feat(tvix/repl): Allow binding variables at the top-level r/8346

Allow binding variables at the REPL's toplevel in the same way the Nix
REPL does, using the syntax <ident> = <expr>. This fully, strictly
evaluates the value and sets it in the repl's "env", which gets passed
in at the toplevel when evaluating expressions.

The laziness behavior differs from Nix's, but I think this is good:

    ❯ nix repl
    Welcome to Nix version 2.3.18. Type :? for help.

    nix-repl> x = builtins.trace "x" 1

    nix-repl> x
    trace: x
    1

    nix-repl> x
    1

vs tvix:

    tvix-repl> x = builtins.trace "x" 1
    trace: "x" :: string
    tvix-repl> x
    => 1 :: int
    tvix-repl> x
    => 1 :: int

Bug: https://b.tvl.fyi/issues/371
Change-Id: Ieb2d626b7195fa87be638c9a4dae2eee45eb9ab1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11954
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: aspen <root@gws.fyi>

1 files changed, 2 insertions, 0 deletions

diff --git a/tvix/Cargo.lock b/tvix/Cargo.lock
index 731731e72a1b..bbdeaeefb262 100644
--- a/tvix/Cargo.lock
+++ b/tvix/Cargo.lock
@@ -4367,7 +4367,9 @@ dependencies = [
  "dirs",
  "nix-compat",
  "rnix",
+ "rowan",
  "rustyline",
+ "smol_str",
  "thiserror",
  "tikv-jemallocator",
  "tokio",