feat(tvix/eval): add builtins.stringLength

Fairly straightforward, only thing of note is that we coerce (weakly) to
string here as well.

Change-Id: I03b427e657e402f1f9eb0f795b689bbf5092aba1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6745
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/builtins/mod.rs

@@ -234,6 +234,11 @@ fn pure_builtins() -> Vec<Builtin> {
                 .collect::<Vec<Value>>();
             Ok(Value::List(NixList::construct(parts.len(), parts)))
         }),
+        Builtin::new("stringLength", &[false], |args, vm| {
+            // also forces the value
+            let s = args[0].coerce_to_string(CoercionKind::Weak, vm)?;
+            Ok(Value::Integer(s.as_str().len() as i64))
+        }),
         Builtin::new(
             "sub",
             &[false, false],

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.exp

@@ -0,0 +1 @@
+[ 3 "hello" 9 4 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.nix

@@ -0,0 +1,10 @@
+[
+  (builtins.stringLength "foo")
+  (let s = "hello"; in (builtins.substring 0 (builtins.stringLength s) s))
+  (builtins.stringLength ("foo" + "${"bar" + "baz"}"))
+
+  # feel free to delete this test case at any time, it's just to show: This is a
+  # thing at the moment. We may want to break compatibility with this aspect of
+  # the C++ Nix implementation at any time.
+  (builtins.stringLength "😀")
+]