feat(tvix/eval): Initial resolution of <...> paths

This commit implements (lazy) resolution of `<...>` paths via either the
NIX_PATH environment variable, or the -I command-line flag - both
handled via EvalOptions. As a result, EvalOptions can no longer derive
Copy, meaning we have to clone it at each line of the repl - this is
probably not a huge deal as repl performance is not exactly an inner
loop and we're not cloning very much.

Internally, this works by creating a thunk which pushes a constant
containing the string inside the brackets to the stack, then a new
opcode to resolve that path via the `NixPath`. To get that opcode to
work, we now have to pass in the NixPath when constructing the VM.

This (intentionally) leaves out proper implementation of path resolution
via `findFile` (cppnix just calls whatever identifier called findFile is
in scope!!!) as that's widely considered a bit of a misfeature, but if
we do decide to implement that down the road it likely wouldn't be more
than a few extra ops within the thunk introduced here.

Change-Id: Ibc979b7e425b65cbe88599940520239a4a10cee2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6918
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/mod.rs

@@ -188,7 +188,7 @@ impl Compiler<'_> {
     fn compile(&mut self, slot: LocalIdx, expr: &ast::Expr) {
         match expr {
             ast::Expr::Literal(literal) => self.compile_literal(literal),
-            ast::Expr::Path(path) => self.compile_path(path),
+            ast::Expr::Path(path) => self.compile_path(slot, path),
             ast::Expr::Str(s) => self.compile_str(slot, s),
 
             ast::Expr::UnaryOp(op) => self.compile_unary_op(slot, op),
@@ -249,7 +249,7 @@ impl Compiler<'_> {
         self.emit_constant(value, node);
     }
 
-    fn compile_path(&mut self, node: &ast::Path) {
+    fn compile_path(&mut self, slot: LocalIdx, node: &ast::Path) {
         // TODO(tazjin): placeholder implementation while waiting for
         // https://github.com/nix-community/rnix-parser/pull/96
 
@@ -274,13 +274,24 @@ impl Compiler<'_> {
             let mut buf = self.root_dir.clone();
             buf.push(&raw_path);
             buf
-        } else {
+        } else if raw_path.starts_with('<') {
             // TODO: decide what to do with findFile
+            if raw_path.len() == 2 {
+                return self.emit_error(
+                    node,
+                    ErrorKind::PathResolution("Empty <> path not allowed".into()),
+                );
+            }
+            let path = &raw_path[1..(raw_path.len() - 1)];
+            // Make a thunk to resolve the path (without using `findFile`, at least for now?)
+            return self.thunk(slot, node, move |c, _| {
+                c.emit_constant(path.into(), node);
+                c.push_op(OpCode::OpFindFile, node);
+            });
+        } else {
             self.emit_error(
                 node,
-                ErrorKind::NotImplemented(
-                    "other path types (e.g. <...> lookups) not yet implemented",
-                ),
+                ErrorKind::NotImplemented("other path types not yet implemented"),
             );
             return;
         };