token: add helper functions

src/lex/token.rs

@@ -82,6 +82,82 @@ pub enum Kind {
     Comment,
 }
 
+impl Kind {
+    pub fn is_start_of_lhs_expr(&self) -> bool {
+        match self {
+            Kind::Ident => true,
+            Kind::IntLiteral => true,
+            Kind::StringLiteral => true,
+            _ => false
+        }
+    }
+
+    pub fn is_start_of_expr(&self) -> bool {
+        match self {
+            k if k.is_start_of_lhs_expr() => true,
+            Kind::Minus => true,
+            _ => false
+        }
+    }
+
+    pub fn is_assignment_op(&self) -> bool {
+        match self {
+            Kind::Eq => true,
+            Kind::PlusEq => true,
+            Kind::MinusEq => true,
+            Kind::AsteriskEq => true,
+            Kind::AsteriskAsteriskEq => true,
+            Kind::SlashEq => true,
+            Kind::PercentEq => true,
+            Kind::GtGtEq => true,
+            Kind::LtLtEq => true,
+            Kind::PipeEq => true,
+            Kind::AmpEq => true,
+            Kind::CaretEq => true,
+            _ => false,
+        }
+    }
+
+    /// If this token is a binary operator, return its precedence value when
+    /// interpreted as such.  See `operator-precedence.txt` for details.
+    pub fn binary_op_precedence(&self) -> Option<u32> {
+        match self {
+            Kind::Eq => Some(2),
+            Kind::PlusEq => Some(2),
+            Kind::MinusEq => Some(2),
+            Kind::AsteriskEq => Some(2),
+            Kind::AsteriskAsteriskEq => Some(2),
+            Kind::SlashEq => Some(2),
+            Kind::PercentEq => Some(2),
+            Kind::AmpEq => Some(2),
+            Kind::PipeEq => Some(2),
+            Kind::CaretEq => Some(2),
+            Kind::GtGtEq => Some(2),
+            Kind::LtLtEq => Some(2),
+            Kind::PipePipe => Some(3),
+            Kind::AmpAmp => Some(4),
+            Kind::EqEq => Some(5),
+            Kind::BangEq => Some(5),
+            Kind::Gt => Some(5),
+            Kind::GtEq => Some(5),
+            Kind::Lt => Some(5),
+            Kind::LtEq => Some(5),
+            Kind::Pipe => Some(6),
+            Kind::Caret => Some(7),
+            Kind::Amp => Some(8),
+            Kind::GtGt => Some(9),
+            Kind::LtLt => Some(9),
+            Kind::Plus => Some(10),
+            Kind::Minus => Some(10),
+            Kind::Asterisk => Some(11),
+            Kind::Slash => Some(11),
+            Kind::Percent => Some(11),
+            Kind::AsteriskAsterisk => Some(12),
+            _ => None
+        }
+    }
+}
+
 impl fmt::Display for Kind {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(