feat(tvix/eval): Implement builtins.sort

This is a bit tricky because the comparator can throw errors, so we
need to propagate them out if they exist and try to avoid sorting
forever by returning a reasonable ordering in this case (as
short-circuiting is not available).

Co-Authored-By: Vincent Ambo <tazjin@tvl.su>
Change-Id: Icae1d30f43ec1ae64b2ba51e73ee467605686792
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7072
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -628,10 +628,47 @@ fn pure_builtins() -> Vec<Builtin> {
             },
         ),
         Builtin::new("seq", &[true, true], |mut args: Vec<Value>, _: &mut VM| {
-            // The builtin calling infra has already forced both args for us, so we just return the
+            // The builtin calling infra has already forced both args for us, so
-            // second and ignore the first
+            // we just return the second and ignore the first
             Ok(args.pop().unwrap())
         }),
+        Builtin::new("sort", &[true, true], |args: Vec<Value>, vm: &mut VM| {
+            let mut list = args[1].to_list()?;
+            let comparator = &args[0];
+
+            // Used to let errors "escape" from the sorting closure. If anything
+            // ends up setting an error, it is returned from this function.
+            let mut error: Option<ErrorKind> = None;
+
+            list.sort_by(|lhs, rhs| {
+                let result = vm
+                    .call_with(comparator, [lhs.clone(), rhs.clone()])
+                    .map_err(|err| ErrorKind::ThunkForce(Box::new(err)))
+                    .and_then(|v| v.force(vm)?.as_bool());
+
+                match (&error, result) {
+                    // The contained closure only returns a "less
+                    // than?"-boolean, no way to yield "equal".
+                    (None, Ok(true)) => Ordering::Less,
+                    (None, Ok(false)) => Ordering::Greater,
+
+                    // Closest thing to short-circuiting out if an error was
+                    // thrown.
+                    (Some(_), _) => Ordering::Equal,
+
+                    // Propagate the error if one was encountered.
+                    (_, Err(e)) => {
+                        error = Some(e);
+                        Ordering::Equal
+                    }
+                }
+            });
+
+            match error {
+                None => Ok(Value::List(list)),
+                Some(e) => Err(e),
+            }
+        }),
         Builtin::new("splitVersion", &[true], |args: Vec<Value>, _: &mut VM| {
             let s = args[0].to_str()?;
             let s = VersionPartsIter::new(s.as_str());

tvix/eval/src/value/list.rs

@@ -1,5 +1,5 @@
 //! This module implements Nix lists.
-use std::ops::Deref;
+use std::ops::{Deref, DerefMut};
 
 use crate::errors::ErrorKind;
 use crate::vm::VM;
@@ -135,3 +135,9 @@ impl Deref for NixList {
         &self.0
     }
 }
+
+impl DerefMut for NixList {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}