feat(tvix/eval): introduce mechanism for defining builtins

Adds a new builtins module in which builtins can be constructed. The
functions in this module should return a correctly structured value to
be passed to the compiler's `globals`.

This is wired up all the way to the compiler with an example
`toString` builtin, available as a global. Note that this does not yet
actually behave like the real toString, which has some differences
from `Display`.

Change-Id: Ibb5f6fbe6207782fdf2434435567fc1bd80039a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6254
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -0,0 +1,26 @@
+//! This module implements the builtins exposed in the Nix language.
+//!
+//! See //tvix/eval/docs/builtins.md for a some context on the
+//! available builtins in Nix.
+
+use std::collections::HashMap;
+
+use crate::value::{Builtin, Value};
+
+macro_rules! builtin {
+    ( $map:ident, $name:literal, $arity:literal, $body:expr ) => {
+        $map.insert($name, Value::Builtin(Builtin::new($name, $arity, $body)));
+    };
+}
+
+/// Set of Nix builtins that are globally available.
+pub fn global_builtins() -> HashMap<&'static str, Value> {
+    let mut globals = HashMap::new();
+
+    builtin!(globals, "toString", 1, |args| {
+        // TODO: toString is actually not the same as Display
+        Ok(Value::String(format!("{}", args[0]).into()))
+    });
+
+    globals
+}