Router.PreferExactMatches ends up limiting trimming unnecessarily

[stephentoub]

Router.PreferExactMatches has this comment on it:

aspnetcore/src/Components/Components/src/Routing/Router.cs

Lines 85 to 87 in dfe625f

	/// Important: all applications should explicitly set this to true. The option to set it to false
	/// (or leave unset, which defaults to false) is only provided for backward compatibility.
	/// In .NET 6, this option will be removed and the router will always prefer exact matches.

As of now, though, the option is still available and still false by default. The net effect is the "LegacyRoute" support constructed here:

aspnetcore/src/Components/Components/src/Routing/Router.cs

Lines 159 to 161 in dfe625f

	Routes = PreferExactMatches
	? RouteTableFactory.Create(assemblies)
	: LegacyRouteTableFactory.Create(assemblies);

ends of referencing and preventing a bunch of APIs that could otherwise by trimmed by the linker from getting trimmed in a default Blazor wasm app.

cc: @eerhardt, @javiercn

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[pranavkm]

@SteveSandersonMS / @javiercn, what's our plan for this? If we want to remove legacy routing (I'd be open to it), we should do it right away. If we want to wait until 7.0 to remove legacy routing, we should consider enabling legacy routing behind a feature switch so that we have the ability to trim it away.

[SteveSandersonMS]

If possible it would be great to drop the legacy code paths ASAP. If we get any signs of dissatisfaction, we could ship the legacy router OOB in a separate package.

Of course it would need to be announced as a breaking change.

[pranavkm]

Cool. I have a PR out for it - #31295 - that's part of my no-linq work. Let me see if I can separate the two since they're not really dependent.