feat(tvix/eval): implement correct toString behavior r/4857

Implement C++ Nix's `EvalState::coerceToString` minus some of the Path
/ store handling. This is currently only used for `toString` which does
all possible coercions, but we've already prepared the weaker coercion
variant which is e.g. used for builtins that expect string arguments.

`EvalState::coerceToPath` is still missing for builtins that need a
path, but it'll be easy to build on top of this.

Change-Id: I78d15576b18921791d04b6b1e964b951fdef22c6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6571
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 4 insertions, 4 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 0fadb738d5..74215cd37c 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -10,7 +10,7 @@ use std::{
 
 use crate::{
     errors::ErrorKind,
-    value::{Builtin, NixAttrs, NixList, NixString, Value},
+    value::{Builtin, CoercionKind, NixAttrs, NixList, NixString, Value},
 };
 
 use crate::arithmetic_op;
@@ -121,9 +121,9 @@ fn pure_builtins() -> Vec<Builtin> {
             ));
         }),
         Builtin::new("toString", 1, |args, vm| {
-            force!(vm, &args[0], value, {
-                Ok(Value::String(format!("{}", value).into()))
-            })
+            args[0]
+                .coerce_to_string(CoercionKind::Strong, vm)
+                .map(|s| Value::String(s))
         }),
         Builtin::new("typeOf", 1, |args, vm| {
             force!(vm, &args[0], value, {