fix(tvix/eval): detect cycles when printing infinite values r/5178

Using the same method as in Thunk::deep_force, detect cycles when
printing values by maintaining a set of already seen thunks.

With this, display of infinite values matches that of Nix:

    > nix-instantiate --eval --strict -E 'let as = { x = 123; y = as; }; in as'
    { x = 123; y = { x = 123; y = <CYCLE>; }; }

    > tvix-eval -E 'let as = { x = 123; y = as; }; in as'
    => { x = 123; y = { x = 123; y = <CYCLE>; }; } :: set

Change-Id: I007b918d5131d82c28884e46e46ff365ef691aa8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7056
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

1 files changed, 5 insertions, 5 deletions

diff --git a/tvix/eval/src/value/list.rs b/tvix/eval/src/value/list.rs
index 42d91b6b26..d13e220bbe 100644
--- a/tvix/eval/src/value/list.rs
+++ b/tvix/eval/src/value/list.rs
@@ -1,21 +1,21 @@
 //! This module implements Nix lists.
-use std::fmt::Display;
-
 use crate::errors::ErrorKind;
 use crate::vm::VM;
 
+use super::thunk::ThunkSet;
+use super::TotalDisplay;
 use super::Value;
 
 #[repr(transparent)]
 #[derive(Clone, Debug, PartialEq)]
 pub struct NixList(Vec<Value>);
 
-impl Display for NixList {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl TotalDisplay for NixList {
+    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
         f.write_str("[ ")?;
 
         for v in &self.0 {
-            v.fmt(f)?;
+            v.total_fmt(f, set)?;
             f.write_str(" ")?;
         }