finished resolver (chapter 11) and started classes (chapter 12)

frontend/src/lexer/error.rs

@@ -0,0 +1,19 @@
+use thiserror::Error;
+
+use super::CodePos;
+
+#[derive(Error, Debug)]
+pub enum LexerError {
+    #[error("Unexpected character '{c}' at {code_pos}.")]
+    UnexpectedCharacter { c: char, code_pos: CodePos },
+    #[error("Unterminated string literal starting at {code_pos}.")]
+    UnterminatedStringLiteral { code_pos: CodePos },
+    #[error("Unterminated block comment starting at {code_pos}.")]
+    UnterminatedBlockComment { code_pos: CodePos },
+    #[error("Invalid number literal {lexeme} at {code_pos}: {msg}")]
+    InvalidNumberLiteral {
+        lexeme: String,
+        msg: String,
+        code_pos: CodePos,
+    },
+}