[Analyzer] stringBuilder.Append(string.Substring(...)) to stringBuilder.Append(string, ...)

[stephentoub]

This pattern shows up reasonably frequently and ends up allocating an additional string:
https://grep.app/search?q=%5C.Append%5C%28%5Cw%2A%5C.Substring&regexp=true&filter[lang][0]=C%23

We should have an analyzer that flags use like:

stringBuilder.Append(string.Substring(offset));
stringBuilder.Append(string.Substring(offset, length));

and a fixer that transforms it into:

stringBuilder.Append(string, offset, string.Length - offset);
stringBuilder.Append(string, offset, length);

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

[jilles-sg]

Rule CA1846 will propose changing Substring to AsSpan which likewise eliminates the allocation. Do we need another analyzer for this case?

[stephentoub]

AsSpan won't work / be flagged in a library that targets netstandard, whereas the Append overload that accepts offset and length is available everywhere.