Allow Regex engine to be trimmed

[eerhardt]

Move the lazy Regex creation code to a delegate that is only set with the RegexRouteConstraint constructor that takes a string regexPattern. This allows for the Regex engine to be trimmed when the regexPattern constructor is trimmed.

Fix #46142

Publishing dotnet new api -aot app on win-x64:

main: 12.1 MB (12,725,760 bytes)
PR: 11.3 MB (11,909,120 bytes)

[davidfowl]

I do not understand.

[JamesNK]

I do not understand.

We want to trim away new Regex(string, ...). We can't have it lazily called in the regex property because the property is always used by Match. Moving lazily creating the regex to the new RegexRouteConstraint(string) constructor means it's trimmed away along with the constructor.

FYI the alpha constraint still inherits from RegexRouteConstraint, but it doesn't use the base string ctor:

aspnetcore/src/Http/Routing/src/Constraints/AlphaRouteConstraint.cs

Lines 12 to 23 in ce8e0ce

	public partial class AlphaRouteConstraint : RegexRouteConstraint, ICachableParameterPolicy
	{
	/// <summary>
	/// Initializes a new instance of the <see cref="AlphaRouteConstraint" /> class.
	/// </summary>
	public AlphaRouteConstraint() : base(GetAlphaRouteRegex())
	{
	}
	[GeneratedRegex(@"^[A-Za-z]*$")]
	private static partial Regex GetAlphaRouteRegex();
	}