feat(tvix/eval): NixList::concat(): avoid an unnecessary move r/5158

In `a++b`, the previous implementation would move `b` (i.e. memcpy
its elements) twice.  Let's do that only once.

We sure do call NixList.clone() a whole lot.  At some point in the
future we probably want to do a SmolStr-type split for NixList into
a two-variant enum where one side is an Rc<Vec<Value>> for lists
longer than a certain length.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: I32154d18785a1f663454a8b9d4afd3e78bffdf9c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7040
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 3 insertions, 4 deletions

diff --git a/tvix/eval/src/value/list.rs b/tvix/eval/src/value/list.rs
index 486b0c3fca..3be5d41457 100644
--- a/tvix/eval/src/value/list.rs
+++ b/tvix/eval/src/value/list.rs
@@ -50,10 +50,9 @@ mod arbitrary {
 
 impl NixList {
     pub fn concat(&self, other: &Self) -> Self {
-        let mut lhs = self.clone();
-        let mut rhs = other.clone();
-        lhs.0.append(&mut rhs.0);
-        lhs
+        let mut ret = self.clone();
+        ret.0.extend_from_slice(&other.0);
+        ret
     }
 
     pub fn len(&self) -> usize {