Update GeneratedRegex to support partial properties

[stephentoub]

Partial properties are destined for support in C# 13. When that happens, we should utilize them in the regex generator (we wanted this experience from the get-go).

This will entail:

• Updating GeneratedRegexAttribute to target properties:

-[AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
 public sealed class GeneratedRegexAttribute : Attribute

• Updating the generator to support instance and static get-only properties, e.g. if a user writes:

[GeneratedRegex("...")]
private partial Regex MyProp1 { get; }

[GeneratedRegex("...")]
public static partial Regex MyProp2 { get; }

the generator emits:

private partial global::System.Text.RegularExpressions.Regex MyProp1 => global::System.Text.RegularExpressions.Generated.MyProp1_0.Instance;
public static partial global::System.Text.RegularExpressions.Regex MyProp2 => global::System.Text.RegularExpressions.Generated.MyProp2_0.Instance;

We will only support get-only auto props. Others will result in an error diagnostic.

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Issue Details

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Partial properties are possibly destined for support in C# 12. If/when that happens, we should utilize them in the regex generator (we wanted this experience from the get-go).

This will entail:

• Updating GeneratedRegexAttribute to target properties:

-[AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
 public sealed class GeneratedRegexAttribute : Attribute

• Updating the generator to support instance and static get-only properties, e.g. if a user writes:

[GeneratedRegex("...")]
private partial Regex MyProp1 { get; }

[GeneratedRegex("...")]
public static partial Regex MyProp2 { get; }

the generator emits:

private partial global::System.Text.RegularExpressions.Regex MyProp1 => global::System.Text.RegularExpressions.Generated.MyProp1_0.Instance;
public static partial global::System.Text.RegularExpressions.Regex MyProp2 => global::System.Text.RegularExpressions.Generated.MyProp2_0.Instance;

• Deciding if/how we want to support settable properties. (My default position would be to simply error on them, and only support get-only props.)

• Erroring on non-auto-props with GeneratedRegex on them.

This will all obviously be subject to whatever the design for partial properties ends up being.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	8.0.0

[joperezr]

Calling this out since it wasn't mentioned above as the things this would entail, but we should also consider at the time this gets added whether or not our Roslyn Analyzer and Code Fixers should suggest using the static partial method or the properties instead.

[stephentoub]

FYI @RikkiGibson, @333fred

[RikkiGibson]

dotnet/csharplang#6420

[teo-tsirpanis]

whether or not our Roslyn Analyzer and Code Fixers should suggest using the static partial method or the properties instead

I think we should suggest the partial properties when we can; they are one parentheses pair easier to use than methods and the returned Regex is a cached singleton and matches the expectation that property accesses are cheap.

[ericstj]

Moved this out since partial properties are not available yet.

[bartonjs]

Video

Looks good as proposed

-[AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
 public sealed class GeneratedRegexAttribute : Attribute