refactor(tvix/eval): remove use of imbl::Vector r/8520

This vector type has served us well for now, but it contains internal refcounts
which are incompatible with upcoming changes related to garbage collection.

The performance impact of this change within all benchmarks I ran was within the
margin of error:

[nix-shell:/tmp/perf]$ hyperfine "./before -E '(import <nixpkgs> {}).firefox.outPath' --log-level ERROR --no-warnings"
Benchmark 1: ./u64 -E '(import <nixpkgs> {}).firefox.outPath' --log-level ERROR --no-warnings
  Time (mean ± σ):      7.528 s ±  0.272 s    [User: 6.578 s, System: 0.631 s]
  Range (min … max):    7.160 s …  8.012 s    10 runs

nix-shell:/tmp/perf]$ hyperfine "./std-vec -E '(import <nixpkgs> {}).firefox.outPath' --log-level ERROR --no-warnings"
Benchmark 1: ./std-vec -E '(import <nixpkgs> {}).firefox.outPath' --log-level ERROR --no-warnings
  Time (mean ± σ):      7.515 s ±  0.178 s    [User: 6.508 s, System: 0.652 s]
  Range (min … max):    7.276 s …  7.861 s    10 runs

Change-Id: Ib95f871956e336a1e5771f6293583854b1efb276
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12197
Reviewed-by: aspen <root@gws.fyi>
Tested-by: BuildkiteCI

1 files changed, 3 insertions, 2 deletions

diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 7ac6d493fa1f..8d89020363e2 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -779,8 +779,9 @@ where
                 Op::Concat => lifted_pop! {
                     self(rhs, lhs) => {
                         let rhs = rhs.to_list().with_span(&frame, self)?.into_inner();
-                        let lhs = lhs.to_list().with_span(&frame, self)?.into_inner();
-                        self.stack.push(Value::List(NixList::from(lhs + rhs)))
+                        let mut lhs = lhs.to_list().with_span(&frame, self)?.into_inner();
+                        lhs.extend(rhs.into_iter());
+                        self.stack.push(Value::List(lhs.into()))
                     }
                 },