fixing decrementation and testing

Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "Boa"
 version = "0.1.5"

src/lib/syntax/lexer.rs

@@ -2,14 +2,14 @@
 //!
 //! The Lexer splits its input source code into a sequence of input elements called tokens, represented by the [Token](../ast/token/struct.Token.html) structure.
 //! It also removes whitespace and comments and attaches them to the next token.
+use crate::syntax::ast::punc::Punctuator;
+use crate::syntax::ast::token::{Token, TokenData};
 use std::char::from_u32;
 use std::error;
 use std::fmt;
 use std::iter::Peekable;
 use std::str::Chars;
 use std::str::FromStr;
-use crate::syntax::ast::punc::Punctuator;
-use crate::syntax::ast::token::{Token, TokenData};
 
 #[allow(unused)]
 macro_rules! vop {
@@ -390,8 +390,11 @@ impl<'a> Lexer<'a> {
                 '+' => op!(self, Punctuator::AssignAdd, Punctuator::Add, {
                     '+' => Punctuator::Inc
                 }),
-                '-' => op!(self, Punctuator::AssignSub, Punctuator::AssignSub, {
+                '-' => op!(self, Punctuator::AssignSub, Punctuator::Sub, {
-                    '-' => Punctuator::Dec
+                    '-' => {
+                        self.next()?;
+                        Punctuator::Dec
+                    }
                 }),
                 '%' => op!(self, Punctuator::AssignMod, Punctuator::Mod),
                 '|' => op!(self, Punctuator::AssignOr, Punctuator::Or, {

tests/lexer_test.rs

@@ -52,3 +52,19 @@ fn check_positions() {
     assert_eq!(lexer.tokens[6].pos.column_number, 27);
     assert_eq!(lexer.tokens[6].pos.line_number, 1);
 }
+
+// Increment/Decrement
+#[test]
+fn check_decrement_advances_lexer_2_places() {
+    // Here we want an example of decrementing an integer
+    let s = &String::from("let a = b--;");
+    let mut lexer = Lexer::new(s);
+    lexer.lex().expect("finished");
+    assert_eq!(lexer.tokens[4].data, TokenData::Punctuator(Punctuator::Dec));
+    // Decrementing means adding 2 characters '--', the lexer should consume it as a single token
+    // and move the curser forward by 2, meaning the next token should be a semicolon
+    assert_eq!(
+        lexer.tokens[5].data,
+        TokenData::Punctuator(Punctuator::Semicolon)
+    );
+}