feat(tazjin/rlox): Implement early return from functions

In the book this is implemented via exceptions as control flow, and
I'm sticking somewhat closely to that by doing it via an error
variant.

Change-Id: I9c7b84d6bb28265ab94021ea681df84f16a53da2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2395
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/parser.rs

@@ -79,6 +79,11 @@ pub struct Var {
     pub initialiser: Option<Expr>,
 }
 
+#[derive(Debug)]
+pub struct Return {
+    pub value: Expr,
+}
+
 #[derive(Debug)]
 pub struct If {
     pub condition: Expr,
@@ -110,6 +115,7 @@ pub enum Statement {
     If(If),
     While(While),
     Function(Rc<Function>),
+    Return(Return),
 }
 
 // Parser
@@ -130,6 +136,7 @@ statement      → exprStmt
                | forStmt
                | ifStmt
                | printStmt
+               | returnStmt
                | whileStmt
                | block ;
 
@@ -137,6 +144,8 @@ forStmt        → "for" "(" ( varDecl | exprStmt | ";" )
                  expression? ";"
                  expression? ")" statement ;
 
+returnStmt     → "return" expression? ";" ;
+
 whileStmt      → "while" "(" expression ")" statement ;
 
 exprStmt       → expression ";" ;
@@ -256,6 +265,8 @@ impl Parser {
             self.while_statement()
         } else if self.match_token(&TokenKind::For) {
             self.for_statement()
+        } else if self.match_token(&TokenKind::Return) {
+            self.return_statement()
         } else {
             self.expr_statement()
         }
@@ -379,6 +390,12 @@ impl Parser {
         Ok(body)
     }
 
+    fn return_statement(&mut self) -> StmtResult {
+        let value = self.expression()?;
+        self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;
+        Ok(Statement::Return(Return { value }))
+    }
+
     fn expr_statement(&mut self) -> StmtResult {
         let expr = self.expression()?;
         self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;