Fix T0010 FP: Allow "_" discard like parameter name

[martin-strecker-sonarsource]

_ => 42 should be allowed, because _ is more descriptive than x in this case and signals: "I do not care about the parameter." Rational: In (_, _) => 42 _ is a real discard, while in _ => 42 it isn't only for back compatibility reasons.

See e.g. here:
https://sonarcloud.io/project/issues?resolved=false&pullRequest=8950&id=sonaranalyzer-dotnet&open=AY9NLK1CHaBGYjI6fPmk

_ is a better name than x in such a case.

[martin-strecker-sonarsource]

/azp run

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[sonarcloud]

Quality Gate passed for 'Sonar .NET Java Plugin'

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[sonarcloud]

Quality Gate passed for 'SonarAnalyzer for .NET'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[pavel-mikula-sonarsource]

LGTM

[pavel-mikula-sonarsource]

What about leaving a comment here that in C# 13, it will be a real discard and the "_" will not be needed anymore?

[martin-strecker-sonarsource]

Even if parsed as a discard, it is still an IdenifierToken:
sharplab.io

So I think it will still be needed.

[pavel-mikula-sonarsource]

Do you know if there's a branch available with that feature? I didn't find any in a quick search.

It might not be IdentifierToken, but some kind of discard designator, like here:
https://sharplab.io/#v2:C4LghgzsA0AmIGoA+ABATARgLACgUGYACdQgYUIG9dCbiiUAWQgWQAoBKS62ngNzABOhVgH1ohAB6cAvIQAqHANzcaAXxWENABwEBLfsACmw3QDsYhM8E4KZAPkKxDAMzABXADbBlOVUA===

[martin-strecker-sonarsource]

I'm not aware of a branch for it. The closest I could find on SharpLab is "Semi Auto-properties," but that one hasn't been updated since Oct 22.

https://sharplab.io/#v2:EYLgZgpghgLgrgJwgZwLTIgWwJaqnGAe1QAcFCTkAaGEKZGKgExAGoAfAAQCYBGAWABQnAMwACHmIDCYgN5CxiieOwA7GGIAK5EnLEBzCBoC8APjFhsEADZMA3GIwwHAXwVL3i0RIAsYgLIAFACUcp5KXrzcADxqMOZQAMYw2ISqYsZiAPoZ5rIuduFugi5AA===

The DiscardDesignation is an interesting find. The significant difference here is that the distinguishing happens on the node level, while for parameters, it would likely need to occur on the token level.

There are cases like this already with, e.g., IdentifierNameSyntax where an Identifier property can be either an IdentiferToken or a GlobalKeyword.
There can be IdentifierToken or the ArgListKeyword for parameters. Maybe we see an UnderScore here as well in the future.

    <Node Name="ParameterSyntax" Base="BaseParameterSyntax">
        <Kind Name="Parameter"/>
        <! -- ... -->
        <Field Name="Identifier" Type="SyntaxToken">
            <PropertyComment>
                <summary>Gets the identifier.</summary>
            </PropertyComment>
            <Kind Name="IdentifierToken"/>
            <Kind Name="ArgListKeyword"/>
        </Field>
        <! -- ... -->
    </Node>