Analyzer proposal: Use String.Contains(char) instead of String.Contains(String)

[xtqqczze]

Suggested category: Performance
Suggested default severity: Suggestion
Fix is breaking or non-breaking: Non-breaking

Cause

This rule locates calls to Contains(String) when the string consists of a single char, and suggests using Contains(char) instead

Rule description

When Contains(String) is used with a single char, the call can be safely substituted with Contains(char).

Additional context

#24068
dotnet/coreclr#15740
#36310

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mavasani]

Tagging @carlossanlop @buyaa-n - .NET API analyzer suggestion.

[carlossanlop]

Seems pretty straightforward. There was a previous effort to manually replace instances of String.Contains(string) with String.Contains(char), so an analyzer should automatically find any new instances for us from now on.

Suggested severity: Warning
Suggested category: Performance
Suggested milestone: Future (but we welcome contributions for 6.0)
Analyzer size: Small
Fixer size: Small

Should cover all overloads, and only where a literal is passed as argument:

string str = "whatever";
string find = "a";

// Yes
str.Contains("a");
str.Contains("a", StringComparison.InvariantCultureIgnoreCase);

// No
str.Contains(find);
str.Contains(find, StringComparison.InvariantCultureIgnoreCase);

[GrabYourPitchforks]

This as proposed is not a breaking change, but we need to take care if we extend this analyzer to other methods (IndexOf, etc.). Switching from string to char in those methods is an observable behavioral change.