disable api compat for design time build

[Anipik]

Related to #32146

Design time build is failing as it tries to run apicompat even for all the target frameworks. ApiCompat will still run for building inside the visual studio

[eerhardt]

I was thinking a bit more about this. Why is ApiCompat hooked into $(TargetsTriggeredByCompilation)? That doesn't seem like the ideal place for it to be hooked into the build. I would have expected it to be an AfterTargets="Build" or AfterTargets="CoreBuild", something along those lines.

If we are going to keep it hooked into $(TargetsTriggeredByCompilation) and if we didn't want to depend on the $(DesignTimeBuild) property, it looks like a different property we could check is $(SkipCompilerExecution). This is the property that tells CoreCompile to not actually compile the C# code during these design-time builds. It is paired with $(ProvideCommandLineArgs), which tells the Csc task to just output the full csc.exe command line args, which VS uses. It seems reasonable to not run ApiCompat when $(SkipCompilerExecution) == true.

[eerhardt]

Looking at https://github.com/dotnet/project-system/blob/master/docs/design-time-builds.md#determining-whether-a-target-is-running-in-a-design-time-build, it looks like depending on $(DesignTimeBuild) is acceptable.

QUESTION: Should this change also go into https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.ApiCompat/build/Microsoft.DotNet.ApiCompat.targets#L21? Does it ever make sense to run ApiCompat during a DesignTimeBuild? Reading that project-system doc:

Targets that dynamically change references, source files or compilation options must run during design-time builds to avoid unexpected behavior in Visual Studio. In contrast, if a target does not contribute these items, then it should actively avoid running in these builds to ensure design-time builds are as fast as possible.

I don't think ApiCompat changes references, source files or compilation options, right? So it would seem best to default it to off during a DesignTimeBuild.

[ViktorHofer]

I agree with @eerhardt on that. Let's move the fix over to arcade and change the defaults in ApiCompat.targets to not run on design time builds. I would not merge this in to not add unnecessary complexity.

[eerhardt]

Here's my Build Logging window when ApiCompat is running during DesignTimeBuild:

And when I shut it off:

Notice that the time went from 2 seconds down to under 0.1 seconds for the netstandard2.0 design time build. If you had a few projects in a single solution, doing ApiCompat would be taking a lot of time.

[ViktorHofer]

Superseded by dotnet/arcade#4829