feat(tvix/eval): add initial barebones compiler

This compiler can only take care of very trivial literals so far.

Change-Id: I9dfac75a801b7235f868061a979ae24159fe1425
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6070
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/compiler.rs

@@ -0,0 +1,65 @@
+//! This module implements a compiler for compiling the rnix AST
+//! representation to Tvix bytecode.
+
+use crate::chunk::Chunk;
+use crate::errors::EvalResult;
+use crate::opcode::OpCode;
+use crate::value::Value;
+use rnix;
+
+struct Compiler {
+    chunk: Chunk,
+}
+
+impl Compiler {
+    fn compile(&mut self, node: &rnix::SyntaxNode) -> EvalResult<()> {
+        match node.kind() {
+            // Root of a file contains no content, it's just a marker
+            // type.
+            rnix::SyntaxKind::NODE_ROOT => self.compile(&node.first_child().expect("TODO")),
+
+            // Literals contain a single token comprising of the
+            // literal itself.
+            rnix::SyntaxKind::NODE_LITERAL => {
+                let token = node.first_token().expect("TODO");
+                self.compile_literal(token)
+            }
+
+            kind => {
+                println!("visiting unsupported node: {:?}", kind);
+                Ok(())
+            }
+        }
+    }
+
+    fn compile_literal(&mut self, token: rnix::SyntaxToken) -> EvalResult<()> {
+        let value = rnix::value::Value::from_token(token.kind(), token.text()).expect("TODO");
+
+        match value {
+            rnix::NixValue::Float(f) => {
+                let idx = self.chunk.add_constant(Value::Float(f));
+                self.chunk.add_op(OpCode::OpConstant(idx));
+                Ok(())
+            }
+
+            rnix::NixValue::Integer(i) => {
+                let idx = self.chunk.add_constant(Value::Integer(i));
+                self.chunk.add_op(OpCode::OpConstant(idx));
+                Ok(())
+            }
+
+            rnix::NixValue::String(_) => todo!(),
+            rnix::NixValue::Path(_, _) => todo!(),
+        }
+    }
+}
+
+pub fn compile(ast: rnix::AST) -> EvalResult<Chunk> {
+    let mut c = Compiler {
+        chunk: Chunk::default(),
+    };
+
+    c.compile(&ast.node())?;
+
+    Ok(c.chunk)
+}

tvix/eval/src/main.rs

@@ -5,6 +5,7 @@ use std::{
 };
 
 mod chunk;
+mod compiler;
 mod errors;
 mod eval;
 mod opcode;