lex: fix erroneous keyword recognition

I tried to parse a function called
"fn_with_callback" for the upcoming function
support and realized that the lexer mistook the
first two letters for the respective keyword.
So that is fixed now, i guess.

src/lex/mod.rs

@@ -192,7 +192,11 @@ impl Lexer {
                 if kinds.contains(&t.kind) {
                     Ok(t)
                 } else {
-                    self.syntax_error(format!("Expected one of {:?}, got {}", kinds, t.kind))
+                    self.syntax_error(if kinds.len() == 1 {
+                        format!("Expected {}, got {}", kinds[0], t.kind)
+                    } else {
+                        format!("Expected one of {:?}, got {}", kinds, t.kind)
+                    })
                 }
             }
             Some(Err(e)) => Err(e),
@@ -213,7 +217,18 @@ impl Lexer {
             // keywords are always at least 2 characters long as per the language spec
             let first_char = kw.raw.chars().next().unwrap();
             if current == first_char && self.skip_if_match(&kw.raw[1..]) {
-                return self.token_ok(kw.kind);
+                // We need to account for identifiers that just happen to start
+                // with a keyword name.  For example, "settings" begins with the
+                // "set" keyword, but is obviously still an identifier.
+                let is_really_a_keyword = match self.cursor.peek() {
+                    Some(c) if c.is_ident_part() => false,
+                    _ => true,
+                };
+                if is_really_a_keyword {
+                    return self.token_ok(kw.kind);
+                } else {
+                    break;
+                }
             }
         }
 
@@ -222,7 +237,7 @@ impl Lexer {
 
     fn read_ident(&mut self) -> Result<Token, Error> {
         for c in &mut self.cursor {
-            if !c.is_ascii_alphanumeric() && c != '_' {
+            if !c.is_ident_part() {
                 self.cursor.prev();
                 break;
             }
@@ -326,3 +341,17 @@ impl Lexer {
         ))
     }
 }
+
+trait IsIdentifier {
+    fn is_ident_start(&self) -> bool;
+    fn is_ident_part(&self) -> bool;
+}
+
+impl IsIdentifier for char {
+    fn is_ident_start(&self) -> bool {
+        self.is_ascii_alphabetic() || self == &'_'
+    }
+    fn is_ident_part(&self) -> bool {
+        self.is_ascii_alphanumeric() || self == &'_'
+    }
+}