SA1027 (TabsMustNotBeUsed) code fix ignores leading whitespace

[sharwell]

Suppose you have the source code which is represented by the following C# string literal:

"public\tclass\tFoo{}"

• If you attempt to only fix the first tab, the result is correctly indented.
• If you attempt to fix the first tab and then the second tab, the result is correctly indented.
• If you attempt to correct only the second tab, it is indented one space too many.

This occurs because the code fix assumes each character preceding the whitespace which is being fixed has a width of 1 character.

This problem is most likely to appear when using the fix all provider. The batch fixer applies each change in isolation and merges the result, so lines containing multiple tab characters separated by non-whitespace characters are frequently incorrect in the result.