduced the
+    // number of `memcpy` operations in `try_evaluate_obligations`. So the size of this pair is
+    // much less important than it was, but still shouldn't be changed without some thought.
     static_assert_size!((PredicateObligation<'_>, Option<GoalStalledOn<TyCtxt<'_>>>), 104);
     // tidy-alphabetical-end
 }