parser: support prefix expressions like !

phpast/samples/bang.php

@@ -0,0 +1,3 @@
+<?php
+
+! $foo;

trunk_parser/src/ast.rs

@@ -273,6 +273,7 @@ pub enum Expression {
     Bool(bool),
     ArrayIndex(Box<Self>, Option<Box<Self>>),
     Null,
+    BooleanNot(Box<Self>),
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize)]

trunk_parser/src/parser/mod.rs

@@ -912,6 +912,16 @@ impl Parser {
 
                 Expression::New(Box::new(target), args)
             },
+            _ if is_prefix(&self.current.kind) => {
+                let op = self.current.kind.clone();
+
+                self.next();
+
+                let rbp = prefix_binding_power(&op);
+                let rhs = self.expression(rbp)?;
+
+                prefix(&op, rhs)
+            },
             _ => todo!("expr lhs: {:?}", self.current.kind),
         };
 
@@ -1077,6 +1087,27 @@ impl Parser {
     }
 }
 
+fn is_prefix(op: &TokenKind) -> bool {
+    match op {
+        TokenKind::Bang => true,
+        _ => false
+    }
+}
+
+fn prefix_binding_power(op: &TokenKind) -> u8 {
+    match op {
+        TokenKind::Bang => 99,
+        _ => unreachable!()
+    }
+}
+
+fn prefix(op: &TokenKind, rhs: Expression) -> Expression {
+    match op {
+        TokenKind::Bang => Expression::BooleanNot(Box::new(rhs)),
+        _ => unreachable!()
+    }
+}
+
 fn infix(lhs: Expression, op: TokenKind, rhs: Expression) -> Expression {
     if op == TokenKind::Equals {
         return Expression::Assign(Box::new(lhs), Box::new(rhs));