Fix comma placement after lambda block in variable declarations

[Copilot]

Fix comma placement after lambda block in variable declarations

Formatting incorrectly moved commas to a new line when multiple lambda or anonymous method variables were declared in a single statement:

// Before fix - comma forced to new line
Action a = () => { }
,
       b = () => { };

// After fix - comma stays on same line
Action a = () => { },
       b = () => { };

Root Cause

Lambda/anonymous method blocks have BlockSyntax parent (not ExpressionSyntax), so IsCloseBraceOfExpression() returns false. This triggered newline insertion logic, which checked exceptions for commas in initializers, arguments, tuples, and collections—but missed commas in variable declarations.

Changes

• FormattingHelpers.cs: Added IsCommaInVariableDeclaration() helper
• TokenBasedFormattingRule.cs: Added exception check for !currentToken.IsCommaInVariableDeclaration()
• FormattingTests.cs: Added tests for lambda lists, multiple variables, anonymous methods, and expression-bodied lambdas

Testing

• All 654 formatting tests pass
• Added WorkItem attribute to all test methods referencing issue Incorrect comma placement when formatting lambda list #10526
• Added test for expression-bodied (non-block) lambdas

Fixes #10526

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Incorrect comma placement when formatting lambda list</issue_title>
<issue_description>Version Used: VS 2015 Update 2

Steps to Reproduce:

1. Add using System;
2. Write the following code:

Action a = () => { }, 
       b = () => { };

1. Format the document

Expected Behavior: Nothing changes

Actual Behavior: The comma goes on its own line:

Action a = () => { }
,
       b = () => { };

---

Might be related to #7704, another lambda formatting bug I reported.
</issue_description>

<agent_instructions>@copilot see if you can figure out what's causing us to force a new-line here.</agent_instructions>

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

• Fixes Incorrect comma placement when formatting lambda list #10526

---

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[CyrusNajmabadi]

@copilot add [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/10526")] to all added tests.

[Copilot]

Done in 2e74af8.

[CyrusNajmabadi]

@copilot add tests with non-block lambdas:

                    Action a = () => x,
                           b = () => y;

[Copilot]

Done in 23bc2683.

[CyrusNajmabadi]

@copilot add tests with non-block lambdas:

                    Action a = () => x,
                           b = () => y;

[Copilot]

Done in 4893298.