feat(3p/nix): Statically pass bindings capacity where possible r/1285

To aid in making the decision of where to (currently just statically)
use a vector or btree as the backing implementation, add an extra
constructor argument to Bindings::NewGC for a capacity, and use
a (currently hardcoded at 32, for no good reason other than it felt like
a reasonable number) pivot to switch between our possible backing
implementations. Then, update all the call sites where it feels
reasonable that we know the capacity statically to *pass* that capacity
to the constructor.

Paired-With: Luke Granger-Brown <git@lukegb.com>
Paired-With: Vincent Ambo <mail@tazj.in>
Paired-With: Perry Lorier <isomer@tvl.fyi>
Change-Id: I1858c161301a1cd0e83aeeb9a58839378869e71d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1124
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: isomer <isomer@tvl.fyi>

1 files changed, 2 insertions, 2 deletions

diff --git a/third_party/nix/src/libexpr/eval.cc b/third_party/nix/src/libexpr/eval.cc
index f733a86a89..56e4006204 100644
--- a/third_party/nix/src/libexpr/eval.cc
+++ b/third_party/nix/src/libexpr/eval.cc
@@ -1240,9 +1240,9 @@ void ExprOpUpdate::eval(EvalState& state, Env& env, Value& dest) {
 
   state.nrOpUpdates++;
 
-  state.mkAttrs(dest, /* capacity = */ 0);
+  state.mkAttrs(dest, v1.attrs->size() + v2.attrs->size());
 
-  /* Merge the sets, preferring values from the second set. */
+  // Merge the sets, preferring values from the second set.
   dest.attrs->merge(*v1.attrs);
   dest.attrs->merge(*v2.attrs);
 }