feat(tazjin/rlox): Set up precedence parsing scaffolding

Defines a new precedence levels enum which can be used to restrict the
parser precedence in any given location. As an example, unary
expressions and grouping are implemented, as these have a different
precedence from e.g. expression()

Change-Id: I91f299fc77530f76c3aba717f638985428104ee5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2558
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/bytecode/compiler.rs

@@ -1,101 +0,0 @@
-use super::chunk::Chunk;
-use super::errors::{Error, ErrorKind, LoxResult};
-use super::opcode::OpCode;
-use crate::scanner;
-
-struct Compiler<T: Iterator<Item = scanner::Token>> {
-    tokens: T,
-    chunk: Chunk,
-    panic: bool,
-    errors: Vec<Error>,
-
-    // TODO(tazjin): Restructure so that these don't need to be Option?
-    current: Option<scanner::Token>,
-    previous: Option<scanner::Token>,
-}
-
-impl<T: Iterator<Item = scanner::Token>> Compiler<T> {
-    fn compile(&mut self) -> LoxResult<()> {
-        self.advance();
-        self.expression();
-        self.consume(
-            &scanner::TokenKind::Eof,
-            ErrorKind::ExpectedToken("Expected end of expression"),
-        )?;
-
-        self.end_compiler()
-    }
-
-    fn advance(&mut self) {
-        self.previous = self.current.take();
-        self.current = self.tokens.next();
-    }
-
-    fn expression(&mut self) {
-        unimplemented!()
-    }
-
-    fn consume(
-        &mut self,
-        expected: &scanner::TokenKind,
-        err: ErrorKind,
-    ) -> LoxResult<()> {
-        unimplemented!()
-    }
-
-    fn current_chunk(&mut self) -> &mut Chunk {
-        &mut self.chunk
-    }
-
-    fn end_compiler(&mut self) -> LoxResult<()> {
-        self.emit_op(OpCode::OpReturn);
-        Ok(())
-    }
-
-    fn emit_op(&mut self, op: OpCode) {
-        let line = self.previous().line;
-        self.current_chunk().add_op(op, line);
-    }
-
-    fn previous(&self) -> &scanner::Token {
-        self.previous
-            .as_ref()
-            .expect("invalid internal compiler state: missing previous token")
-    }
-
-    fn error_at(&mut self, token: &scanner::Token, kind: ErrorKind) {
-        if self.panic {
-            return;
-        }
-
-        self.panic = true;
-        self.errors.push(Error {
-            kind,
-            line: token.line,
-        })
-    }
-}
-
-pub fn compile(code: &str) -> Result<Chunk, Vec<Error>> {
-    let chars = code.chars().collect::<Vec<char>>();
-    let tokens = scanner::scan(&chars).map_err(|errors| {
-        errors.into_iter().map(Into::into).collect::<Vec<Error>>()
-    })?;
-
-    let mut compiler = Compiler {
-        tokens: tokens.into_iter().peekable(),
-        chunk: Default::default(),
-        panic: false,
-        errors: vec![],
-        current: None,
-        previous: None,
-    };
-
-    compiler.compile()?;
-
-    if compiler.errors.is_empty() {
-        Ok(unimplemented!())
-    } else {
-        Err(compiler.errors)
-    }
-}