Add feature-switch for invariant TimeZone support

[marek-safar]

For size sensitive apps it'd be very helpful to have an option (feature switch) to exclude full time-zones support. This is something that is supported by Blazor 3.2 as an opt-in feature which we should not regress on.

@eerhardt

[Clockwork-Muse]

... This is implying that we'd be distributing our own copy of timezone data? Or is this something else? Traditionally, we've pulled from the OS for desktop/server deployments, which would mean the size of the timezone data with an app is 0.

Note that the full tzdb is < 400kb, and is trivially amenable to CDN delivery/caching. Perhaps Blazor should also add the ability to specify a web resource for this, so it can be delivered out-of-band from the app itself?

[joperezr]

@eerhardt at least the setting seems to already be present in the SDK so isn't this one done already? https://github.com/dotnet/sdk/blob/408fe82020f979ec65862add4d5b1dd85fef5d80/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets#L396-L399

What am I missing?

@Clockwork-Muse The point here is to have a switch so that consumers can choose to always treat the Timezone as Invariant with the intent of reducing the application final size once it gets linked since all of the code in the framework that takes Timezones and compares them will be optimized out.

[eerhardt]

@eerhardt at least the setting seems to already be present in the SDK so isn't this one done already?
What am I missing?

The InvariantGlobalization setting only affects the globalization data that we read from ICU. In relation to TimeZones, the only thing InvariantGlobalization affects is how we look up time zone names - we will use a non-localized name from the tzdata file. We will need a new setting here since we've shipped InvariantGlobalization already with the current TimeZone behavior.

This is something that is supported by Blazor 3.2 as an opt-in feature which we should not regress on.

In Blazor WASM 3.2, we shipped an MSBuild property named BlazorEnableTimeZoneSupport, which removes the dotnet.timezones.dat file from the app. That functionality is in 5.0-preview8 currently.

This work item is to introduce a new feature switch in the BCL which will disable Time Zone support altogether (even without the app being linked, as feature switches can be configured by AppContext). The feature switch will also be used to trim the TimeZoneInfo code in Blazor WASM, when enabled.

Since this is net-new functionality that changes behavior, and .NET 5 is closing down, I don't believe this is strictly necessary to ship .NET 5. The behavior from 3.2 is not regressed with the current bits.

In 6.0, we can introduce a new feature switch in this area.

cc @pranavkm

[SamMonoRT]

@eerhardt @pranavkm - Still seems to be in Uncommitted state - do we think this is possible to fit in for .NET6 ?