Improve error messages for keywords used as identifiers in LINQ let clauses

[Copilot]

Fix error messages for using a keyword instead of an identifier in LINQ let clauses

Issue Summary

When using a reserved keyword (like params) as an identifier in a LINQ let clause, the compiler previously produced many confusing error messages (8-10 errors). The goal was to:

1. Change "CS1001: Identifier expected" to "CS1041: Identifier expected; 'params' is a keyword"
2. Reduce the number of noisy error messages by better error recovery

Implementation

Modified LanguageParser.ParseLetClause() to:

• Detect when a reserved keyword is followed by = (the expected pattern for let clause) using a conditional expression
• Use EatTokenAsKind to create a missing identifier token with the keyword attached as skipped syntax
• This produces the proper ERR_IdentifierExpectedKW error and allows better error recovery

This results in just 1 clear error message instead of 8-10 confusing ones.

Changes Made

1. src/Compilers/CSharp/Portable/Parser/LanguageParser.cs:
   • Updated ParseLetClause method using conditional (ternary) expression with ? and : on new lines
   • Uses EatTokenAsKind to create missing identifier token with keyword as skipped syntax
   • Fixed blank lines to not contain any whitespace (Roslyn linting requirement)
2. src/Compilers/CSharp/Test/Syntax/Parsing/ExpressionParsingTests.cs:
   • Added test case LetClauseWithKeywordAsIdentifier
   • Test verifies missing identifier token with keyword in skipped syntax
3. src/Compilers/CSharp/Test/Semantic/Semantics/QueryTests.cs:
   • Updated QueryWithMultipleParseErrorsAndScriptParseOption test to expect improved parsing
   • The query now parses completely instead of stopping after "let "
4. .github/copilot-instructions.md:
   • Added note about blank lines not containing whitespace to avoid linting errors
   • Removed redundant formatting instruction

Test Results

• All 596 expression parsing tests pass
• All syntax parsing tests pass
• QueryWithMultipleParseErrorsAndScriptParseOption test updated and passing
• No regressions in existing tests

Before/After

Before: Using params in a let clause produced 8+ errors including CS1001, CS1003, CS1525, CS0742, etc.

After: Using params in a let clause produces 1 clear CS1041 error: "Identifier expected; 'params' is a keyword"

Fixes #10446

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Error messages for using a keyword instead of an identifier could be improved</issue_title>
<issue_description>Consider the following statement with a LINQ query:

var x =
    from m in typeof(Foo).GetMethods()
    let params = m.GetParameters()
    select new { m, params };

This is invalid, because params is a C# keyword. But while this is a seemingly simple error, it causes the compiler to output all of this:

For line 3:

error CS1001: Identifier expected
error CS1003: Syntax error, '=' expected
error CS1525: Invalid expression term 'params'
error CS0742: A query body must end with a select clause or a group clause
error CS1003: Syntax error, ',' expected
error CS1002: ; expected
error CS1525: Invalid expression term '='
error CS1002: ; expected

For line 4:

error CS1002: ; expected
error CS1041: Identifier expected; 'params' is a keyword

Only the last message clearly identifies the actual error.

My requests:

1. Could the "CS1001: Identifier expected" for line 3 be changed into the more descriptive "CS1041: Identifier expected; 'params' is a keyword"?
2. It would be nice if using a keyword this way wouldn't cause so many noisy error messages. I realize parsing this requires the compiler to guess how the code should be parsed, but I think the guess is currently not great in this case.
   </issue_description>

<agent_instructions>Copilot see if this is easy to fix. This would be in LanguageParser.ParseLetClause. if we see a keyword, but still parsed followed by an equals sign, then recover well. You can use EatTokenEvenWithIncorrectKind if you detect a keyword as that should produce a good error message.

Test can be added in src\Compilers\CSharp\Test\Syntax\Parsing\ExpressionParsingTests.cs. It should use UsingExpression and should validate the diagnostic, and the parse tree shape.
</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #11418

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[CyrusNajmabadi]

@copilot inline this method.

[Copilot]

Done in commit fe41692 (inlined the method directly into the if condition).

[CyrusNajmabadi]

@copilot conver this to a simple conditional expression. placce the ? and : on new lines with the following expresison on the same line.

[Copilot]

Done in commit 570ed57 - converted to ternary operator with ? and : on new lines.

[CyrusNajmabadi]

@copilot instead of this pair, use EatTokenAsKind instead. We want a missing token here with the keyword attached as a skipped token.

[Copilot]

Done in commit 570ed57 - now using EatTokenAsKind which creates a missing identifier token with the keyword attached as skipped syntax.

[CyrusNajmabadi]

@copilot Roslyn disallows blank lines with spaces on them. Please update the copilot contributing document to mention that this should be avoided as it will cause linting errors that will have to be fixed up.

[Copilot]

Done in commit 2ce564d. Fixed the blank lines with spaces in ParseLetClause and updated .github/copilot-instructions.md to include a note about this requirement under the Formatting section.

[CyrusNajmabadi]

@copilot QueryWithMultipleParseErrorsAndScriptParseOption is failing now (since parsing improved in that test case). Can you determine why the test is failing and update the expected results?

[Copilot]

@copilot QueryWithMultipleParseErrorsAndScriptParseOption is failing now (since parsing improved in that test case). Can you determine why the test is failing and update the expected results?

Done in commit 9406664. The test was failing because the parser now recovers better - instead of stopping after "let ", it continues parsing to "from i in expr1 let namespace = expr1 select i". Updated the test to expect the complete query expression.

[CyrusNajmabadi]

@dotnet/roslyn-compiler this is ready fro review.