chore: add comments

Author: jhefferman-sfdc

packages/@lwc/template-compiler/src/parser/expression-complex/validate.ts

@@ -201,7 +201,7 @@ export function validateComplexExpression(
     expression: Expression;
     raw: string;
 } {
-    const leadingChars = source.slice(expressionStart + 1, expression.start);
+    const leadingChars = source.slice(expressionStart + OPENING_CURLY_LEN, expression.start);
     const trailingChars = getTrailingChars(source.slice(expression.end));
     const idxOfClosingBracket = expression.end + trailingChars.length;
     // Capture text content between the outer curly braces, inclusive.
@@ -239,7 +239,7 @@ export function validateComplexExpression(
     invariant(
         expression.end === templateExpression.end,
         ParserDiagnostics.TEMPLATE_EXPRESSION_PARSING_ERROR,
-        ['Expression incorrectly formed']
+        ['expression incorrectly formed']
     );
 
     return {

packages/@lwc/template-compiler/src/parser/index.ts

@@ -518,14 +518,16 @@ function parseTextComplex(
                 index
             );
             parsedTextNodes.push(ast.text(parsed.raw, parsed.expression, location));
+
+            // Parse the remainder of the text node for expressions
             index += parsed.raw.length;
             start = index;
             continue;
         }
         index++;
     }
 
-    // Parse any literal that followed the expression
+    // Parse any remaining literal
     if (start < rawText.length) {
         const literalToken = rawText.slice(start, index);
         parsedTextNodes.push(
@@ -1914,6 +1916,13 @@ function getTemplateAttribute(
 
     let attrValue: Literal | Expression;
 
+    /*
+        A complex attribute expression should only be parsed as a complex expression if it has been quoted.
+        Quoting complex expressions ensures that the expression is valid HTML. If the complex expression 
+        has not been quoted, then it is parsed as a legacy expression and it will fail with an appropriate explanation.
+        This ensures backward compatibility with legacy expressions which do not require, or currently permit quotes
+        to be used.
+    */
     const isPotentialComplexExpression =
         quotedExpression && !escapedExpression && value.startsWith(EXPRESSION_SYMBOL_START);
     if (ctx.config.experimentalComplexExpressions && isPotentialComplexExpression) {