Reserve some keywords that might be needed in the future

CHANGELOG.md

@@ -8,6 +8,11 @@ The Koto project adheres to
 
 ## [0.15.0] Unreleased
 
+### Changed
+
+#### Language
+
+- `await`, `const`, and `let` have been reserved as keywords for future use.
 
 ## [0.14.0] 2024.04.17
 

crates/lexer/src/lexer.rs

@@ -58,7 +58,7 @@ pub enum Token {
     Less,
     LessOrEqual,
 
-    // Pipe tokens are created at the parsing time tokens
+    // Pipe is detected by the parser instead of the lexer
     Pipe,
 
     // Keywords
@@ -93,6 +93,11 @@ pub enum Token {
     Until,
     While,
     Yield,
+
+    // Reserved keywords
+    Await,
+    Const,
+    Let,
 }
 
 impl Token {
@@ -639,8 +644,10 @@ impl<'a> TokenLexer<'a> {
         if !matches!(self.previous_token, Some(Token::Dot)) {
             check_keyword!("as", As);
             check_keyword!("and", And);
+            check_keyword!("await", Await);
             check_keyword!("break", Break);
             check_keyword!("catch", Catch);
+            check_keyword!("const", Const);
             check_keyword!("continue", Continue);
             check_keyword!("debug", Debug);
             check_keyword!("export", Export);
@@ -651,6 +658,7 @@ impl<'a> TokenLexer<'a> {
             check_keyword!("if", If);
             check_keyword!("import", Import);
             check_keyword!("in", In);
+            check_keyword!("let", Let);
             check_keyword!("loop", Loop);
             check_keyword!("match", Match);
             check_keyword!("not", Not);

crates/parser/src/error.rs

@@ -169,6 +169,8 @@ pub enum SyntaxError {
     MatchEllipsisOutsideOfNestedPatterns,
     #[error("'else' can only be used in the last arm in a match expression")]
     MatchElseNotInLastArm,
+    #[error("Keyword reserved for future use")]
+    ReservedKeyword,
     #[error("'self' doesn't need to be declared as an argument")]
     SelfArg,
     #[error("'else' can only be used in the last arm in a switch expression")]

crates/parser/src/parser.rs

@@ -868,6 +868,11 @@ impl<'source> Parser<'source> {
             Token::From | Token::Import => self.consume_import(context),
             Token::Export => self.consume_export(context),
             Token::Try => self.consume_try_expression(context),
+            // Reserved keywords
+            Token::Await => self.consume_token_and_error(SyntaxError::ReservedKeyword),
+            Token::Const => self.consume_token_and_error(SyntaxError::ReservedKeyword),
+            Token::Let => self.consume_token_and_error(SyntaxError::ReservedKeyword),
+            // An error occurred in the lexer
             Token::Error => self.consume_token_and_error(SyntaxError::LexerError),
             _ => return Ok(None),
         };

crates/parser/tests/parsing_failures.rs

@@ -380,5 +380,24 @@ switch
                 check_parsing_fails("from foo import bar as");
             }
         }
+
+        mod reserved_keywords {
+            use super::*;
+
+            #[test]
+            fn r#await() {
+                check_parsing_fails("await = 99");
+            }
+
+            #[test]
+            fn r#const() {
+                check_parsing_fails("const = 99");
+            }
+
+            #[test]
+            fn r#let() {
+                check_parsing_fails("let = 99");
+            }
+        }
     }
 }